Epidermal growth factor receptor targeting prevents uncoupling of the Grb2-SOS complex

Inhibition of KDM5A attenuates cisplatin-induced hearing loss via regulation of the MAPK/AKT pathway

The study aimed to investigate the potential role of lysine-specific demethylase 5A (KDM5A) in cisplatin-induced ototoxicity. The effect of the KDM5A inhibitor CPI-455 was assessed by apoptosis assay, immunofluorescence, flow cytometry, seahorse respirometry assay, and auditory brainstem response test. RNA sequencing, qRT-PCR, and CUT&Tag assays were used to explore the mechanism underlying CPI-455-induced protection. Our results demonstrated that the expression of KDM5A was increased in cisplatin-injured cochlear hair cells compared with controls. CPI-455 treatment markedly declined KDM5A and elevated H3K4 trimethylation levels in cisplatin-injured cochlear hair cells. Moreover, CPI-455 effectively prevented the death of hair cells and spiral ganglion neurons and increased the number of ribbon synapses in a cisplatin-induced ototoxicity mouse model both in vitro and in vivo. In HEI-OC1 cells, KDM5A knockdown reduced reactive oxygen species accumulation and improved mitochondrial membrane potential and oxidative phosphorylation under cisplatin-induced stress. Mechanistically, through transcriptomics and epigenomics analyses, a set of apoptosis-related genes, including Sos1, Sos2, and Map3k3, were regulated by CPI-455. Altogether, our findings indicate that inhibition of KDM5A may represent an effective epigenetic therapeutic target for preventing cisplatin-induced hearing loss.

The proliferative effects of Pyropia yezoensis peptide on IEC-6 cells are mediated through the epidermal growth factor receptor signaling pathway

For a number of years, seaweed has been used as a functional food in Asian countries, particularly in Korea, Japan and China. Pyropia yezoensis is a marine red alga that has potentially beneficial biological activities. In this study, we examined the mechanisms through which a Pyropia yezoensis peptide [PYP1 (1–20)] induces the proliferation of IEC-6 cells, a rat intestinal epithelial cell line, and the involvement of the epidermal growth factor receptor (EGFR) signaling pathway. First, cell viability assay revealed that PYP1 (1–20) induced cell proliferation in a concentration-dependent manner. Subsequently, we examined the mechanisms responsible for this induction of proliferation induced by PYP1 (1–20). EGFR is widely expressed in mammalian epithelial tissues, and the binding of this ligand affects a variety of cell physiological parameters, such as cell growth and proliferation. PYP1 (1–20) increased the expression of EGFR, Shc, growth factor receptor-bound protein 2 (Grb2) and son of sevenless (SOS). EGFR also induced the activation of the Ras signaling pathway through Raf, MEK and extracellular signal-regulated kinase (ERK) phosphorylation. In addition, cell cycle analysis revealed the expression of cell cycle-related proteins. The results demonstrated an increased number of cells in the G1 phase and an enhanced cell proliferation. In addition, the upregulation of cyclin D, cyclin E, Cdk2, Cdk4 and Cdk6 was observed accompanied by a decreased in p21 and p27 expression. These findings suggest that PYP1 (1–20) stimulates the proliferation of rat IEC-6 cells by activating the EGFR signaling pathway. Therefore, PYP1 (1–20) may be a potential source for the development of bio-functional foods which promotes the proliferation of intestinal epithelial cells.

Nucleobindin-2 is a positive regulator for insulin-stimulated glucose transporter 4 translocation in fenofibrate treated E11 podocytes

The physiology of insulin signaling under normal and disease conditions is well studied in classical insulin target tissues, but not in podocytes. To examine insulin stimulation of podocyte GLUT4 translocation, we established a protocol involving treatment with the PPARα agonist fenofibrate to induce E11 podocyte differentiation within 48 hours rather than 7-10 days, which is required for differentiation under the reported protocol. This allowed us to transiently introduce GLUT4 reporter cDNA and RNAi and thereby to examine the regulatory pathway involved. Here we demonstrate that treatment with 200 μM fenofibrate for 36 hours following transfection had a dramatic effect on podocyte morphology, induced several podocyte specific protein expression markers (G protein-coupled receptor 137B, chloride intracellular channel 5, and nephrin) and resulted in insulin-stimulated GLUT4 translocation. In addition, Nucleobindin-2 was found to constitutively associate with Septin 7 (the repressor of GLUT4 translocation), and knockdown of Nucleobindin-2 was found to completely abrogate insulin-stimulated GLUT4 translocation. Together, these data suggest that Nucleobindin-2 may repress Septin7-induced inhibition of insulin-stimulated GLUT4 translocation in podocytes.

Understanding SOS (Son of Sevenless)

Son of Sevenless (SOS) was discovered in Drosophila melanogaster. Essential for normal eye development in Drosophila, SOS has two human homologues, SOS1 and SOS2. The SOS1 gene encodes the Son of Sevenless 1 protein, a Ras and Rac guanine nucleotide exchange factor. This protein is composed of several important domains. The CDC25 and REM domains provide the catalytic activity of SOS1 towards Ras and the histone fold DH/PH (Dbl homology and Pleckstrin homology) domains function, in tandem, to stimulate GTP/GDP exchange for Rac. In contrast to Ras, there have been few studies that implicate SOS1 in human disease and, initially, less attention was given to this gene. However, mutations in SOS1 have been reported recently in Noonan syndrome and in type 1 hereditary gingival fibromatosis. Although, there have been very few studies that focus on the regulation of this important gene by physiological or exogenous factors, we recently found that the SOS1 gene was induced by the environmental toxin, dioxin, and that this effect was mediated by the aryl hydrocarbon receptor (AhR). These recent observations raise the possibility that alterations in the expression of the SOS1 gene and, consequently, in the activity of the SOS1 protein may affect toxicological endpoints and lead to clinical disease. These possibilities, thus, have stimulated much interest in SOS1 recently. In this article, we review the functions of SOS1 and the evidence for its roles in physiology and pathology across species.

Src-dependent TrkA transactivation is required for pituitary adenylate cyclase-activating polypeptide 38-mediated Rit activation and neuronal differentiation

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a potent neuropeptide that possesses both neurotrophic and neurodevelopmental effects. Recently, the Rit GTPase was found to be activated by a novel Galpha/cAMP/exchange protein activated by cyclic AMP (Epac)-dependent signaling pathway and required for PACAP-dependent cAMP response element-binding protein activation and neuronal differentiation. However, Epac did not function as a Rit guanine nucleotide exchange factor (GEF), and the nature of the PACAP regulatory cascade remained unclear. Here, we show that PACAP-mediated Rit activation involves Src family kinase-dependent TrkA receptor transactivation. PACAP receptor (PACR1) stimulation triggered both G(i)alpha and G(s)alpha/cAMP/Epac regulatory cascades resulting in Src kinase activity, which in turn induced TrkA kinase tyrosine phosphorylation. Importantly, Src inhibition, or the lack of functional Trk receptors, was found to inhibit PACAP-mediated Rit activation, whereas constitutively active Src alone was sufficient to stimulate Rit-guanosine triphosphate levels. A single tyrosine (Y(499)) phosphorylation event was identified as critical to both PACAP-mediated transactivation and TrkA-dependent Rit activation. Accordingly, PACAP stimulation resulted in TrkA-dependent phosphorylation of both the Shc adaptor and son of sevenless (SOS)1/2 GEFs, and Rit activation was inhibited by RNA interference silencing of SOS1/2, implicating a TrkA/Shc/SOS signaling complex in Rit regulation. Together, these observations expand upon the nature of PACR1-mediated transactivation and identify TrkA-Rit signaling as a key contributor to PACAP-dependent neuronal differentiation.

Topological analysis of MAPK cascade for kinetic ErbB signaling

Ligand-induced homo- and hetero-dimer formation of ErbB receptors results in different biological outcomes irrespective of recruitment and activation of similar effector proteins. Earlier experimental research indicated that cells expressing both EGFR (epidermal growth factor receptor) and the ErbB4 receptor (E1/4 cells) induced E1/4 cell-specific B-Raf activation and higher extracellular signal-regulated kinase (ERK) activation, followed by cellular transformation, than cells solely expressing EGFR (E1 cells) in Chinese hamster ovary (CHO) cells. Since our experimental data revealed the presence of positive feedback by ERK on upstream pathways, it was estimated that the cross-talk/feedback pathway structure of the Raf-MEK-ERK cascade might affect ERK activation dynamics in our cell system. To uncover the regulatory mechanism concerning the ERK dynamics, we used topological models and performed parameter estimation for all candidate structures that possessed ERK-mediated positive feedback regulation of Raf. The structure that reliably reproduced a series of experimental data regarding signal amplitude and duration of the signaling molecules was selected as a solution. We found that the pathway structure is characterized by ERK-mediated positive feedback regulation of B-Raf and B-Raf-mediated negative regulation of Raf-1. Steady-state analysis of the estimated structure indicated that the amplitude of Ras activity might critically affect ERK activity through ERK-B-Raf positive feedback coordination with sustained B-Raf activation in E1/4 cells. However, Rap1 that positively regulates B-Raf activity might be less effective concerning ERK and B-Raf activity. Furthermore, we investigated how such Ras activity in E1/4 cells can be regulated by EGFR/ErbB4 heterodimer-mediated signaling. From a sensitivity analysis of the detailed upstream model for Ras activation, we concluded that Ras activation dynamics is dominated by heterodimer-mediated signaling coordination with a large initial speed of dimerization when the concentration of the ErbB4 receptor is considerably high. Such characteristics of the signaling cause the preferential binding of the Grb2-SOS complex to heterodimer-mediated signaling molecules.

Prediction and validation of the distinct dynamics of transient and sustained ERK activation

To elucidate the hidden dynamics of extracellular-signal-regulated kinase (ERK) signalling networks, we developed a simulation model of ERK signalling networks by constraining in silico dynamics based on in vivo dynamics in PC12 cells. We predicted and validated that transient ERK activation depends on rapid increases of epidermal growth factor and nerve growth factor (NGF) but not on their final concentrations, whereas sustained ERK activation depends on the final concentration of NGF but not on the temporal rate of increase. These ERK dynamics depend on Ras and Rap1 dynamics, the inactivation processes of which are growth-factor-dependent and -independent, respectively. Therefore, the Ras and Rap1 systems capture the temporal rate and concentration of growth factors, and encode these distinct physical properties into transient and sustained ERK activation, respectively.

Bile acids induce mitochondrial ROS, which promote activation of receptor tyrosine kinases and signaling pathways in rat hepatocytes

Previous studies have demonstrated in hepatocytes that deoxycholic acid (DCA) promotes inactivation of protein tyrosine phosphatases (PTPases) and activation of ERBB1 and the extracellular-regulated kinase (ERK) 1/2 pathway. The present studies have determined the biochemical mechanism(s) through which these events occur. DCA and taurodeoxycholic acid (TDCA) (100 micromol/L) caused activation of ERBB1, insulin receptor, and the ERK1/2 and AKT pathways in primary rodent hepatocytes. DCA- and TDCA-induced receptor and signaling pathway activations were blocked by the reactive oxygen species (ROS) scavengers N-acetyl cysteine (NAC) and Trolox (TX), as well as by cyclosporin A (CsA) and bongkrekic acid (BKA). DCA activated the ERK1/2 pathway in HuH7 human hepatoma cells that was blocked by the incubation of cells with an ERBB1 inhibitor, NAC, TX, CsA, or BKA. DCA did not activate the ERK1/2 pathway in mitochondria-defective HuH7 Rho 0 cells. In HuH7 cells and primary hepatocytes, DCA enhanced the production of ROS, an effect that was abolished in Rho 0 cells and by prior incubation of cells with CsA or BKA. In hepatocytes and HuH7 cells, DCA inhibited PTPase activity. Incubation of hepatocytes with either CsA or BKA prevented DCA-induced inhibition of PTPase activity. Loss of mitochondrial function in Rho 0 cells also abolished the inhibitory effects of DCA on PTPase activity. In conclusion, DCA and TDCA cause ROS generation in hepatocytes that is dependent on metabolically active mitochondria. The generation of ROS is essential for PTPase inactivation, receptor tyrosine kinase activation, and enhanced signaling down the ERK1/2 and AKT pathways.

A novel proteomic screen for peptide-protein interactions

Regulated interactions between short, unstructured amino acid sequences and modular protein domains are central to cell signaling. Here we use synthetic peptides in "active" (e.g. phosphorylated) and "control" (e.g. non-phosphorylated) forms as baits in affinity pull-down experiments to determine such interactions by quantitative proteomics. Stable isotope labeling by amino acids in cell culture distinguishes specific binders directly by the isotope ratios determined by mass spectrometry (Blagoev, B., Kratchmarova, I., Ong, S.-E., Nielsen, M., Foster, L. J., and Mann, M. (2003) Nat. Biotechnol. 21, 315-318). A tyrosine-phosphorylated peptide of the epidermal growth factor receptor specifically retrieved the Src homology domain (SH) 2- and SH3 domain-containing adapter protein Grb2. A proline-rich sequence of Son of Sevenless also specifically bound Grb2, demonstrating that the screen maintains specificity with low affinity interactions. The proline-rich Sos peptide retrieved only SH3 domain containing proteins as specific binding partners. Two of these, Pacsin 3 and Sorting Nexin 9, were confirmed by immunoprecipitation. Our data are consistent with a change in the role of Sos from Ras-dependent signaling to actin remodeling/endocytic signaling events by a proline-SH3 domain switch.

Human cytomegalovirus infection inhibits epidermal growth factor (EGF) signalling by targeting EGF receptors

Infection with human cytomegalovirus (HCMV) is known to involve complex interactions between viral and cellular factors resulting in perturbation of a number of cellular functions. Specifically, HCMV infection targets control of the cell cycle, cellular transcription and immunoregulation, presumably to optimize the cellular environment for virus persistence and productive infection. Here, we show that HCMV infection also prevents external signalling to the cell by disrupting the function of epidermal growth factor receptor (EGFR). Infection with HCMV resulted in a decrease in cell-surface expression of EGFR. This decrease was correlated with a concomitant decrease in steady-state levels of EGFR protein. Consistent with this, HCMV inhibited EGF-mediated receptor autophosphorylation. Infection with a mutant HCMV deleted of all viral gene products known to be involved in down-regulation of MHC Class I receptors still resulted in this down-regulation, implying that EGFR down-regulation by HCMV is mediated by a novel virus function. We suggest that a primary goal of HCMV is to 'isolate' the infected cell from host-mediated signals so that the cell responds solely to an array of virus-specific signals which optimize the cell for virus production.

Hematopoietic progenitor kinase 1 associates physically and functionally with the adaptor proteins B cell linker protein and SLP-76 in lymphocytes

B cell linker protein (BLNK) is a SLP-76-related adaptor protein essential for signal transduction from the BCR. To identify components of BLNK-associated signaling pathways, we performed a phosphorylation-dependent yeast two-hybrid analysis using BLNK probes. Here we report that the serine/threonine kinase hematopoietic progenitor kinase 1 (HPK1), which is activated upon antigen-receptor stimulation and which has been implicated in the regulation of MAP kinase pathways, interacts physically and functionally with BLNK in B cells and with SLP-76 in T cells. This interaction requires Tyr(379) of HPK1 and the Src homology 2 (SH2) domain of BLNK/SLP-76. Via homology modeling, we defined a consensus binding site within ligands for SLP family SH2 domains. We further demonstrate that the SH2 domain of SLP-76 participates in the regulation of AP-1 and NFAT activation in response to T cell receptor (TCR) stimulation and that HPK1 inhibits AP-1 activation in a manner partially dependent on its interaction with SLP-76. Our data are consistent with a model in which full activation of HPK1 requires its own phosphorylation on tyrosine and subsequent interaction with adaptors of the SLP family, providing a mechanistic basis for the integration of this kinase into antigen receptor signaling cascades.

Re-localization of activated EGF receptor and its signal transducers to multivesicular compartments downstream of early endosomes in response to EGF

The rapid internalization of receptor tyrosine kinases after ligand binding has been assumed to be a negative modulation of signal transduction. However, accumulating data indicate that signal transduction from internalized cell surface receptors also occurs from endosomes. We show that a substantial fraction of tyrosine-phosphorylated epidermal growth factor receptor (EGFR) and Shc, Grb2 and Cbl after internalization relocates from early endosomes to compartments which are negative for the early endosomes, recycling vesicle markers EEA1 and transferrin in EGF-stimulated cells. These compartments contained the multivesicular body and late endosome marker CD63, and the late endosome and lysosome marker LAMP-1, and showed a multivesicular morphology. Subcellular fractionation revealed that activated EGFR, adaptor proteins and activated ERK 1 and 2 were located in EEA1-negative and LAMP-1-positive fractions. Co-immunoprecipitations showed EGFR in complex with both Shc, Grb2 and Cbl. Treatment with the weak base chloroquine or inhibitors of lysosomal enzymes after EGF stimulation induced an accumulation of tyrosine-phosphorylated EGFR and Shc in EEA1-negative and CD63-positive vesicles after a 120-min chase period. This was accompanied by a sustained activation of ERK 1 and 2. These results suggest that EGFR signaling is not spatially restricted to the plasma membrane, primary vesicles and early endosomes, but is continuing from late endocytic trafficking organelles maturing from early endosomes.

High Affinity IgG Receptor Activation of Src Family Kinases Is Required for Modulation of the Shc-Grb2-Sos Complex and the Downstream Activation of the Nicotinamide Adenine Dinucleotide Phosphate (Reduced) Oxidase

We used the U937 cell line to examine the modulation of adaptor protein interactions (Shc, Grb2, and Cbl) after high affinity IgG receptor (FcγRI) cross-linking, leading to the formation of the Grb2-Sos complex, the activation of Ras, and the regulation of the respiratory burst. Cross-linking of FcγRI induced the conversion of GDP-Ras to GTP-Ras reaching a maximum 5 min after stimulation. Concomitant with Ras activation, Sos underwent an electrophoretic mobility shift and the Sos-Grb2 association was increased (6-fold). The Grb2-Sos complex was present only in the membrane fraction and was augmented after FcγRI stimulation. Tyrosine-phosphorylated Shc, mainly the p52 isoform, was observed to transiently onload to the membrane Grb2-Sos complex on FcγRI stimulation. Cross-linking of FcγRI induces the tyrosine phosphorylation of Cbl, which forms a complex with Grb2 and Shc via the Cbl C terminus. Kinetic experiments confirm that Cbl-Grb2 is relatively stable, whereas Grb2-Sos, Grb2-Shc, and Cbl-Shc interactions are highly inducible. The Src family tyrosine kinase inhibitor, PP1, was shown to completely inhibit Shc tyrosine phosphorylation, the Shc-Grb2 interaction, and the FcγR-induced respiratory burst. Our results provide the first evidence that the upstream activation of Src kinases is required for the modulation of the Shc-Grb2 interaction and the myeloid NADPH oxidase response.

Internalized epidermal growth factor receptors participate in the activation of p21(ras) in fibroblasts

Regulated activation of the highly conserved Ras GTPase is a central event in the stimulation of cell proliferation, motility, and differentiation elicited by receptor tyrosine kinases, such as the epidermal growth factor receptor (EGFR). In fibroblasts, this involves formation and membrane localization of Shc.Grb2.Sos complexes, which increases the rate of Ras guanine nucleotide exchange. In order to control Ras-mediated cell responses, this activity is regulated by receptor down-regulation and a feedback loop involving the dual specificity kinase mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK). We investigated the role of EGFR endocytosis in the regulation of Ras activation. Of fundamental interest is whether activated receptors in endosomes can participate in the stimulation of Ras guanine nucleotide exchange, because the constitutive membrane localization of Ras may affect its compartmentalization. By exploiting the differences in postendocytic signaling of two EGFR ligands, epidermal growth factor and transforming growth factor-alpha, we found that activated EGFR located at the cell surface and in internal compartments contribute equally to the membrane recruitment and tyrosine phosphorylation of Shc in NR6 fibroblasts expressing wild-type EGFR. Importantly, both the rate of Ras-specific guanine nucleotide exchange and the level of Ras-GTP were depressed to near basal values on the time scale of receptor trafficking. Using the selective MEK inhibitor PD098059, we were able to block the feedback desensitization pathway and maintain activation of Ras. Under these conditions, the generation of Ras-GTP was not significantly affected by the subcellular location of activated EGFR. In conjunction with our previous analysis of the phospholipase C pathway in the same cell line, this suggests a selective continuation of specific signaling activities and cessation of others upon receptor endocytosis.

c-Raf-mediated inhibition of epidermal growth factor-stimulated cell migration

Epidermal growth factor stimulates migration of a number of cell types, yet the signaling pathways that regulate epidermal growth factor-stimulated migration are poorly defined. In this report, we employ a transient transfection migration assay to assess the role of components of the Ras-mitogen-activated protein (MAP) kinase signaling pathway in epidermal growth factor-stimulated chemotaxis of rat embryo fibroblasts. Expression of dominant negative Ras blocks epidermal growth factor-mediated chemotaxis, while constitutively active Ras has no effect on chemokinesis or chemotaxis. PD98059 and U0126, inhibitors of MAP kinase kinase (MEK) activity, decreased epidermal growth factor-stimulated migration, while kinase-defective MEK1, an inhibitor of MAP kinase activation, enhanced migration. To understand the paradoxical effects of these molecules on epidermal growth factor-induced migration, we examined the role of c-Raf on migration. Expression of either wild type c-Raf or the catalytic domain of c-Raf effectively inhibited epidermal growth factor-stimulated cell migration. We suggest that, whereas Ras activity is necessary to promote epidermal growth factor-stimulated migration, sustained activation of c-Raf may be important in down-regulating migratory signaling pathways triggered by epidermal growth factor receptor activation. Further, activation of c-Raf upon inhibition of the MEK-MAP kinase pathway may contribute to the inhibition of cell migration observed with pharmacological MEK inhibitors.

Dominant negative EGFR-CD533 and inhibition of MAPK modify JNK1 activation and enhance radiation toxicity of human mammary carcinoma cells

Exposure of MDA-MB-231 human mammary carcinoma cells to an ionizing radiation dose of 2 Gy results in immediate activation and Tyr phosphorylation of the epidermal growth factor receptor (EGFR). Doxycycline induced expression of a dominant negative EGFR-CD533 mutant, lacking the COOH-terminal 533 amino acids, in MDA-TR15-EGFR-CD533 cells was used to characterize intracellular signaling responses following irradiation. Within 10 min, radiation exposure caused an immediate, transient activation of mitogen activated protein kinase (MAPK) which was completely blocked by expression of EGFR-CD533. The same radiation treatment also induced an immediate activation of the c-Jun-NH2-terminal kinase 1 (JNK1) pathway that was followed by an extended rise in kinase activity after 30 min. Expression of EGFR-CD533 did not block the immediate JNK1 response but completely inhibited the later activation. Treatment of MDA-TR15-EGFR-CD533 cells with the MEK1/2 inhibitor, PD98059, resulted in approximately 70% inhibition of radiation-induced MAPK activity, and potentiated the radiation-induced increase of immediate JNK1 activation twofold. Inhibition of Ras farnesylation with a concomitant inhibition of Ras function completely blocked radiation-induced MAPK and JNK1 activation. Modulation of EGFR and MAPK functions also altered overall cellular responses of growth and apoptosis. Induction of EGFR-CD533 or treatment with PD98059 caused a 3-5-fold increase in radiation toxicity in a novel repeated radiation exposure growth assay by interfering with cell proliferation and potentiating apoptosis. In summary, this data demonstrates that both MAPK and JNK1 activation in response to radiation occur through EGFR-dependent and -independent mechanisms, and are mediated by signaling through Ras. Furthermore, we have demonstrated that radiation-induced activation of EGFR results in downstream activation of MAPK which may affect the radiosensitivity of carcinoma cells.

Membrane targeting and stabilization of sarcospan is mediated by the sarcoglycan subcomplex

The dystrophin-glycoprotein complex (DGC) is a multisubunit complex that spans the muscle plasma membrane and forms a link between the F-actin cytoskeleton and the extracellular matrix. The proteins of the DGC are structurally organized into distinct subcomplexes, and genetic mutations in many individual components are manifested as muscular dystrophy. We recently identified a unique tetraspan-like dystrophin-associated protein, which we have named sarcospan (SPN) for its multiple sarcolemma spanning domains (Crosbie, R.H., J. Heighway, D.P. Venzke, J.C. Lee, and K.P. Campbell. 1997. J. Biol. Chem. 272:31221-31224). To probe molecular associations of SPN within the DGC, we investigated SPN expression in normal muscle as a baseline for comparison to SPN's expression in animal models of muscular dystrophy. We show that, in addition to its sarcolemma localization, SPN is enriched at the myotendinous junction (MTJ) and neuromuscular junction (NMJ), where it is a component of both the dystrophin- and utrophin-glycoprotein complexes. We demonstrate that SPN is preferentially associated with the sarcoglycan (SG) subcomplex, and this interaction is critical for stable localization of SPN to the sarcolemma, NMJ, and MTJ. Our experiments indicate that assembly of the SG subcomplex is a prerequisite for targeting SPN to the sarcolemma. In addition, the SG- SPN subcomplex functions to stabilize alpha-dystroglycan to the muscle plasma membrane. Taken together, our data provide important information about assembly and function of the SG-SPN subcomplex.

Assembly of the sarcoglycan complex. Insights for muscular dystrophy

Four unique transmembrane glycoproteins comprise the sarcoglycan complex in striated muscle. The sarcoglycan complex contributes to maintenance of sarcolemma integrity. A shared feature of four types of autosomal recessive limb girdle muscular dystrophy (LGMD) is that mutations in a single sarcoglycan gene result in the loss of all sarcoglycans at the sarcolemma. The mechanism of destabilization is unknown. We report here our findings of sarcoglycan complex biosynthesis in a heterologous cell system. We demonstrate that the sarcoglycans are glycosylated and assemble into a complex that resides in the plasma membrane. Complex assembly was dependent on the simultaneous synthesis of all four sarcoglycans. Mutant sarcoglycans block complex formation and insertion of the sarcoglycans into the plasma membrane. This constitutes the first biochemical evidence to support the idea that the molecular defect in sarcoglycan-deficient LGMD is because of aberrant sarcoglycan complex assembly and trafficking, which leads to the absence of the complex from the sarcolemma.

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.

Phosphorylation of CrkII adaptor protein at tyrosine 221 by epidermal growth factor receptor

CrkII adaptor protein becomes tyrosine-phosphorylated upon various types of stimulation. We examined whether tyrosine 221, which has been shown to be phosphorylated by c-Abl, was phosphorylated also by other tyrosine kinases, such as epidermal growth factor (EGF) receptor. For this purpose, we developed an antibody that specifically recognizes Tyr221-phosphorylated CrkII, and we demonstrated that CrkII was phosphorylated on Tyr221 upon EGF stimulation. When NRK cells were stimulated with EGF, the tyrosine-phosphorylated CrkII was detected at the periphery of the cells, where ruffling is prominent, suggesting that signaling to CrkII may be involved in EGF-dependent cytoskeletal reorganization. The EGF-dependent phosphorylation of CrkII was also detected in a c-Abl-deficient cell line. Moreover, recombinant CrkII protein was phosphorylated in vitro by EGF receptor. These results strongly suggest that EGF receptor directly phosphorylates CrkII. Mutational analysis revealed that the src homology 2 domain was essential for the phosphorylation of CrkII by EGF receptor but not by c-Abl, arguing that these kinases phosphorylate CrkII by different phosphorylation mechanisms. Finally, we found that the CrkII protein phosphorylated upon EGF stimulation did not bind to the phosphotyrosine-containing peptide and that CrkII initiated dissociation from EGF receptor within 3 min even with the sustained tyrosine phosphorylation of EGF receptor. This result implicated phosphorylation of Tyr221 in the negative regulation of the src homology 2-mediated binding of CrkII to EGF receptor.

Insulin regulates the dynamic balance between Ras and Rap1 signaling by coordinating the assembly states of the Grb2-SOS and CrkII-C3G complexes

Insulin stimulation of Chinese hamster ovary cells expressing the human insulin receptor resulted in a time-dependent decrease in the amount of GTP bound to Rap1. The inactivation of Rap1 was associated with an insulin-stimulated decrease in the amount of Rap1 that was bound to Raf1. In parallel with the dissociation of Raf1 from Rap1, there was an increased association of Raf1 with Ras. Concomitant with the inactivation of Rap1 and decrease in Rap1-Raf1 binding, we observed a rapid insulin-stimulated dissociation of the CrkII-C3G complex which occurred in a Ras-independent manner. The dissociation of the CrkII-C3G was recapitulated in vitro using a GST-C3G fusion protein to precipitate CrkII from whole cell detergent extracts. The association of GST-C3G with CrkII was also dose dependent and demonstrated that insulin reduced the affinity of CrkII for C3G without any effect on CrkII protein levels. Furthermore, the reduction in CrkII binding affinity was reversible by tyrosine dephosphorylation with PTP1B and by mutation of Tyr221 to phenylalanine. Together, these data demonstrate that insulin treatment results in the de-repression of Rap1 inhibitory function on the Raf1 kinase concomitant with Ras activation and stimulation of the downstream Raf1/MEK/ERK cascade.

Uncoupling of hepatic, epidermal growth factor-mediated mitogen-activated protein kinase activation in the fetal rat

Stimulation of cell proliferation by mitogens involves tyrosine phosphorylation of proteins at the cell membrane by receptor tyrosine kinases. This promotes formation of multi-protein complexes that can activate the small G-protein, Ras. Activation of Ras, in turn, leads to sequential activation of the following three serine-threonine kinases: Raf, extracellular signal-regulated kinase kinase (MEK), and members of the family of mitogen-activated protein (MAP) kinases. Prior studies have shown that intraperitoneal injection of epidermal growth factor (EGF) leads to rapid activation of hepatic MAP kinases in adult rats but not in late gestation (E19) fetal rats (Boylan, J. M., and Gruppuso, P. A. (1996) Cell Growth & Differ. 7, 1261-1269). The present studies were undertaken to determine the mechanism for this "uncoupling" of the MAP kinase pathway. E19 fetal rats and adult male rats were injected with EGF (0.5 microg/g body weight, intraperitoneally) or with saline. After 15 min, livers were removed and prepared for kinase analyses. EGF injection led to a rapid and marked activation of hepatic Raf and MEK in both fetal and adult rats, whereas MAP kinase activation was minimal in fetal as opposed to adult rats. Examination of the ontogeny of this dissociation of MAP kinase activation from MEK activation showed gradual acquisition of intact signaling as an adult hepatocyte phenotype was attained during the first 4 postnatal weeks. Over this period, MAP kinase content as determined by Western immunoblotting was constant. Recombination experiments using partially purified fetal and adult rat liver MEK and MAP kinase showed intact MAP kinase activation in vitro, indicating that neither enzyme was irreversibly altered in the fetus. In studies using primary cultures of E19 fetal rat hepatocytes, uncoupling of MAP kinase activation from MEK activation could be induced by incubation of fetal hepatocytes for 24 h with a potent fetal hepatocyte mitogen, transforming growth factor-alpha. These findings indicate that a novel negative feedback mechanism for MAP kinase regulation may be active in developing rat hepatocytes.

Insulin and epidermal growth factor stimulate a conformational change in Rap1 and dissociation of the CrkII-C3G complex

Insulin and epidermal growth factor (EGF) stimulation of Chinese hamster ovary cells expressing the human insulin and EGF receptors resulted in a time-dependent decrease in the ability of a Rap1 antibody (amino acid epitope 121-136) to immunoprecipitate Rap1 from whole cell detergent extracts. This was due to an apparent masking of Rap1 as heat denaturation of the whole cell detergent extracts (5 min at 100 degrees C) resulted in equal immunoprecipitation of Rap1 with this epitope-specific antibody. The time-dependent change in Rap1 immunoreactivity was paralleled with an insulin-stimulated dissociation of the CrkII-C3G complex. Similarly, EGF treatment also resulted in a time-dependent dissociation of the CrkII-C3G complex that occurred concomitant with the masking of the 121-136 Rap1 epitope. Furthermore, pretreatment of the cells with the tyrosine kinase inhibitor, genistein, decreased both the basal and insulin-stimulated tyrosine phosphorylation of CrkII that directly correlated with the amount of CrkII that was immunoprecipitated with C3G. Together, these data suggest that insulin and EGF stimulation result in the dissociation of the CrkII-C3G complex, thereby inducing an apparent conformation change in Rap1.

Signal transducer and activator of transcription-3 serine phosphorylation by insulin is mediated by a Ras/Raf/MEK-dependent pathway

We recently reported that insulin stimulation results in the serine phosphorylation of STAT3 (signal transducer and activator of transcription-3). In the present study, we identified serine 727 as the site of insulin-stimulated STAT3 serine phosphorylation. This phosphorylation event occurs independent of tyrosine phosphorylation. Furthermore, interleukin-6-induced tyrosine phosphorylation can occur independent of serine phosphorylation, demonstrating that these two phosphorylation pathways are mechanistically unrelated. Selective activation of the JNK and p38 family of mitogen-activated protein (MAP) kinases by anisomycin treatment did not result in the phosphorylation of STAT3. In contrast, activation of the ERK MAP kinase pathway with both insulin and osmotic shock resulted in the serine phosphorylation of STAT3. In addition, expression of a dominant-interfering Ras mutant (N17Ras) or treatment with the specific MEK inhibitor (PD98059) prevented the insulin stimulation of STAT3 serine phosphorylation. Blockade of ERK activation by expression of the MAP kinase phosphatase (MKP-1) had no effect on insulin-stimulated STAT3 serine phosphorylation. Together, these data demonstrate that the insulin-stimulated serine phosphorylation of STAT3 occurs by a MEK-dependent pathway that is independent of ERK activation.

Signaling molecules involved in coupling growth hormone receptor to mitogen-activated protein kinase activation

We have shown previously that GH stimulates the mitogen-activated protein (MAP) kinases designated ERKs (extracellular signal-regulated kinases) 1 and 2. To examine pathways coupling GH receptor (GHR) to MAP kinase activation, we have determined the effects of GH on SHC-growth factor receptor bound 2-son of Sevenless (SHC-Grb2-SOS) association and activation of Ras, Raf, and MAP-ERK kinase (MEK). GH promoted the rapid, transient association of SHC with the Grb2-SOS complex, which correlated with the time course of Ras, Raf, and MEK activation. Despite the continuous presence of GH, these activation events were transient with Ras, Raf, and MEK returning to near basal activity by 15 or 30 min. The inactivation of Ras, Raf, and MEK directly correlated with the serine/threonine phosphorylation of SOS and dissociation of SOS from Grb2 but not Grb2 from tyrosine-phosphorylated SHC. Phosphorylation was blocked by the MEK inhibitor, PD98059. Based upon the established functions of the MAP kinase pathway, these data indicate that GH stimulation results in the assembly of a SHC-Grb2-SOS complex that serves to activate Ras and thereby engage the Raf-MEK-ERK pathway. Activation of this pathway generates a feedback kinase cascade that phosphorylates SOS resulting in the dissociation of SHC-Grb2 complexes from SOS, thereby causing a more rapid termination of the signaling pathway than would result from SHC dephosphorylation.

Impaired plasma membrane targeting of Grb2-murine son of sevenless (mSOS) complex and differential activation of the Fyn-T cell receptor (TCR)-zeta-Cbl pathway mediate T cell hyporesponsiveness in autoimmune nonobese diabetic mice

Nonobese diabetic (NOD) mouse thymocytes are hyporesponsive to T cell antigen receptor (TCR)-mediated stimulation of proliferation, and this T cell hyporesponsiveness may be causal to the onset of autoimmune diabetes in NOD mice. We previously showed that TCR-induced NOD T cell hyporesponsiveness is associated with a block in Ras activation and defective signaling along the PKC/Ras/MAPK pathway. Here, we report that several sequential changes in TCR-proximal signaling events may mediate this block in Ras activation. We demonstrate that NOD T cell hyporesponsiveness is associated with the (a) enhanced TCR-beta-associated Fyn kinase activity and the differential activation of the Fyn-TCR-zeta-Cbl pathway, which may account for the impaired recruitment of ZAP70 to membrane-bound TCR-zeta; (b) relative inability of the murine son of sevenless (mSOS) Ras GDP releasing factor activity to translocate from the cytoplasm to the plasma membrane; and (c) exclusion of mSOS and PLC-gamma1 from the TCR-zeta-associated Grb2/pp36-38/ZAP70 signaling complex. Our data suggest that altered tyrosine phosphorylation and targeting of the Grb2/pp36-38/ZAP70 complex to the plasma membrane and cytoskeleton and the deficient association of mSOS with this Grb2-containing complex may block the downstream activation of Ras and Ras-mediated amplification of TCR/CD3-mediated signals in hyporesponsive NOD T cells. These findings implicate mSOS as an important mediator of downregulation of Ras signaling in hyporesponsive NOD T cells.

Signal transduction pathway of human fibroblast growth factor receptor 3. Identification of a novel 66-kDa phosphoprotein

Stimulation of fibroblast growth factor receptor 3 (FGFR3) results in a variety of functional effects, including regulation of epithelial cell growth and differentiation. In order to characterize the signaling pathway through which FGFR3 regulates cell growth, L6 cells lacking any endogenous FGFR were stably transfected with the two different human isoforms, FGFR3 IIIb and FGFR3 IIIc, that result from alternative splicing of exon III of the FGFR3 gene encoding the ligand binding domain. Expression of FGFR3 IIIc in stably transfected L6 cells conferred growth responses to several members of the FGF family including FGF-1, -2, -4, and -6, while FGFR3 IIIb-expressing cells responded only to FGF-1. Activation of FGFR3 upon ligand binding resulted in activation of mitogen-activated protein kinase pathway. FGFR3 utilizes two different pools of adapter protein GRB2 to link to Ras. Activated FGFR3 predominantly interacts with GRB2.Sos in complex with a previously identified 90-kDa protein and designated protein 80K-H. In addition, 80K-H.GRB2. Sos complex was found to contain a novel 66-kDa protein. Tyrosine phophorylation of the 66-kDa protein was dependent on ligand activation of FGFR3, suggesting that the 66-kDa protein may play an important role in FGFR3-specific signaling. In addition to this unique pathway, FGFR3 also links to GRB2.Sos complex via the adapter protein Shc. Furthermore, activated FGFR3 was not able to induce dissociation of GRB2.Sos complex following Sos phosphorylation. In summary, FGFR3 signaling pathway utilizes two GRB2-containing complexes; Shc.GRB2.Sos and 80K-H.pp66.GRB2.Sos; these two complexes may alternatively link FGFG3 to mitogen-activated protein kinase. Finally, activated FGFR3 was also found to result in phosphorylation of phospholipase C-gamma but reduced phosphorylation of c-Src.

Interactions between Src homology (SH) 2/SH3 adapter proteins and the guanylnucleotide exchange factor SOS are differentially regulated by insulin and epidermal growth factor

Co-immunoprecipitation of whole cell extracts demonstrated that the guanylnucleotide exchange factor SOS was associated with the small adapter proteins Grb2, CrkII, and Nck. In vitro binding indicated a similar binding affinity of SOS for all three adapter proteins but with a slightly lower Kd for Grb2 (approximately 2.5-fold) compared with Nck and CrkII. Insulin stimulation resulted in co-immunoprecipitation of tyrosine-phosphorylated IRS1 with Grb2 and to a lesser extent CrkII. Although Grb2 also associated with tyrosine-phosphorylated Shc, there was no detectable interaction of CrkII with Shc. In contrast, EGF stimulation resulted in the predominant co-immunoprecipitation of Grb2 with the EGF receptor, whereas CrkII primarily associated with an unidentified 120-130-kDa protein. Similar to the ability of insulin to induce the dissociation of the Grb2-SOS complex, there was a concomitant time-dependent dissociation of the CrkII-SOS and Nck-SOS complexes. However, EGF stimulation had no effect on the association state of the Grb2-SOS or the Nck-SOS complexes but did result in a time-dependent dissociation of the CrkII from SOS. Together, these data demonstrate that different cellular pools of SOS associate with different adapter proteins forming various signaling complexes, each undergoing distinct patterns of assembly/disassembly following growth factor stimulation.

Insulin and epidermal growth factor receptors regulate distinct pools of Grb2-SOS in the control of Ras activation

Insulin and epidermal growth factor (EGF) stimulate a rapid but transient increase in the amount of GTP bound to Ras that returns to the basal GDP-bound state within 10-30 min. Although insulin stimulation resulted in a dissociation of the Grb2.SOS complex, EGF did not affect the Grb2.SOS complex but instead induced dissociation of Grb2-SOS from tyrosine-phosphorylated Shc. The dissociation of Grb2-SOS from Shc was not due to dephosphorylation as Shc remained persistently tyrosine-phosphorylated during this time. Furthermore, there was no decrease in the extent of insulin receptor substrate 1, insulin receptor, or EGF receptor tyrosine phosphorylation. Surprisingly, however, despite the EGF-induced decrease in the amount of Grb2-SOS bound to Shc, the extent of Grb2 associated with Shc remained constant, and there was a concomitant increase in the amount of SOS associated with Grb2. In addition, after the insulin-stimulated dissociation of Grb2 from SOS, EGF treatment induced the reassociation of the Grb2.SOS complex. Quantitative immunoprecipitation demonstrated that only a small fraction of the total cellular pool of Grb2 was associated with SOS. Similarly, only a small fraction of SOS and Grb2 were co-immunoprecipitated with Shc. Together, these data suggest the presence of distinct Grb2-SOS pools that are independently utilized by insulin and EGF in their recruitment to tyrosine-phosphorylated Shc.