Tyrosine phosphorylation of Grb2 by Bcr/Abl and epidermal growth factor receptor: a novel regulatory mechanism for tyrosine kinase signaling.

Growth factor receptor-binding protein-2 (Grb2) plays a key role in signal transduction initiated by Bcr/Abl oncoproteins and growth factors, functioning as an adaptor protein through its Src homology 2 and 3 (SH2 and SH3) domains. We found that Grb2 was tyrosine-phosphorylated in cells expressing BCR/ABL and in A431 cells stimulated with epidermal growth factor (EGF). Phosphorylation of Grb2 by Bcr/Abl or EGF receptor reduced its SH3-dependent binding to Sos in vivo, but not its SH2-dependent binding to Bcr/Abl. Tyr209 within the C-terminal SH3 domain of Grb2 was identified as one of the tyrosine phosphorylation sites, and phosphorylation of Tyr209 abolished the binding of the SH3 domain to a proline-rich Sos peptide in vitro. In vivo expression of a Grb2 mutant where Tyr209 was changed to phenylalanine enhanced BCR/ABL-induced ERK activation and fibroblast transformation, and potentiated and prolonged Grb2-mediated activation of Ras, mitogen-activated protein kinase and c-Jun N-terminal kinase in response to EGF stimulation. These results suggest that tyrosine phosphorylation of Grb2 is a novel mechanism of down-regulation of tyrosine kinase signaling.     

A new function for a phosphotyrosine phosphatase: linking GRB2-Sos to a receptor tyrosine kinase.

Autophosphorylated growth factor receptors provide binding sites for the src homology 2 domains of intracellular signaling molecules. In response to epidermal growth factor (EGF), the activated EGF receptor binds to a complex containing the signaling protein GRB2 and the Ras guanine nucleotide-releasing factor Sos, leading to activation of the Ras signaling pathway. We have investigated whether the platelet-derived growth factor (PDGF) receptor binds GRB2-Sos. In contrast with the EGF receptor, the GRB2 does not bind to the PDGF receptor directly. Instead, PDGF stimulation induces the formation of a complex containing GRB2; 70-, 80-, and 110-kDa tyrosine-phosphorylated proteins; and the PDGF receptor. Moreover, GRB2 binds directly to the 70-kDa protein but not to the PDGF receptor. Using a panel of PDGF beta-receptor mutants with altered tyrosine phosphorylation sites, we identified Tyr-1009 in the PDGF receptor as required for GRB2 binding. Binding is inhibited by a phosphopeptide containing a YXNX motif. The protein tyrosine phosphatase Syp/PTP1D/SHPTP2/PTP2C is approximately 70 kDa, binds to the PDGF receptor via Tyr-1009, and contains several YXNX sequences. We found that the 70-kDa protein that binds to the PDGF receptor and to GRB2 comigrates with Syp and is recognized by anti-Syp antibodies. Furthermore, both GRB2 and Sos coimmunoprecipitate with Syp from lysates of PDGF-stimulated cells, and GRB2 binds directly to tyrosine-phosphorylated Syp in vitro. These results indicate that GRB2 interacts with different growth factor receptors by different mechanisms and the cytoplasmic phosphotyrosine phosphatase Syp acts as an adapter between the PDGF receptor and the GRB2-Sos complex.     

HPK1, a hematopoietic protein kinase activating the SAPK/JNK pathway.

In mammalian cells, a specific stress-activated protein kinase (SAPK/JNK) pathway is activated in response to inflammatory cytokines, injury from heat, chemotherapeutic drugs and UV or ionizing radiation. The mechanisms that link these stimuli to activation of the SAPK/JNK pathway in different tissues remain to be identified. We have developed and applied a PCR-based subtraction strategy to identify novel genes that are differentially expressed at specific developmental points in hematopoiesis. We show that one such gene, hematopoietic progenitor kinase 1 (hpk1), encodes a serine/threonine kinase sharing similarity with the kinase domain of Ste20. HPK1 specifically activates the SAPK/JNK pathway after transfection into COS1 cells, but does not stimulate the p38/RK or mitogen-activated ERK signaling pathways. Activation of SAPK requires a functional HPK1 kinase domain and HPK1 signals via the SH3-containing mixed lineage kinase MLK-3 and the known SAPK activator SEK1. HPK1 therefore provides an example of a cell type-specific input into the SAPK/JNK pathway. The developmental specificity of its expression suggests a potential role in hematopoietic lineage decisions and growth regulation.     

Human JIK, a novel member of the STE20 kinase family that inhibits JNK and is negatively regulated by epidermal growth factor

Mammalian members related to Saccharomyces cerevisiae serine/threonine kinase STE20 can be divided into two subfamilies based on their structure and function. The PAK subfamily is characterized by an N-terminal p21-binding domain (also known as CRIB domain), a C-terminal kinase domain, and is regulated by the small GTP-binding proteins Rac1 and Cdc42Hs. The second group is represented by the GCK-like members, which contain an N-terminal catalytic domain and lack the p21-binding domain. Some of them have been demonstrated to induce c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) cascade, while others have been shown to be activated by a subset of stress conditions or apoptotic agents, although little is known about their specific function. Here, we have identified a novel human STE20-related serine/threonine kinase, belonging to the GCK-like subfamily. This kinase does not induce the JNK/SAPK pathway, but, instead, inhibits the basal activity of JNK/SAPK, and diminishes its activation in response to human epidermal growth factor (EGF). Therefore, we designated this molecule JIK for JNK/SAPK-inhibitory kinase. The inhibition of JNK/SAPK signaling pathway by JIK was found to occur between the EGF receptor and the small GTP-binding proteins Rac1 and Cdc42Hs. In contrast, JIK does not activate nor does it inhibit ERK2, ERK6, p38, or ERK5. Furthermore, JIK kinase activity is not modulated by any exogenous stimuli, but, interestingly, it is dramatically decreased upon EGF receptor activation. Thus, JIK might represent the first member of the STE20 kinase family whose activity can be negatively regulated by tyrosine kinase receptors, and whose downstream targets inhibit, rather than enhance, JNK/SAPK activation.     

Interaction of lp-dlg/KIAA0583, a membrane-associated guanylate kinase family protein,with vinexin and beta-catenin at sites of cell-cell contact

Vinexin is a recently identified cytoskeletal protein and plays a key role in the regulation of cytoskeletal organization and signal transduction. Vinexin localizes at sites of cell-extracellular matrix adhesion in NIH3T3 fibroblasts and at sites of cell-cell contact in epithelial LLC-PK1 cells. Expression of vinexin promotes the formation of actin stress fiber, but the role of vinexin at sites of cell-cell contact is unclear. Here we identified lp-dlg/KIAA0583 as a novel binding partner for vinexin by using yeast two-hybrid screening. lp-dlg/KIAA0583 has a NH2-terminal coiled-coil-like domain, in addition to four PDZ domains, an Src homology (SH) 3 domain, and a guanylate kinase domain, which are conserved structures in membrane-associated guanylate kinase family proteins. The third SH3 domain of vinexin bound to the region between the second and third PDZ domain of lp-dlg, which contains a proline-rich sequence. lp-dlg colocalized with vinexin at sites of cell-cell contact in LLC-PK1 cells. Furthermore, lp-dlg colocalized with beta-catenin, a major adherens junction protein, in LLC-PK1 cells. Co-immunoprecipitation experiments revealed that both endogenous and epitope-tagged deletion mutants of lp-dlg/KIAA0583 associated with beta-catenin. We also showed that these three proteins could form a ternary complex. Together these findings suggest that lp-dlg/KIAA0583 is a novel scaffolding protein that can link the vinexin-vinculin complex and beta-catenin at sites of cell-cell contact.     

Cell-type specific phosphorylation of threonines T654 and T669 by PKD defines the signal capacity of the EGF receptor.

In Rat-1 fibroblasts epidermal growth factor (EGF), but not platelet-derived growth factor (PDGF) stimulates the activity of the c-Jun N-terminal kinase (JNK). Moreover, PDGF induced suppression of EGF-mediated JNK activation, apparently through protein kinase C (PKC) activation. Further analysis revealed that PKD was specifically activated by PDGF but not EGF in Rat-1 cells. In SF126 glioblastoma cells, however, EGF and PDGF synergistically activated JNK, while neither PDGF nor EGF stimulated PKD activity. In this cell line, overexpression of PKD blocked EGF- and PDGF-induced JNK activation. Mutational analysis further revealed that the EGFR mutant (T654/669E) was incapable of activating JNK and provided evidence that PKD-mediated dual phosphorylation of these critical threonine residues leads to suppression of EGF-induced JNK activation. Our results establish a novel crosstalk mechanism which allows signal integration and definition in cells with many different RTKs.     

Activation of extracellular signal-regulated kinase by ultraviolet is mediated through Src-dependent epidermal growth factor receptor phosphorylation. Its implication in an anti-apoptotic function.

Ultraviolet (UV) irradiation stimulates stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK), which is a member of the mitogen-activated protein kinase (MAPK) superfamily and implicated in stress-induced apoptosis. UV also induces the activation of another MAPK member, extracellular signal-regulated kinase (ERK), which is typically involved in a growth-signaling cascade. However, the UV-induced signaling pathway leading to ERK activation, together with the physiological role, has remained unknown. Here we examined the molecular mechanism and physiological function of UV-induced ERK activation in human epidermoid carcinoma A431 cells that retain a high number of epidermal growth factor (EGF) receptors. UV-induced ERK activation was accompanied with the Tyr phosphorylation of EGF receptors, and both responses were completely abolished in the presence of a selective EGF receptor inhibitor (AG1478) or the Src inhibitor PP2 and by the expression of a kinase-dead Src mutant. On the other hand, SAPK/JNK activation by UV was partially inhibited by these inhibitors. UV stimulated Src activity in a manner similar to the ERK activation, but the Src activation was insensitive to AG1478. UV-induced cell apoptosis measured by DNA fragmentation and caspase 3 activation was enhanced by AG1478 and an ERK kinase inhibitor (U0126) but inhibited by EGF receptor stimulation by the agonist. These results indicate that UV-induced ERK activation, which provides a survival signal against stress-induced apoptosis, is mediated through Src-dependent Tyr phosphorylation of EGF receptors.     

Design of GFB-111, a platelet-derived growth factor binding molecule with antiangiogenic and anticancer activity against human tumors in mice

We have designed a molecule, GFB-111, that binds to platelet-derived growth factor (PDGF), prevents it from binding to its receptor tyrosine kinase, and blocks PDGF-induced receptor autophosphorylation, activation of Erk1 and Erk2 kinases, and DNA synthesis. GFB-111 is highly potent (IC50 = 250 nM) and selective for PDGF over EGF, IGF-1, aFGF, bFGF, and HRGbeta (IC50 values > 100 microM), but inhibits VEGF-induced Flk-1 tyrosine phosphorylation and Erk1/Erk2 activation with an IC50 of 10 microM. GFB-111 treatment of nude mice bearing human tumors resulted in significant inhibition of tumor growth and angiogenesis. The results demonstrate the feasibility of designing novel growth factor-binding molecules with potent anticancer and antiangiogenic activity.     

Involvement of c‐Jun N‐terminal kinase and extracellular signal‐regulated kinase 1/2 in EGF‐induced angiogenesis

Angiogenesis is a process during which endothelial cells divide and migrate to form new capillaries from the preexisting blood vessels. The present study was designed to investigate whether MAPKs (mitogen‐activated protein kinases) play crucial roles in regulating EGF (epidermal growth factor)‐induced endothelial cell angiogenesis. Our results showed that EGF stimulated HUVEC (human umbilical vein endothelial cells) proliferation in a concentration‐dependent manner, of which the maximum effective concentration of EGF was 10 ng/ml. Western blot analysis showed that EGF at 10 ng/ml significantly induced the phosphorylation of ERK1/2 (extracellular signal‐regulated kinase 1 and 2) and p38 kinase at 5 min, while it induced the phosphorylation of JNK/SAPK (c‐Jun N‐terminal kinase/stress‐activated protein kinase) at 15 min. Further results showed that a JNK/SAPK inhibitor, SP600125, and a specific siRNA JNK/SAPK could both significantly inhibit EGF‐induced tube formation in HUVEC cells, and an ERK1/2 inhibitor PD098059 could also block the tube formation in some content, while a p38 inhibitor SB203580 failed to do so. Furthermore, only SP600125 significantly inhibited EGF‐induced HUVEC cell proliferation under no cytotoxic concentration, so did JNK/SAPK siRNA. In conclusion, JNK/SAPK and ERK1/2 signals therefore play critical roles in EGF‐mediated HUVEC cell angiogenesis.     

Intersectin can regulate the Ras/MAP kinase pathway independent of its role in endocytosis

We previously identified intersectin, a multiple EH and SH3 domain-containing protein, as a component of the endocytic machinery. Overexpression of the SH3 domains of intersectin blocks transferrin receptor endocytosis, possibly by disrupting targeting of accessory proteins of clathrin-coated pit formation. More recently, we identified mammalian Sos, a guanine-nucleotide exchange factor for Ras, as an intersectin SH3 domain-binding partner. We now demonstrate that overexpression of intersectin's SH3 domains blocks activation of Ras and MAP kinase in various cell lines. Several studies suggest that activation of MAP kinase downstream of multiple receptor types is dependent on endocytosis. Thus, the dominant-negative effect of the SH3 domains on Ras/MAP kinase activation may be indirectly mediated through a block in endocytosis. Consistent with this idea, incubating cells at 4 degrees C or with phenylarsine oxide, treatments previously established to inhibit EGF receptor endocytosis, blocks EGF-dependent activation of MAP kinase. However, under these conditions, Ras activity is unaffected and overexpression of the SH3 domains of intersectin is still able to block Ras activation. Thus, intersectin SH3 domain overexpression can effect EGF-mediated MAP kinase activation directly through a block in Ras, consistent with a functional role for intersectin in Ras activation.     

Differential signalling pathways for EGF versus PDGF activation of Erk1/2 MAP kinase and cell proliferation in brown pre-adipocytes

Stimulation by both adrenergic and non-adrenergic pathways can induce proliferation of brown pre-adipocytes. To understand the signalling pathways involved in non-adrenergic stimulation of cell proliferation, we examined Erk1/2 activation. In primary cultures of mouse brown pre-adipocytes, both EGF (epidermal growth factor) and PDGF (platelet-derived growth factor) induced Erk1/2 activation. EGF-stimulated Erk1/2 activation involved Src tyrosine kinases, but not PKC or PI3K, whereas in PDGF-induced Erk1/2 activation, PI3K, PKC (probably the atypical ζ isoform) and Src were involved sequentially. Both EGF and PDGF induced PI3K-dependent Akt activation that was not involved in Erk1/2 activation. By comparing effects of signalling inhibitors (wortmannin, SH-6, TPA, Gö6983, PP2, PD98059) on EGF- and PDGF-induced Erk1/2 activation and cell proliferation (³H-thymidine incorporation), we conclude that while the signal transduction pathways initiated by these growth factors are clearly markedly different, their effects on cell proliferation can be fully explained through their stimulation of Erk1/2 activation; thus Erk1/2 is a common, essential step for stimulation of proliferation in these cells.     

MEK kinase 2 and the adaptor protein Lad regulate extracellular signal-regulated kinase 5 activation by epidermal growth factor via Src

Lad is an SH2 domain-containing adaptor protein that binds MEK kinase 2 (MEKK2), a mitogen-activated protein kinase (MAPK) kinase kinase for the extracellular signal-regulated kinase 5 (ERK5) and JNK pathways. Lad and MEKK2 are in a complex in resting cells. Antisense knockdown of Lad expression and targeted gene disruption of MEKK2 expression results in loss of epidermal growth factor (EGF) and stress stimuli-induced activation of ERK5. Activation of MEKK2 and the ERK5 pathway by EGF and stress stimuli is dependent on Src kinase activity. The Lad-binding motif is encoded within amino acids 228 to 282 in the N terminus of MEKK2, and expression of this motif blocks Lad-MEKK2 interaction, resulting in inhibition of Src-dependent activation of MEKK2 and ERK5. JNK activation by EGF is similarly inhibited by loss of Lad or MEKK2 expression and by blocking the interaction of MEKK2 and Lad. Our studies demonstrate that Src kinase activity is required for ERK5 activation in response to EGF, MEKK2 expression is required for ERK5 activation by Src, Lad and MEKK2 association is required for Src activation of ERK5, and EGF and Src stimulation of ERK5-regulated MEF2-dependent promoter activity requires a functional Lad-MEKK2 signaling complex.     

Analysis of platelet-derived growth factor receptor domain function using a novel chimeric receptor approach.

A chimeric receptor consisting of an epidermal growth factor (EGF) receptor ligand-binding domain and platelet-derived growth factor (PDGF) receptor transmembrane and cytoplasmic signalling domains has been constructed and shown to be fully functional in phosphorylation, mitogenesis, transformation, Ca2+ release, and pH change assays. Expression of this receptor in EGF receptor-deficient, PDGF-responsive NIH 3T3 cells allows the activation of PDGF signalling pathways by EGF. This system was used to examine the function of kinase insertion sequences (KIS). While a mutant with a KIS deletion of 83 amino acids displayed a significant but reduced ability to induce mitogenic, transforming, and Ca2+ release responses in transfected cells, deletion of 20 additional amino acids resulted in abolishment of such activities. This differential loss of signalling potential correlated with the reduced or abolished potential of these receptor mutants to phosphorylate cellular substrates such as PLC gamma. Our results suggest an integral role for KIS in PDGF receptor cytoplasmic domain conformation and an involvement in substrate interaction, but provide no evidence for an exclusive role of KIS in the mediation of biological signals.     

Nck-2, a Novel Src Homology2/3-containing Adaptor Protein That Interacts with the LIM-only Protein PINCH and Components of Growth Factor Receptor Kinase-signaling Pathways

Many of the protein–protein interactions that are essential for eukaryotic intracellular signal transduction are mediated by protein binding modules including SH2, SH3, and LIM domains. Nck is a SH3- and SH2-containing adaptor protein implicated in coordinating various signaling pathways, including those of growth factor receptors and cell adhesion receptors. We report here the identification, cloning, and characterization of a widely expressed, Nck-related adaptor protein termed Nck-2. Nck-2 comprises primarily three N-terminal SH3 domains and one C-terminal SH2 domain. We show that Nck-2 interacts with PINCH, a LIM-only protein implicated in integrin-linked kinase signaling. The PINCH-Nck-2 interaction is mediated by the fourth LIM domain of PINCH and the third SH3 domain of Nck-2. Furthermore, we show that Nck-2 is capable of recognizing several key components of growth factor receptor kinase-signaling pathways including EGF receptors, PDGF receptor-β, and IRS-1. The association of Nck-2 with EGF receptors was regulated by EGF stimulation and involved largely the SH2 domain of Nck-2, although the SH3 domains of Nck-2 also contributed to the complex formation. The association of Nck-2 with PDGF receptor-β was dependent on PDGF activation and was mediated solely by the SH2 domain of Nck-2. Additionally, we have detected a stable association between Nck-2 and IRS-1 that was mediated primarily via the second and third SH3 domain of Nck-2. Thus, Nck-2 associates with PINCH and components of different growth factor receptor-signaling pathways via distinct mechanisms. Finally, we provide evidence indicating that a fraction of the Nck-2 and/or Nck-1 proteins are associated with the cytoskeleton. These results identify a novel Nck-related SH2- and SH3-domain–containing protein and suggest that it may function as an adaptor protein connecting the growth factor receptor-signaling pathways with the integrin-signaling pathways.     

Overexpression of the type II transforming growth factor-beta receptor inhibits fibroblasts proliferation and activates extracellular signal regulated kinase and c-Jun N-terminal kinase

Transforming growth factor-beta (TGF-beta) is a bimodal regulator of cellular growth. The cellular effects of TGF-beta depend on the intensity of signals emanating from TGF-beta receptors. Low levels of receptor activity are sufficient to stimulate cell proliferation, while higher degrees of receptor activation are associated with growth inhibition. To study the mechanisms of these effects, a tetracycline-inducible expression system was used to overexpress type II TGF-beta receptors in NIH 3T3 fibroblasts. Overexpressed type II TGF-beta receptors suppressed fibroblast proliferation elicited by TGF-beta1, fibroblast growth factor (FGF) or platelet-derived growth factor (PDGF). Accompanying these anti-proliferative effects, increases in extracellular-signal regulated kinase (ERK) and c-Jun N-terminal kinase (JNK) activity were detected. Furthermore, PDGF alpha-, but not PDGF beta-receptor protein levels were reduced by type II TGF-beta receptor overexpression. In conclusion, our system is an excellent tool to study the molecular mechanisms of growth inhibition by TGF-beta in fibroblasts. Activation of JNK and ERK, or modulation of PDGF receptor expression may be involved in this process.     

Molecular mechanism for the Shp-2 tyrosine phosphatase function in promoting growth factor stimulation of Erk activity

We have previously shown that activation of extracellular signal-regulated kinase (Erk) by epidermal growth factor (EGF) treatment was significantly decreased in mouse fibroblast cells expressing a mutant Shp-2 molecule lacking 65 amino acids in the SH2-N domain, Shp-2(Delta46-110). To address the molecular mechanism for the positive role of Shp-2 in mediating Erk induction, we evaluated the activation of signaling components upstream of Erk in Shp-2 mutant cells. EGF-stimulated Ras, Raf, and Mek activation was significantly attenuated in Shp-2 mutant cells, suggesting that Shp-2 acts to promote Ras activation or to suppress the down-regulation of activated Ras. Biochemical analyses indicate that upon EGF stimulation, Shp-2 is recruited into a multiprotein complex assembled on the Gab1 docking molecule and that Shp-2 seems to exert its biological function by specifically dephosphorylating an unidentified molecule of 90 kDa in the complex. The mutant Shp-2(Delta46-110) molecule failed to participate in the Gab1-organized complex for dephosphorylation of p90, correlating with a defective activation of the Ras-Raf-Mek-Erk cascade in EGF-treated Shp-2 mutant cells. Evidence is also presented that Shp-2 does not appear to modulate the signal relay from EGF receptor to Ras through the Shc, Grb2, and Sos proteins. These results begin to elucidate the mechanism of Shp-2 function downstream of a receptor tyrosine kinase to promote the activation of the Ras-Erk pathway, with potential therapeutic applications in cancer treatment.