Not all Shc's roads lead to Ras

Bifurcation of cell migratory and proliferative signaling by the adaptor protein Shc

Cytokines and extracellular matrix proteins initiate signaling cascades that regulate cell migration and proliferation. Evidence is provided that the adaptor protein Shc can differentially regulate these processes. Specifically, under growth factor-limiting conditions, Shc stimulates haptotactic cell migration without affecting anchorage-dependent proliferation. However, when growth factors are present, Shc no longer influences cell migration; rather, Shc is crucial for DNA synthesis. Mutational analysis of Shc demonstrates that, while tyrosine phosphorylation is required for both DNA synthesis and cell migration, the switch in Shc signaling is associated with differential use of Shc's phosphotyrosine interacting domains; the PTB domain regulates haptotaxis, while the SH2 domain is selectively required for proliferation.

Brain insulin receptors and spatial memory. Correlated changes in gene expression, tyrosine phosphorylation, and signaling molecules in the hippocampus of water maze trained rats

Evidence accumulated from clinical and basic research has indirectly implicated the insulin receptor (IR) in brain cognitive functions, including learning and memory (Wickelgren, I. (1998) Science 280, 517-519). The present study investigates correlative changes in IR expression, phosphorylation, and associated signaling molecules in the rat hippocampus following water maze training. Although the distribution of IR protein matched that of IR mRNA in most forebrain regions, a dissociation of the IR mRNA and protein expression patterns was found in the cerebellar cortex. After training, IR mRNA in the CA1 and dentate gyrus of the hippocampus was up-regulated, and there was increased accumulation of IR protein in the hippocampal crude synaptic membrane fraction. In the CA1 pyramidal neurons, changes in the distribution pattern of IR in particular cellular compartments, such as the nucleus and dendritic regions, was observed only in trained animals. Although IR showed a low level of in vivo tyrosine phosphorylation, an insulin-stimulated increase of in vitro Tyr phosphorylation of IR was detected in trained animals, suggesting that learning may induce IR functional changes, such as enhanced receptor sensitivity. Furthermore, a training-induced co-immunoprecipitation of IR with Shc-66 was detected, along with changes in in vivo Tyr phosphorylation of Shc and mitogen-activated protein kinase, as well as accumulation of Shc-66, Shc-52, and Grb-2 in hippocampal synaptic membrane fractions following training. These findings suggest that IR may participate in memory processing through activation of its receptor Tyr kinase activity, and they suggest possible engagement of Shc/Grb-2/Ras/mitogen-activated protein kinase cascades.

Accelerated mammary tumor development in mutant polyomavirus middle T transgenic mice expressing elevated levels of either the Shc or Grb2 adapter protein

The Grb2 and Shc adapter proteins play critical roles in coupling activated growth factor receptors to several cellular signaling pathways. To assess the role of these molecules in mammary epithelial development and tumorigenesis, we have generated transgenic mice which individually express the Grb2 and Shc proteins in the mammary epithelium. Although mammary epithelial cell-specific expression of Grb2 or Shc accelerated ductal morphogenesis, mammary tumors were rarely observed in these strains. To explore the potential role of these adapter proteins in mammary tumorigenesis, mice coexpressing either Shc or Grb2 and a mutant form of polyomavirus middle T (PyV mT) antigen in the mammary epithelium were generated. Coexpression of either Shc or Grb2 with the mutant PyV mT antigen resulted in a dramatic acceleration of mammary tumorigenesis compared to parental mutant PyV mT strain. The increased rate of tumor formation observed in these mice was correlated with activation of the epidermal growth factor receptor family and mitogen-activated protein kinase pathway. These observations suggest that elevated levels of the Grb2 or Shc adapter protein can accelerate mammary tumor progression by sensitizing the mammary epithelial cell to growth factor receptor signaling.

The p66shc adaptor protein controls oxidative stress response and life span in mammals

Gene mutations in invertebrates have been identified that extend life span and enhance resistance to environmental stresses such as ultraviolet light or reactive oxygen species. In mammals, the mechanisms that regulate stress response are poorly understood and no genes are known to increase individual life span. Here we report that targeted mutation of the mouse p66shc gene induces stress resistance and prolongs life span. p66shc is a splice variant of p52shc/p46shc (ref. 2), a cytoplasmic signal transducer involved in the transmission of mitogenic signals from activated receptors to Ras. We show that: (1) p66shc is serine phosphorylated upon treatment with hydrogen peroxide (H2O2) or irradiation with ultraviolet light; (2) ablation of p66shc enhances cellular resistance to apoptosis induced by H2O2 or ultraviolet light; (3) a serine-phosphorylation defective mutant of p66shc cannot restore the normal stress response in p66shc-/- cells; (4) the p53 and p21 stress response is impaired in p66shc-/- cells; (5) p66shc-/- mice have increased resistance to paraquat and a 30% increase in life span. We propose that p66shc is part of a signal transduction pathway that regulates stress apoptotic responses and life span in mammals.

Cytoskeletal reorganization leads to induction of the urokinase-type plasminogen activator gene by activating FAK and Src and subsequently the Ras/Erk signaling pathway

Previously, we showed that cytoskeletal reorganization (CSR) induced by colchicine or cyochalasins leads to activation of the urokinase-type plasminogen activator (uPA) gene in LLC-PK(1) cells via the Ras/Erk signaling pathway [Irigoyen et al. (1997) J. Biol. Chem. 272, 1904]. It remained to be seen how CSR activates Ras/Erk signaling. Changes in cell morphology triggered by extracellular signals are often mediated by integrin-associated proteins, such as focal adhesion kinase (FAK) and Src. We found that CSR induced the activation of FAK and Src and the association of FAK and Shc, a signaling molecule linking growth factor receptor tyrosine kinase and Grb2. Furthermore, expression of either FRNK, a kinase-minus FAK-like molecule acting as a dominant negative FAK, or a dominant negative Src suppressed CSR-induced uPA gene promoter activation. These results suggest that cells respond to a morphology change, using the cytoskeleton as a sensor, by activating FAK and Src and subsequently the Ras/Erk signaling pathway.

Obligatory cross-talk with the tyrosine kinases assembled with the TCR/CD3 complex in CD4 signal transduction

Dissection of the CD4 signal transduction pathway has revealed striking similarities with the TCR/CD3 pathway. Furthermore, downstream signaling by CD4 is impaired in cells lacking surface TCR, suggesting a role for the TCR/CD3 complex in CD4 signal transduction. We have investigated the molecular basis for the dependence of CD4 signaling on TCR/CD3 expression. Using the phosphotyrosine binding domains of the Shc adaptor and the Fyn kinase, which both participate in CD4 signaling, as baits, we show that CD4 induces tyrosine phosphorylation of a subset of the proteins phosphorylated in response to TCR/CD3 engagement. The phosphoprotein patterns were dramatically altered in cells defective for TCR/CD3 expression, and were recoverable by reconstitution of correctly assembled TCR, suggesting that CD4 uses TCR/CD3-associated tyrosine kinases to signal. Among the tyrosine kinases associated with the resting TCR/CD3 complex, only Fyn is activated following CD4 engagement. The failure of Fyn to become phosphorylated in cells defective for TCR expression underlines the unique role of TCR/CD3 associated Fyn in CD4 signal transduction. While no calcium mobilization was measurable in cells defective for TCR/CD3 expression in response to CD4 engagement, the Ras/MAP kinase pathway could be partially activated. Thus, CD4 activates at least two signaling pathways, and tyrosine kinases associated with the TCR/CD3 complex are key components of one of these pathways.

Distinct calcium-dependent pathways of epidermal growth factor receptor transactivation and PYK2 tyrosine phosphorylation in PC12 cells

Recently, we have demonstrated that in PC12 cells activation of the Ras/extracellular signal-regulated kinase pathway in response to membrane depolarization or bradykinin is mediated by calcium-dependent transactivation of the epidermal growth factor receptor (EGFR). Here we address the question whether Ca(2+)-calmodulin-dependent protein kinase (CaM kinase) has a role in the EGFR transactivation signal. Using compounds that selectively interfere with either CaM kinase activity or calmodulin function, we show that KCl-mediated membrane depolarization-triggered, but not bradykinin-mediated signals involve CaM kinase function upstream of the EGFR. Although both depolarization-induced calcium influx and bradykinin stimulation of PC12 cells were found to induce c-fos transcription through EGFR activation, the former signal is CaM kinase-dependent and the latter was shown to be independent. As PYK2 is also activated upon elevation of intracellular calcium, we investigated the potential involvement of this cytoplasmic tyrosine kinase in EGFR transactivation. Interestingly, we observed that inhibition of CaM kinase activity in PC12 cells abrogated tyrosine phosphorylation of PYK2 upon KCl but not bradykinin treatment. Nevertheless, PYK2 activation in response to both stimuli appeared to be mediated by pathways parallel to EGFR transactivation. Our data demonstrate the existence of two distinct calcium-dependent mechanisms leading either to EGFR-mediated extracellular signal-regulated activation or to PYK2 tyrosine phosphorylation. Both pathways either in concert or independently might contribute to the definition of biological responses in neuronal cell types.

The Listeria monocytogenes protein InlB is an agonist of mammalian phosphoinositide 3-kinase

The Gram-positive pathogen Listeria monocytogenes induces its own internalization into some non-phagocytic mammalian cells by stimulating host tyrosine phosphorylation, phosphoinositide (PI) 3-kinase activity, and rearrangements in the actin cytoskeleton. Entry into many cultured cell lines is mediated by the bacterial protein InlB. Here we investigate the role of InlB in regulating mammalian signal transduction and cytoskeletal structure. Treatment of Vero cells with purified InlB caused rapid and transient increases in the lipid products of the PI 3-kinase p85-p110, tyrosine phosphorylation of the mammalian adaptor proteins Gab1, Cbl, and Shc, and association of these proteins with p85. InlB also stimulated large scale changes in the actin cytoskeleton (membrane ruffling), which were PI 3-kinase-dependent. These results identify InlB as the first reported non-mammalian agonist of PI 3-kinase and demonstrate similarities in the signal transduction events elicited by this bacterial protein and known agonists such as epidermal growth factor.

Insulin-like growth factor I synergizes with interleukin 4 for hematopoietic cell proliferation independent of insulin receptor substrate expression

In the present study, we investigated the potential role of insulin-like growth factor I (IGF-I) receptor (IGF-IR) in cell proliferation by overexpressing it in 32D myeloid progenitor cells. The overexpression of IGF-IR caused the transfectants to proliferate in response to IGF-I in the absence of insulin receptor substrate (IRS) expression. The activation of overexpressed wild-type IGF-IR, but not that of an ATP-binding mutant of IGF-IR, resulted in the increased tyrosine phosphorylation of several intracellular proteins, including SHC, Src homology 2-containing inositol-5-phosphatase, protein kinase C-delta, and Erk2. Grb2 association with SHC and mitogen-activated protein kinase (MAPK) activity was also enhanced in response to IGF-I stimulation. Interestingly, the stimulation of the IGF-IR transfectants with interleukin 4 (IL-4) also resulted in strong mitogenesis independent of IRS expression. Moreover, IGF-I and/or IL-4 induced long-term cell growth of the IGF-IR transfectants. IL-4 was able to synergize with IGF-I for DNA synthesis, even in the parental 32D cells and a pro-B-cell line, Baf3, indicating the physiological importance of the two growth factors in hematopoietic cell proliferation. IL-4 stimulation of the IGF-IR transfectants resulted in enhanced tyrosine phosphorylation of SHC, Erk2, and signal transducer and activator of transcription 6 (STAT6) proteins. Both IL-4 and IGF-I were able to induce c-myc early response gene expression, and this expression was maximal in the presence of both factors. Finally, we demonstrated that a MAPK kinase inhibitor was able to suppress mitogenesis of the IGF-IR transfectants in response to IGF-I and/or IL-4. Together, our results suggest that IL-4 synergizes with IGF-I for hematopoietic cell proliferation, likely through cross talk between SHC/Grb2/MAPK and STAT6 pathways and through c-myc gene up-regulation.

Protein kinase C regulates integrin-induced activation of the extracellular regulated kinase pathway upstream of Shc

Adhesion of fibroblasts to extracellular matrices via integrin receptors is accompanied by extensive cytoskeletal rearrangements and intracellular signaling events. The protein kinase C (PKC) family of serine/threonine kinases has been implicated in several integrin-mediated events including focal adhesion formation, cell spreading, cell migration, and cytoskeletal rearrangements. However, the mechanism by which PKC regulates integrin function is not known. To characterize the role of PKC family kinases in mediating integrin-induced signaling, we monitored the effects of PKC inhibition on fibronectin-induced signaling events in Cos7 cells using pharmacological and genetic approaches. We found that inhibition of classical and novel isoforms of PKC by down-regulation with 12-0-tetradeconoyl-phorbol-13-acetate or overexpression of dominant-negative mutants of PKC significantly reduced extracellular regulated kinase 2 (Erk2) activation by fibronectin receptors in Cos7 cells. Furthermore, overexpression of constitutively active PKCalpha, PKCdelta, or PKCepsilon was sufficient to rescue 12-0-tetradeconoyl-phorbol-13-acetate-mediated down-regulation of Erk2 activation, and all three of these PKC isoforms were activated following adhesion. PKC was required for maximal activation of mitogen-activated kinase kinase 1, Raf-1, and Ras, tyrosine phosphorylation of Shc, and Shc association with Grb2. PKC inhibition does not appear to have a generalized effect on integrin signaling, because it does not block integrin-induced focal adhesion kinase or paxillin tyrosine phosphorylation. These results indicate that PKC activity enhances Erk2 activation in response to fibronectin by stimulating the Erk/mitogen-activated protein kinase pathway at an early step upstream of Shc.

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.

Polarized distribution of Bcr-Abl in migrating myeloid cells and co-localization of Bcr-Abl and its target proteins

Bcr-Abl plays a critical role in the pathogenesis of Philadelphia chromosome-positive leukemia. Although a large number of substrates and interacting proteins of Bcr-Abl have been identified, it remains unclear whether Bcr-Abl assembles multi-protein complexes and if it does where these complexes are within cells. We have investigated the localization of Bcr-Abl in 32D myeloid cells attached to the extracellular matrix. We have found that Bcr-Abl displays a polarized distribution, colocalizing with a subset of filamentous actin at trailing portions of migrating 32D cells, and localizes on the cortical F-actin and on vesicle-like structures in resting 32D cells. Deletion of the actin binding domain of Bcr-Abl (Bcr-AbI-AD) dramatically enhances the localization of Bcr-Abl on the vesicle-like structures. These distinct localization patterns of Bcr-Abl and Bcr-Abl-AD enabled us to examine the localization of Bcr-Abl substrate and interacting proteins in relation to Bcr-Abl. We found that a subset of biochemically defined target proteins of Bcr-Abl redistributed and co-localized with Bcr-Abl on F-actin and on vesicle-like structures. The co-localization of signaling proteins with Bcr-Abl at its sites of localization supports the idea that Bcr-Abl forms a multi-protein signaling complex, while the polarized distribution and vesicle-like localization of Bcr-Abl may play a role in leukemogenesis.

Signal perception and transduction: the role of protein kinases

Cells can react to environmental changes by transduction of extracellular signals, to produce intracellular responses. Membrane-impermeable signal molecules are recognized by receptors, which are localized on the plasma membrane of the cell. Binding of a ligand can result in the stimulation of an intrinsic enzymatic activity of its receptor or the modulation of a transducing protein. The modulation of one or more intracellular transducing proteins can finally lead to the activation or inhibition of a so-called 'effector protein'. In many instances, this also results in altered gene expression. Phosphorylation by protein kinases is one of the most common and important regulatory mechanisms in signal transmission. This review discusses the non-channel transmembrane receptors and their downstream signaling, with special focus on the role of protein kinases.

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.

Role of tyrosine residues and protein interaction domains of SHC adaptor in VEGF receptor 3 signaling

The VEGFR3/FLT4 receptor, which is involved in vasculogenesis and angiogenesis, binds and phosphorylates SHC proteins on tyrosine residues. SHC contains two phosphotyrosine interaction domains: a PTB (Phosphotyrosine Binding) and a SH2 (Src Homology 2) domain. Previous studies have shown that SHC proteins are phosphorylated on Y239/Y240 and Y313 (Y317 in humans) by tyrosine kinases such as the EGF and IL3 receptors. We have investigated which of the SHC tyrosine residues are targeted by the VEGFR3/ FLT4 kinase and the role of the SHC PTB and SH2 domains in this process. Our results show that Y239/ Y240 and Y313 are simultaneously phosphorylated by the kinase, creating GRB2 binding sites. Mutation of SHC PTB, but not SH2, domain interferes with the SHC phosphorylation by VEGFR3/FLT4. Soft agar assay experiments revealed that the VEGFR3/FLT4 transforming capacity is increased by the mutation of Y239/Y240 to phenylalanines in SHC, suggesting that these two residues mediate an inhibitory signal for cell growth. Mutation of the two phosphorylation sites increases this effect, suggesting that they have a synergistic role.

Insulin receptor substrate 2 and Shc play different roles in insulin-like growth factor I signaling

The major substrates for the type I insulin-like growth factor (IGF-I) receptor are Shc and insulin receptor substrate (IRS) proteins. In the current study, we report that IGF-I induces a sustained tyrosine phosphorylation of Shc and its association with Grb2 in SH-SY5Y human neuroblastoma cells. The time course of Shc tyrosine phosphorylation parallels the time course of IGF-I-stimulated activation of extracellular signal-regulated kinase (ERK). Transfection of SH-SY5Y cells with a p52 Shc mutant decreases Shc tyrosine phosphorylation and Shc-Grb2 association. This results in the inhibition of IGF-I-mediated ERK tyrosine phosphorylation and neurite outgrowth. In contrast, IGF-I induces a transient tyrosine phosphorylation of IRS-2 and an association of IRS-2 with Grb2. The time course of IRS-2 tyrosine phosphorylation and IRS-2-Grb2 and IRS-2-p85 association closely resembles the time course of IGF-I-mediated membrane ruffling. Treating cells with the phosphatidylinositol 3'-kinase inhibitors wortmannin and LY294002 blocks IGF-I-induced membrane ruffling. The ERK kinase inhibitor PD98059, as well as transfection with the p52 Shc mutant, has no effect on IGF-I-mediated membrane ruffling. Immunolocalization studies show IRS-2 and Grb2, but not Shc, concentrated at the tip of the extending growth cone where membrane ruffling is most active. Collectively, these results suggest that the association of Shc with Grb2 is essential for IGF-I-mediated neurite outgrowth, whereas the IRS-2-Grb2-phosphatidylinositol 3'-kinase complex may regulate growth cone extension and membrane ruffling.

Convergence of progesterone and epidermal growth factor signaling in breast cancer. Potentiation of mitogen-activated protein kinase pathways

During late stages of breast cancer progression, tumors frequently acquire steroid hormone resistance with concurrent amplification of growth factor receptors; this alteration predicts a poor prognosis. We show here that following treatment with the progestin, R5020, breast cancer cells undergo a "biochemical shift" in the regulation of epidermal growth factor (EGF)-stimulated signaling pathways: R5020 potentiates the effects of EGF by up-regulating EGFR, c-ErbB2 and c-ErbB3 receptors, and by enhancing EGF-stimulated tyrosine phosphorylation of signaling molecules known to associate with activated type I receptors. Independently of EGF, R5020 increases Stat5 protein levels, association of Stat5 with phosphotyrosine-containing proteins, and tyrosine phosphorylation of JAK2 and Shc. Furthermore, progestins "prime" breast cancer cells for growth signals by potentiating EGF-stimulated p42/p44 mitogen-activated protein kinase (MAPK), p38 MAP kinase, and JNK activities. Although the levels of cyclin D1, cyclin E, and p21(WAF1), are up-regulated by R5020 alone, they are synergistically up-regulated by EGF in the presence of R5020. Up-regulation of cell cycle proteins by EGF is blocked by inhibition of p42/p44 MAPK only in the presence of R5020, supporting a shift in the regulation of these cell cycle mediators from MAPK-independent to MAPK-dependent pathways. In summary, progesterone selectively increases the sensitivity of key kinase cascades to growth factors, thereby priming cells for stimulation by latent growth signals. These data support a model in which breast cancer cell growth switches from steroid hormone to growth factor dependence.

Emerging roles for SH2/PTB-containing Shc adaptor proteins in the developing mammalian brain

In mammalian systems, SH2-containing cytoplasmic signalling molecules are known to play an important role in determining cell responsiveness to the environment. In particular, following activation of a receptor protein tyrosine kinase (RPTK), proteins like Shc and Grb2 bind to phosphotyrosine residues of stimulated receptors, thereby activating downstream components of specific signalling pathways. The ShcA gene was identified in 1992 and was found to encode three proteins with properties of adaptor molecules coupling RPTKs to Ras. Early data obtained in non-neuronal cells have revealed that Shc and Grb2 proteins are highly expressed and activated in all cells. However, recent analyses of ShcA mRNA and protein in the developing brain revealed progressive downregulation of their expression during differentiation from neuroblasts to neurons. Conversely, the two newly identified Shc homologues (ShcB/Sli and ShcC/Rai) are highly expressed in the mature brain.Thus, variations in the intracellular levels of adaptor proteins might represent one of the mechanisms by which a differentiating cell changes its ability to respond to a given factor, allowing a cell to choose between proliferation and differentiation.

The IL-2 Receptor Promotes Proliferation, bcl-2 and bcl-x Induction, But Not Cell Viability Through the Adapter Molecule Shc

IL-2, the principal mitogenic factor for activated T cells, delivers a proliferative signal through ligation of the heterotrimeric IL-2R. This proliferative signal is critically dependent upon cytoplasmic tyrosines on the β-chain of this receptor (IL-2Rβ) becoming phosphorylated in response to ligand. We found that at least one of these tyrosines (Y338) also mediates cell survival and induction of bcl-2, bcl-x, and c-myc in the murine T cell line CTLL-2. Since the adapter molecule Shc binds to phosphorylated Y338, the specific contribution of Shc to these events was evaluated. An IL-2Rβ/Shc fusion protein, in which Shc was covalently tethered to a truncated version of IL-2Rβ lacking all cytoplasmic tyrosines, revealed a robust proliferative signal mediated through Shc. This Shc-mediated signal induced expression of c-myc as well as the antiapoptotic genes bcl-2 and bcl-x with normal magnitude and kinetics. Nonetheless, signals from this fusion protein failed to sustain the long-term viability of CTLL-2 cells. Thus, induction of bcl family genes and delivery of a competent proliferative signal are not sufficient to promote cell survival and mediate the antiapoptotic effects associated with a complete IL-2 signal.

Integration of growth factor, extracellular matrix, and retinoid signals during bronchial epithelial cell differentiation

Epithelial cell differentiation is regulated by specific combinations of growth factors, hormones, and extracellular matrix (ECM). How these divergent signals are integrated is largely unknown. We used primary cultures of normal human bronchial epithelial cells (NHBEs) to investigate mechanisms of signal integration. In defined, serum-free media, NHBEs undergo mucosecretory differentiation only when grown in the presence of retinoids and on the appropriate substratum (collagen gels). We identified the retinoic acid receptor beta (RARbeta) gene as an early marker of NHBE differentiation. In contrast to immortalized cell lines, in NHBEs strong retinoid-induced RARbeta transcription occurs only when cells are grown on collagen gels, and it requires new protein synthesis and a cis-acting element that maps outside the known RARbeta promoter elements. NHBEs grown on collagen gels exhibit reduced epidermal growth factor (EGF)-induced Raf, MEK, and mitogen-activated protein kinase (MAPK) activity. This correlates with a specific inability to achieve high levels of p66(SHC) tyrosyl phosphorylation and association of p66(SHC) with GRB2, despite high levels of EGF receptor (EGFR) autophosphorylation. Notably, inhibition of EGFR or MEK/MAPK activation replaces the ECM requirement for RARbeta induction. Our results strongly suggest that a key mechanism by which specific ECMs facilitate retinoid-induced mucosecretory differentiation of NHBEs is by restricting the level of EGFR-dependent MEK/MAPK activation evoked by autocrine and/or paracrine EGFR ligands.

Identification of the rat adapter Grb14 as an inhibitor of insulin actions

We cloned by interaction with the beta-subunit of the insulin receptor the rat variant of the human adapter Grb14 (rGrb14). rGrb14 is specifically expressed in rat insulin-sensitive tissues and in the brain. The binding of rGrb14 to insulin receptors is insulin-dependent in vivo in Chinese hamster ovary (CHO) cells overexpressing both proteins and importantly, in rat liver expressing physiological levels of proteins. However, rGrb14 is not a substrate of the tyrosine kinase of the receptor. In the two-hybrid system, two domains of rGrb14 can mediate the interaction with insulin receptors: the Src homology 2 (SH2) domain and a region between the PH and SH2 domains that we named PIR (for phosphorylated insulin receptor-interacting region). In vitro interaction assays using deletion mutants of rGrb14 show that the PIR, but not the SH2 domain, is able to coprecipitate insulin receptors, suggesting that the PIR is the major binding domain of rGrb14. The interaction between rGrb14 and the insulin receptors is almost abolished by mutating tyrosine residue Tyr1150 or Tyr1151 of the receptor. The overexpression of rGrb14 in CHO-IR cells decreases insulin stimulation of both DNA and glycogen synthesis. These effects are accompanied by a decrease in insulin-stimulated tyrosine phosphorylation of IRS-1, but insulin receptor autophosphorylation is unaltered. These findings suggest that rGrb14 could be a new downstream signaling component of the insulin-mediated pathways.

Antibody cross-linking of the glycosylphosphatidylinositol-linked protein CD59 on hematopoietic cells induces signaling pathways resembling activation by complement

CD59 is a glycosylphosphatidylinositol-anchored cell surface glycoprotein involved in protecting cells from host-mediated complement attack. Studies have shown that antibody cross-linking of CD59 induces a series of intracellular signaling events including the activation of protein-tyrosine kinases (PTK). To further characterize these events, antibodies and complement 8, one of the natural ligands of CD59, were used to activate CD59. Antibody-induced cross-linking of CD59 on the surface of THP-1 and U937 hematopoietic cell lines as well as exposure to complement 8 induces a rapid increase in the tyrosine phosphorylation of several proteins within the cell. Consistent with an early role for the Src family PTKs in these signaling events, we found that transient activation of Hck- and CD59-mediated signaling was abrogated in the presence of the Src family PTK-selective inhibitor PP1. Although the molecular mechanism by which CD59 communicates to Hck is unknown, cellular fractionation studies indicated that both CD59 and Hck are compartmentalized in plasma membrane microdomains. We also detected tyrosine phosphorylation of the adaptor proteins p120 and Shc, and the cytoplasmic nonreceptor tyrosine kinase Syk. The identification of CD59-mediated signaling events may help explain why paroxysmal nocturnal hemoglobinuria patients, who are deficient in glycosylphosphatidylinositol-linked proteins including CD59, are susceptible to proliferative disorders.

Involvement of Shc in the signaling response of human prostate tumor cell lines to epidermal growth factor

Autocrine growth factors for the epidermal growth factor receptor (EGFR) have been identified in prostate tumors, implicating a role for EGFR in the progression of prostate cancer. To investigate early signaling mechanisms used by the EGFR in prostate tumor cells, we have characterized the involvement of the Shc (src homology 2/x-collagen related) adapter protein in EGFR signaling in several human prostate tumor cell lines. In androgen-responsive lymph node-prostate cancer (LNCaP) cells and androgen-insensitive PC3, DU145 and PPC-I cells, Shc was identified as one of the most prominent phosphotyrosine proteins to be elevated in response to EGF. Equivalent levels of the 46- and 52-kDa Shc isoforms were detected in all of the tumor cell lines tested. However, levels of the 66-kDa isoform were variable among the cell lines. In all of the tumor cell lines, EGF caused an association between Shc and Grb2, another adapter protein linked to cellular ras activation. Additionally, several phosphotyrosine proteins, including a 115-120-kDa protein in EGF-treated LNCaP cells, co-associated with Shc. The profile of these Shc-associating proteins, however, differed among the tumor cell lines. Our results indicate that Shc is a common downstream element of EGFR signaling in prostate tumor cells and suggest multiple functions for Shc in prostate tumorigenesis.

Tumor promoter arsenite activates extracellular signal-regulated kinase through a signaling pathway mediated by epidermal growth factor receptor and Shc

Although arsenite is an established carcinogen, the mechanisms underlying its tumor-promoting properties are poorly understood. Previously, we reported that arsenite treatment leads to the activation of the extracellular signal-regulated kinase (ERK) in rat PC12 cells through a Ras-dependent pathway. To identify potential mediators of the upstream signaling cascade, we examined the tyrosine phosphorylation profile in cells exposed to arsenite. Arsenite treatment rapidly stimulated tyrosine phosphorylation of several proteins in a Ras-independent manner, with a pattern similar to that seen in response to epidermal growth factor (EGF) treatment. Among these phosphorylated proteins were three isoforms of the proto-oncoprotein Shc as well as the EGF receptor (EGFR). Tyrosine phosphorylation of Shc allowed for enhanced interactions between Shc and Grb2 as identified by coimmunoprecipitation experiments. The arsenite-induced tyrosine phosphorylation of Shc, enhancement of Shc and Grb2 interactions, and activation of ERK were all drastically reduced by treatment of cells with either the general growth factor receptor poison suramin or the EGFR-selective inhibitor tyrphostin AG1478. Down-regulation of EGFR expression through pretreatment of cells with EGF also attenuated ERK activation and Shc tyrosine phosphorylation in response to arsenite treatment. These results demonstrate that the EGFR and Shc are critical mediators in the activation of the Ras/ERK signaling cascade by arsenite and suggest that arsenite acts as a tumor promoter largely by usurping this growth factor signaling pathway.

Insulin-like growth factor-I receptor signal transduction: at the interface between physiology and cell biology

The insulin-like growth factor-I receptor (IGF-IR) mediates the biological actions of IGF-I and IGF-II. The IGFs play a critical role in promoting development, stimulating growth and organogenesis via mitogenic, antiapoptotic and chemotactic activity. Recent research has focused on the events that occur intracellularly upon receptor activation. Several pathways have been shown to be important. The insulin-receptor substrate (IRS), SHC, GRB2, CRKII and CRKL adaptor proteins have all been implicated in transmitting signals to the nucleus of the cell. This review outlines some of the signalling pathways believed to be important in converting IGF-IR activation into changes in cell behavior and metabolism.

The src homology 2 and phosphotyrosine binding domains of the ShcC adaptor protein function as inhibitors of mitogenic signaling by the epidermal growth factor receptor

Upon ligand activation, the epidermal growth factor receptor (EGFR) becomes tyrosine-phosphorylated, thereby recruiting intracellular signaling proteins such as Shc. EGFR binding of Shc proteins results in their tyrosine phosphorylation and subsequent activation of the Ras and Erk pathways. Shc interaction with activated receptor tyrosine kinases is mediated by two distinct phosphotyrosine interaction domains, an NH2-terminal phosphotyrosine binding (PTB) domain and a COOH-terminal Src homology 2 (SH2) domain. The relative importance of these two domains for EGFR binding was examined by determining if expression of the isolated SH2 or PTB domain of ShcC would inhibit EGFR signaling. The SH2 domain potently inhibited numerous aspects of EGFR signaling including activation of Erk2 and the Elk-1 transcription factor as well as EGFR-dependent transformation. Furthermore, the SH2 domain inhibited focus formation by the Neu oncoprotein, another EGFR family member. Surprisingly, inhibition of the EGFR by the SH2 domain did not involve stable association with the receptor. In contrast, the PTB domain associated quite well with the receptor yet had little effect on EGFR signaling. Although the EGFR cytoplasmic tail contains consensus binding sites for the PTB and SH2 domains of ShcC, and both domains of ShcC interact with the receptor in vitro, the SH2 domain is more potent for inhibiting receptor function in vivo. However, inhibition is not due to stable association with the receptor, suggesting that the SH2 domain is binding to a heretofore unknown protein(s) necessary for proper EGFR function.

Tyrosine 474 of ZAP-70 is required for association with the Shc adaptor and for T-cell antigen receptor-dependent gene activation

The protein tyrosine kinase ZAP-70 plays a central role in T-cell activation. Following receptor engagement, ZAP-70 is recruited to the phosphorylated subunits of the T-cell antigen receptor (TCR). This event results in ZAP-70 activation and in association of ZAP-70 with a number of signaling proteins. Among these is the Shc adaptor, which couples the activated TCR to Ras. Shc interaction with ZAP-70 is mediated by the Shc PTB domain. The inhibitory effect of a Shc mutant containing the isolated PTB domain suggests that Shc interaction with ZAP-70 might be required for TCR signaling. Here, we show that a point mutation (Phe474) of the putative Shc binding site on ZAP-70, spanning tyrosine 474, prevented ZAP-70 interaction with Shc and the subsequent binding of Shc to phospho-zeta. Neither ZAP-70 catalytic activity nor the pattern of protein phosphorylation induced by TCR triggering was affected by this mutation. However expression of the Phe474 ZAP-70 mutant resulted in impaired TCR-dependent gene activation. ZAP-70 could effectively phosphorylate Shc in vitro. Only the CH domain, which contains the two Grb2 binding sites on Shc, was phosphorylated by ZAP-70. Both Grb2 binding sites were excellent substrates for ZAP-70. The data show that Tyr474 on ZAP-70 is required for TCR signaling and suggest that Shc association with ZAP-70 and the resulting phosphorylation of Shc might be an obligatory step in linking the activated TCR to the Ras pathway.

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.

Polyomavirus middle T antigen as a probe for T cell antigen receptor-coupled signaling pathways

Stimulation of the T cell antigen receptor (TCR) triggers a complex series of signaling events that culminate in T cell activation and proliferation. The complex structure of the TCR has hindered efforts to link specific signaling events induced by TCR cross-linkage to downstream activation responses, such as interleukin-2 (IL-2) gene transcription. Previous studies have shown that the polyomavirus-derived oncoprotein, middle T antigen (mT), transforms rodent fibroblasts by interacting with and activating several cytoplasmic signaling proteins (Src kinases, phospholipase C (PLC)-gamma1, Shc, and phosphoinositide 3-kinase (PI3-K) implicated in cell growth control. In this study, we demonstrate that expression of mT activates Jurkat T cells, as measured by increases in IL-2 promoter- and NFAT (nuclear factor of activated T cells)-dependent reporter gene transcription. The transcriptional response provoked by mT was blocked by the immunosuppressive drug FK506, a potent inhibitor of TCR-mediated IL-2 gene expression. Mutations that disrupted the binding of mT to Src kinases or PLC-gamma1 abrogated the ability of mT to deliver the signals needed for IL-2 promoter activation. In contrast, a mT mutant that failed to bind PI3-K induced a markedly elevated transcriptional response in Jurkat cells, whereas mutation of the Shc binding site in mT had little effect on the transactivating potential of this viral oncoprotein. Additional studies demonstrated that the association of mT with PLC-gamma1 was necessary and sufficient to activate both Ca2+- and Ras-dependent signaling cascades in Jurkat cells. These results indicate that PLC-gamma1 activation plays pivotal and pleiotropic roles in the stimulation of IL-2 gene expression, whereas activation of PI3-K negatively modulates this response in Jurkat T cells.

Agonist occupation of an alpha2A-adrenoreceptor-Gi1alpha fusion protein results in activation of both receptor-linked and endogenous Gi proteins. Comparisons of their contributions to GTPase activity and signal transduction and analysis of receptor-G protein activation stoichiometry

A fusion protein between a pertussis toxin-resistant (C351G) mutant of the alpha subunit of the G protein Gi1 and the porcine alpha2A-adrenoreceptor was stably expressed in Rat 1 fibroblasts. Agonists caused stimulation of high affinity GTPase activity, which was partially prevented by pertussis toxin treatment, demonstrating that the toxin-resistant component of the GTPase activity was derived from the receptor-fused G protein and the remainder from endogenous Gialpha. Half-maximal stimulation of the GTPase activity of endogenous Gi was achieved with lower concentrations of agonist. Although the Km for GTP of the fusion protein-linked Gi was lower than for the endogenous G protein, Vmax measurements demonstrated that adrenaline activated some 5 mol of endogenous Gi/mol of fusion protein-linked Gi. The isolated alpha2A-adrenoreceptor could activate Gs; however, the fusion protein did not. Compared with adrenaline, the efficacy of a range of partial agonists to stimulate endogenous Gialpha was greater than for the fusion protein-constrained C351G Gi1alpha. alpha2A-Adrenoreceptor agonists could stimulate both p44 mitogen-activated protein kinase and p70 S6 kinase and inhibit forskolin-amplified adenylyl cyclase activity in untreated alpha2A-adrenoreceptor-C351G Gi1alpha fusion protein-expressing cells, but these signals were abolished following pertussis toxin treatment. These results demonstrate conclusively, and for the first time, that agonist occupancy of a receptor-G protein fusion protein can result in activation of G proteins other than that physically linked to the receptor. This was selective between G protein classes. Analysis of the contributions of fusion protein-linked and endogenous G proteins to agonist-stimulated GTPase activity provided a direct and original measure of receptor-G protein activation stoichiometry.

The Src and signal transducers and activators of transcription pathways as specific targets for low molecular weight phosphotyrosine-protein phosphatase in platelet-derived growth factor signaling

The low molecular weight phosphotyrosine-protein phosphatase (LMW-PTP) is a cytosolic phosphotyrosine-protein phosphatase specifically interacting with the activated platelet-derived growth factor (PDGF) receptor through its active site. Overexpression of the LMW-PTP results in modulation of PDGF-dependent mitogenesis. In this study we investigated the effects of this tyrosine phosphatase on the signaling pathways relevant for PDGF-dependent DNA synthesis. NIH 3T3 cells were stably transfected with active or dominant negative LMW-PTP. The effects of LMW-PTP were essentially restricted to the G1 phase of the cell cycle. Upon stimulation with PDGF, cells transfected with the dominant negative LMW-PTP showed an increased activation of Src, whereas the active LMW-PTP induced a reduced activation of this proto-oncogene. We observe that c-Src binding to PDGF receptor upon stimulation is prevented by overexpression of LMW-PTP. These effects were associated with parallel changes in myc expression. Moreover, wild-type and dominant negative LMW-PTP differentially regulated STAT1 and STAT3 activation and tyrosine phosphorylation, whereas they did not modify extracellular signal-regulated kinase activity. However, these modifications were associated with changes in fos expression despite the lack of any effect on extracellular signal-regulated kinase activation. Other independent pathways involved in PDGF-induced mitogenesis, such as phosphatidylinositol 3-kinase and phospholipase C-gamma1, were not affected by LMW-PTP. These data indicate that this phosphatase selectively interferes with the Src and the STATs pathways in PDGF downstream signaling. The resulting changes in myc and fos proto-oncogene expression are likely to mediate the modifications observed in the G1 phase of the cell cycle.

Identification and characterization of two putative human arginine methyltransferases (HRMT1L1 and HRMT1L2)

RNA-binding proteins such as heterogeneous nuclear ribonucleoproteins (hnRNPs), which contain the bulk of methylated arginine residues in eukaryotic cells, play many essential roles in the metabolism of nuclear pre-mRNA. Arginine methyltransferase activity has also been implicated in signal transduction events with components of the cellular growth and viral response pathways. We recently characterized a single yeast hnRNP methyltransferase (HMT1). We now present the identification and characterization of two putative human arginine methyltransferases termed HRMT1L1 and HRMT1L2. In addition to methyltransferase similarities, the N-terminal region of the HRMT1L1 protein contains an Src homology 2 domain. HRMT1L1 maps to a YAC containing the telomere of chromosome 21q. Three alternatively spliced HRMT1L2 transcripts with variable 5'-ends were observed, encoding proteins of 343, 347, and 361 amino acids, respectively. HRMT1L2 maps to human chromosome 19q. Recombinant HRMT1L2 protein encoded by the most common 5'-variant exhibited methyltransferase activity in vitro. Furthermore, in vivo activity was demonstrated by complementation of a yeast HMT1 mutant strain. The identification of highly conserved Hmt1p human homologues that function in yeast indicates that analyses of this class of enzymes in yeast may be directly applicable to higher eukaryotes. The possible roles of HRMT1L1 and HRMT1L2 in human disease are currently unknown.

Tyrosine residues 239 and 240 of Shc are phosphatidylinositol 4,5-bisphosphate-dependent phosphorylation sites by c-Src

In the previous study (Sato K.-I. et al. (1997) FEBS Lett. 410, 136-140), we showed that the phosphorylation of Shc protein by c-Src is dependent on the binding of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) to the PTB domain of Shc. In this study, we demonstrate that, in contrast to c-Src, v-Src and epidermal growth factor (EGF) receptor can phosphorylate Shc in a PtdIns(4,5)P2-independent manner and at different phosphorylation sites. To determine the phosphorylation sites in Shc, we used mutant Shc proteins in which tyrosine residues (Y) 317 and/or 239 and 240 were replaced by phenylalanine residues (F). We found that Y317F Shc but not Y239/240F or Y239/240/317F Shc was phosphorylated by c-Src. The reaction was PtdIns(4,5)P2-dependent and inhibited by the addition of PTB domain of Shc. On the other hand, v-Src and EGF receptor were able to phosphorylate both Y317F and Y239/240F but not Y239/240/317F Shc in a PtdIns(4,5)P2-independent manner. These results highlight the difference between c-Src and v-Src or EGF receptor and suggest that c-Src can phosphorylate predominantly on Tyr239/240 of Shc only when Shc PTB domain is bound to PtdIns(4,5)P2.

Critical role of calcium- dependent epidermal growth factor receptor transactivation in PC12 cell membrane depolarization and bradykinin signaling

PC12 cells respond to a variety of external stimuli such as growth factors, neurotransmitters, and membrane depolarization by activating the Ras/mitogen-activated protein kinase pathway. Here we demonstrate that both depolarization-induced calcium influx and treatment with bradykinin stimulate tyrosine phosphorylation of the epidermal growth factor receptor (EGFR). Using a tetracycline-controlled expression system in conjunction with a dominant-negative EGFR mutant, we demonstrate that depolarization and bradykinin triggered signals involve EGFR function upstream of SHC and MAP kinase. Furthermore, bradykinin-stimulated EGFR transactivation is critically dependent on the presence of extracellular calcium, and when triggered by ionophore treatment, calcium influx is already sufficient to induce EGFR tyrosine phosphorylation. Taken together, our results establish calcium-dependent EGFR transactivation as a signaling mechanism mediating activation of the Ras/mitogen-activated protein kinase pathway in neuronal cell types.

Tat protein from HIV-1 activates MAP kinase in granular neurons and glial cells from rat cerebellum

We have investigated the effect of extracellularly applied Tat protein of the human immunodeficiency virus type 1 (HIV-1) on tyrosine phosphorylation processes, which represent a major signal transduction pathway of cells of the central nervous system. Primary cultures of rat cerebellar astrocytes or granule cells were incubated with synthetic Tat (10 ng/ml) for various periods of time and analyzed for their phosphotyrosine content by Western blotting. In both types of cultures Tat was able to induce the phosphorylation of mitogen-activated protein kinase (MAP kinase) on tyrosine residues, although with different kinetics and isoform specificity. In addition, in neuronal cells, but not in astrocytes, Tat increased the phosphotyrosine content of Shc, a protein involved in signal transduction downstream of receptor tyrosine kinase activation. This study shows that Tat applied extracellularly is able to induce the generation of intracellular signals in neuronal as well as glial cells.

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.

The role of the Shc phosphotyrosine interaction/phosphotyrosine binding domain and tyrosine phosphorylation sites in polyoma middle T antigen-mediated cell transformation

The phosphotyrosine interaction (PI)/phosphotyrosine binding (PTB) domain of Shc binds specific tyrosine-phosphorylated motifs found on activated growth factor receptors and proteins such as polyoma virus middle T antigen (MT). Phenylalanine 198 (Phe198) has been identified as a crucial residue involved in the interaction of the Shc PI/PTB with phosphopeptides. In NIH 3T3 cells expressing MT, p52 Shc carrying the F198V mutation is weakly phosphorylated and does not bind MT or Grb2. Overexpression of the PI/PTB domain alone as Shc amino acids 1-238 acted in a dominant interfering fashion blocking MT-induced transformation. However, expression of a slightly longer construct, Shc 1-260, which encompasses Tyr239/Tyr240, a novel Shc tyrosine phosphorylation site, did not block transformation. This was found to be due to the ability of Shc 1-260 to become tyrosine-phosphorylated and bind Grb2. Furthermore, full-length Shc in which Tyr239/Tyr240 had been mutated to phenylalanine did not become tyrosine-phosphorylated or bind Grb2 but did inhibit colony formation in soft agar. Conversely, p52 Shc carrying a mutation in the other tyrosine phosphorylation site, Tyr317, became heavily tyrosine-phosphorylated, bound Grb2, and gave rise to colonies in soft agar.

Expression and activation of SH2/PTB-containing ShcA adaptor protein reflects the pattern of neurogenesis in the mammalian brain

The adult mammalian brain comprises many functionally distinct neuronal types, which are generated during development as a result of a coordinated signaling cascade that drives neuroblasts from proliferation into differentiation. We investigated whether and how ShcA adaptor proteins, which are known to function as initiators of the Ras signaling cascade in various nonneuronal systems where they have been considered to be expressed ubiquitously, are involved in the proliferative and differentiative phases of the developing brain. We found that in the forebrain expression and activation of ShcA proteins were strictly regulated during embryonic development, both temporally and spatially. The mRNAs encoded by the ShcA gene were expressed exclusively within an area to which active proliferation of immature neuroblasts was confined, the ventricular zone. In postnatal and adult brain, ShcA mRNAs and proteins were present only faintly. In the adult olfactory epithelium, in which neuronal cell renewal occurs throughout life, ShcA remained strongly expressed. These phenomena were peculiar to ShcA, since Grb2 adaptor protein remained expressed at constant level throughout development. The embryonically expressed ShcA proteins were functionally active, since p52(ShcA) became phosphorylated on tyrosine and associated with Grb2 following intraventricular injection of epidermal growth factor in the embryonic brain. Our data indicate that, through an orderly pattern of expression, ShcA gene products may play a role in the control of the switch between proliferation and differentiation of brain neuroblasts.

Evidence for a direct interaction between insulin receptor substrate-1 and Shc

Insulin receptor substrate-1 (IRS-1) and Shc are two proteins implicated in intracellular signal transduction. They are activated by an increasing number of extracellular signals, mediated by receptor tyrosine kinases, cytokine receptors, and G protein-coupled receptors. In this study we demonstrate that Shc interacts directly with IRS-1, using the yeast two-hybrid system and an in vitro interaction assay. Deletion analysis of the proteins to map the domains implicated in this interaction shows that the phosphotyrosine binding domain of Shc binds to the region of IRS-1 comprising amino acids 583-661. An in vitro association assay, performed with or without activation of tyrosine kinases, gives evidence that tyrosine phosphorylation of IRS-1 and Shc drastically improves the interaction. Site-directed mutagenesis on IRS-1 583-693 shows that the asparagine, but not the tyrosine residue of the N625GDY628motif domain, is implicated in the IRS-1-Shc-phosphotyrosine binding interaction. Mutation of another tyrosine residue, Tyr608, also induced a 40% decrease in the interaction. This study, describing a phosphotyrosine-dependent interaction between IRS-1 and Shc, suggests that this association might be important in signal transduction.

Phosphatidylinositol 4,5-bisphosphate stimulates phosphorylation of the adaptor protein Shc by c-Src

The adaptor protein Shc was prepared as glutathione S-transferase fusion proteins (GST-Shc) and used as in vitro substrate for c-Src. Since phosphotyrosine-binding domain of Shc has been shown to bind phosphatidyl-inositol 4,5-bisphosphate (PtdIns(4,5)P2) [Zhou et al. (1995) Nature 378, 584-592], effect of PtdIns(4,5)P2 on the phosphorylation of GST-Shc by c-Src was examined. PtdIns(4,5)P2 stimulated the phosphorylation of GST-Shc without any effect on the c-Src activity as judged by both its autophosphorylation and phosphorylation of exogenous substrate, Cdc2 peptide. On the other hand, phosphatidylserine, phosphatidic acid, phosphatidylinositol, and phosphatidylinositol 4-phosphate but not phosphatidylcholine stimulated the c-Src activity itself. Km for GST-Shc in the presence of 1 microM PtdIns(4,5)P2 was calculated to be 90 nM. The PtdIns(4,5)P2-dependent phosphorylation of GST-Shc was inhibited by a GST-fusion protein containing the phosphotyrosine-binding domain of Shc. These results suggest that PtdIns(4,5)P2 can act as a regulator of phosphorylation of Shc by c-Src through its binding to Shc.

Interaction of the adaptor protein Shc and the adhesion molecule cadherin

In mitogenic signaling pathways, Shc participates in the growth factor activation of Ras by interacting with activated receptors and/or the Grb-2.Sos complex. Using several experimental approaches we demonstrate that Shc, through its SH2 domain, forms a complex with the cytoplasmic domain of cadherin, a transmembrane protein involved in the Ca2+-dependent regulation of cell-cell adhesion. This interaction is demonstrated in a yeast two-hybrid assay, by co-precipitation from mammalian cells, and by direct biochemical analysis in vitro. The Shc-cadherin association is phosphotyrosine-dependent and is abrogated by addition of epidermal growth factor to A-431 cells maintained in Ca2+-free medium, a condition that promotes changes in cell shape. Shc may therefore participate in the control of cell-cell adhesion as well as mitogenic signaling through Ras.

The Shc adaptor protein is highly phosphorylated at conserved, twin tyrosine residues (Y239/240) that mediate protein-protein interactions

BACKGROUND

Signal transduction initiated by a wide variety of extracellular signals involves the activation of protein-tyrosine kinases. Phosphorylated tyrosine residues in activated receptors or docking proteins then function as binding sites for the Src homology 2 (SH2) or phosphotyrosine-binding (PTB) domains of cytoplasmic signalling proteins. Shc is an adaptor protein that contains both PTB and SH2 domains and becomes phosphorylated on tyrosine in response to many different extracellular stimuli. These results have suggested that Shc is a prominent effector of protein-tyrosine kinase signalling. Thus far, only a single Shc phosphorylation site, the tyrosine at position 317 (Y317) has been identified. Phosphorylation of Y317 has been implicated in Grb2 binding and activation of the Ras pathway.

RESULTS

Here, we report the identification of two major and novel Shc tyrosine phosphorylation sites, Y239 and Y240. These residues are present in the central proline-rich (CH1) region and are conserved in all isoforms of Shc. Y239/240 are co-ordinately phosphorylated by the Src protein-tyrosine kinase in vitro, and in response to epidermal growth factor stimulation or in v-src-transformed cells in vivo. Mutagenesis studies indicate that Y239/240 make an important contribution to the association of Shc with Grb2. Phosphopeptide-binding studies suggest that these two tyrosine residues may be involved in interactions with a number of cellular proteins.

CONCLUSIONS

Shc is the most prominent general substrate for protein-tyrosine kinases in vivo. The identification of two novel Shc phosphorylation sites indicates that Shc has the potential to interact with multiple downstream effectors. Shc Y239/240 are highly conserved in evolution, suggesting that the phosphorylation of these residues is of fundamental importance. We propose that distinct Shc phosphorylation isomers from different signalling complexes and thereby activate separate downstream signalling cascades.