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Query: EC:2.7.11.24 (mitogen-activated protein kinase)
 95,810 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Formation of a complex of the nucleotide exchange factor Sos, the SH2 and SH3 containing adaptor protein Grb2/Sem-5 and tyrosine phosphorylated EGF receptor and Shc has been implicated in the activation of Ras by epidermal growth factor (EGF) in fibroblasts: related mechanisms for activation of Ras operate in other cell types. An increase in the apparent molecular weight of Sos has been reported to occur after several minutes of receptor stimulation due to phosphorylation by mitogen-activated protein (MAP) kinases. We report here that treatment of human peripheral blood T lymphoblasts with phorbol esters causes a similar shift in mobility of Sos. This modification of Sos does not alter its ability to bind Grb2, but correlates with strong inhibition of the binding of the Sos/Grb2 complex to tyrosine phosphorylated sequences, either a tyrosine phosphopeptide in cell lysates or p36 in intact cells. This effect, along with the mobility shift of Sos, can be mimicked in vitro by phosphorylation of Sos by the mitogen-activated protein kinase, ERK1. A novel negative feedback mechanism therefore exists whereby activation of MAP kinases through Ras results in the uncoupling of the Sos/Grb2 complex from tyrosine kinase substrates without blocking the interaction of Sos with Grb2.
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PMID:Downregulation of the Ras activation pathway by MAP kinase phosphorylation of Sos. 747 53

The mouse protein mSos1 has a central Ras guanine nucleotide exchange domain, and a long proline-rich C-terminal tail which contains several potential binding sites for the SH3 domains of the adaptor protein, Grb2. In fibroblasts, growth factor stimulation results in the recruitment of Grb2-mSos1 into complexes with activated receptors and cytoplasmic phosphoproteins such as Shc, which are apparently involved in Ras activation, and subsequently to an increase in mSos1 phosphorylation on serine and threonine. The catalytic and C-terminal domains of mSos1 contain several potential sites for phosphorylation by mitogen-activated protein kinases. In vitro, purified p42/p44 MAP-kinase selectively phosphorylated the C-terminal tail of mSos1. Comparative tryptic phosphopeptide mapping of mSos1 phosphorylated in vitro by MAP kinase and of mSos1 immunoprecipitated from EGF-stimulated cells, revealed several phosphopeptides in common. These common phosphorylation sites have been mapped to a region encompassing the first three proline (pro)-rich motifs in the tail of mSos1. Furthermore, a region of mSos1 containing the first two pro-rich motifs could associate with MBP kinase activity in vitro. Phosphorylation of mSos1 did not affect binding of Grb2 to mSos1, but appeared to decrease binding of the mSos1-Grb2 complex to Shc and the EGF-receptor. These findings suggest a potential inhibitory role for MAP-kinase in attenuating nucleotide exchange on Ras, by uncoupling mSos1 from membrane-bound receptor complexes that lead to Ras activation.
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PMID:MAP kinase phosphorylation of mSos1 promotes dissociation of mSos1-Shc and mSos1-EGF receptor complexes. 747 66

Previous studies suggested that interleukin-11 (IL-11) induces the activation of mitogen-activated protein kinase (MAPK) in mouse 3T3L1 cells. However, the mechanisms by which IL-11 activates MAPK remain elusive. Our present results show that IL-11 promotes the formation of the active GTP-bound form of Ras, suggesting that IL-11 actions may be transduced in part through the Ras/MAPK signaling pathway. By immunoblotting and immunoprecipitation, we further demonstrate the association of tyrosine phosphoproteins with Grb2, an adaptor protein serving as a key intermediate for Ras activation. These phosphotyrosine-containing proteins have been subsequently identified to be JAK2, Fyn, and Syp. JAK2 and Fyn are transiently associated with Grb2 upon stimulation with IL-11, suggesting that JAK2 and Fyn may be involved in transducing signals from the IL-11 receptor-glycoprotein 130 to the Ras system through Grb2. Taken together, these results suggest that IL-11-induced interactions of JAK2, Fyn, and Grb2 may not only provide a novel mechanism for the activation of the Ras/MAPK system but also indicate cross-talk among diverse signaling pathways.
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PMID:Interleukin-11 induces complex formation of Grb2, Fyn, and JAK2 in 3T3L1 cells. 749 80

Insulin and epidermal growth factor receptors transmit signals for cell proliferation and gene regulation through formation of active GTP-bound p21ras mediated by the guanine nucleotide exchange factor Sos. Sos is constitutively bound to the adaptor protein Grb2 and growth factor stimulation induces association of the Grb2/Sos complex with Shc and movement of Sos to the plasma membrane location of p21ras. Insulin or epidermal growth factor stimulation induces a rapid increase in p21ras levels, but after several minutes levels decline toward basal despite ongoing hormone stimulation. Here we show that deactivation of p21ras correlates closely with phosphorylation of Sos and dissociation of Sos from Grb2, and that inhibition of mitogen-activated protein (MAP) kinase kinase (also known as extracellular signal-related kinase (ERK) kinase, or MEK) blocks both events, resulting in prolonged p21ras activation. These data suggest that a negative feedback loop exists whereby activation of the Raf/MEK/MAP kinase cascade by p21ras causes Sos phosphorylation and, therefore, Sos/Grb2 dissociation, limiting the duration of p21ras activation by growth factors. A serine/threonine kinase downstream of MEK (probably MAP kinase) mediates this desensitization feedback pathway.
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PMID:Negative feedback regulation and desensitization of insulin- and epidermal growth factor-stimulated p21ras activation. 759 90

A single point mutation, Glu627--> Val, equivalent to the activating mutation in the Neu oncogene, was inserted in the transmembrane domain of the human epidermal growth factor (EGF) receptor. Unlike the wild type, Glu627-EGF receptor, transfected in NIH3T3 cells, gave rise to focal transformation and growth in agar even in the absence EGF. Constitutive activity of mutant EGF receptor amounted to 20% of that of wild type receptor stimulated by EGF. In addition, the mutant receptor was more sensitive to EGF, reaching maximum transforming activity at 5 ng/ml EGF. NIH3T3 cells expressing Glu627-EGF receptor showed a transformed phenotype and were not arrested in G0 upon serum deprivation. The mutant receptor was constitutively autophosphorylated, and several other cellular proteins were phosphorylated on tyrosine in absence of the ligand. Among these, the SHC adaptor protein was phosphorylated in absence of EGF, the other adaptor, GRB-2 was constitutively associated with the Glu627-EGF receptor in vivo and in vitro, and mitogen-activated protein kinase was constitutively phosphorylated. In contrast, other EGF receptor substrates, like phospholipase C gamma, were not phosphorylated in absence of EGF. The mutant receptor showed a higher sensitivity to cleavage by calpain both in absence and presence of EGF, appeared as a 170- and 150-kDa doublet in cell extracts, and a specific calpain inhibitor blocked the appearance of the 150-kDa form. Since the calpain cleavage site is located in the receptor cytoplasmic tail, this finding suggests that the Glu627 mutation induces a slightly different conformation in the EGF receptor intracellular domain. In conclusion, our data show that a point mutation in the EGF receptor transmembrane domain was able to constitutively activate the receptor and to induce transformation via constitutive activation of the Ras pathway.
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PMID:SHC and GRB-2 are constitutively by an epidermal growth factor receptor with a point mutation in the transmembrane domain. 764 41

Nerve growth factor (NGF) and epidermal growth factor (EGF) elicit contrasting actions on PC12 pheochromocytoma cells; NGF causes neuronal differentiation, and EGF induces proliferation. However, ectopic expression of the Src homology 2 (SH2) and SH3-containing oncogenic adaptor protein v-Crk in PC12 cells results in EGF-inducible neuronal differentiation (Hempstead, B. L., Birge, R. B., Fajardo, J. E., Glassman, R., Mahadeo, D., Kraemer, R., and Hanafusa, H. (1994) Mol. Cell. Biol. 14, 1964-1971). Here we show that v-Crk complexes with both the tyrosine-phosphorylated EGF receptor and the Ras guanine nucleotide exchange factor SOS in PC12 cells and is involved in an pathway analogous to that of Grb2. Expression of v-Crk results in an enhanced and sustained activation of Ras and mitogen-activated protein (MAP) kinase following EGF or NGF stimulation, implying that v-Crk can couple divergent tyrosine kinase pathways to Ras. To investigate the causal relationship between EGF receptor binding, MAP kinase activation, and neurite outgrowth, we stably expressed two v-Crk SH2 point mutants, v-Crk(R273N) and v-Crk(H294R) in PC12 cells. Mutations within the SH2 domain of v-Crk block binding of v-Crk to the tyrosine phosphorylated EGF receptor, compromise v-Crk's ability to cause EGF-dependent neurite outgrowth, and act in a dominant negative manner for NGF-induced neurite outgrowth. However, the kinetics of MAP kinase activation in EGF- or NGF-treated v-Crk-(R273N)PC12 cells was comparable with that in v-CrkPC12 cells. These data are consistent with a model in which v-Crk regulates the strength of a tyrosine kinase signal leading to prolonged activation of Ras and MAP kinase. However, the experiments with the SH2 mutants suggest that sustained activation, by itself, may not be sufficient to switch the fate of v-CrkPC12 cells from proliferation toward differentiation.
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PMID:v-Crk modulation of growth factor-induced PC12 cell differentiation involves the Src homology 2 domain of v-Crk and sustained activation of the Ras/mitogen-activated protein kinase pathway. 765 47

Ligation of membrane immunoglobulin M (mIgM) receptor in the Ramos B-cell line induced tyrosine phosphorylation of several intracellular substrates, including the adaptor protein. Shc. Phosphorylated Shc could be seen to associate with Grb2 in a complex which included hSOS. Inasmuch as hSOS is involved in p21ras activation, we also demonstrated that mIgM ligation activated a Ras-dependent kinase cascade in which sequential activation of Raf-1 and MEK-1 culminates in the activation of p42 mitogen-activated protein (MAP) kinase (ERK-2). The tumour promoter and protein kinase C agonist, phorbol 12-myristate 13-acetate (PMA), also activated Raf-1, MEK-1, and MAP kinase in Ramos cells, but did not induce tyrosine phosphorylation of Shc or Shc/Grb2 association. Okadaic acid, another tumour promoter and serine/threonine phosphatase inhibitor, activated p42 MAP kinase without activating Raf-1 or MEK-1, suggesting the existence of a serine/threonine phosphatase which directly regulates MAP kinase activity.
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PMID:The membrane immunoglobulin receptor utilizes a Shc/Grb2/hSOS complex for activation of the mitogen-activated protein kinase cascade in a B-cell line. 771 78

The let-60 ras gene of Caenorhabditis elegans is required for multiple aspects of development. The vulvar differentiation pathway is the most intensively studied of these, but the ras pathway has now been shown to also be essential for male spicule development. Using vulval differentiation, molecular genetic techniques are now being used to study structure/function relationships of particular signaling components and to identify new positively and negatively acting proteins of Ras-mediated signaling pathways. Mutations affecting LET-23, a receptor tyrosine kinase homolog, which cause tissue-specific defects have been localized to the carboxyl terminus. SH2 domain specificity has been analyzed through Src/SEM-5 chimeric proteins in transgenic nematodes. A mitogen-activated protein kinase that acts downstream of LET-60 Ras in vulval differentiation has been identified. Negative regulatory genes have been cloned and found to encode novel proteins and a clathrin adaptor protein.
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PMID:Ras pathways in Caenorhabditis elegans. 774 23

We have previously shown that the IL-6R in a growth-responsive B cell line, AF10, induces activation of mitogen-activated protein (MAP) kinase. Here we demonstrate the activation of Raf-1 and MEK-1, which act as a MAP kinase kinase kinase and a MAP kinase kinase, respectively, in the MAP kinase cascade induced by IL-6 in AF10 cells. IL-6 also induced tyrosine phosphorylation of the signaling transducing subunit of the IL-6R in AF10 cells, along with tyrosine phosphorylation of the gp130-associated tyrosine protein kinase JAK1 and the adaptor molecule p52shc. Although induction of tyrosine phosphorylation and activation of MAP kinase by IL-6 in a differentiation-responsive B cell line, SKW 6.4, were below the limits of detection, the phorbol ester PMA did activate Raf-1, MEK-1, and MAP kinase without inducing the phosphorylation of gp130, JAKs, or p52shc. These results suggest that JAK kinase family members associated with the IL-6R may participate in the activation of MAP kinase in AF10 cells by way of an adaptor protein and Ras-dependent kinase cascade.
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PMID:Involvement of Janus kinases, p52shc, Raf-1, and MEK-1 in the IL-6-induced mitogen-activated protein kinase cascade of a growth-responsive B cell line. 796 20

The c-erbB-2 proto-oncogene encodes a receptor tyrosine kinase (RTK) closely related to the epidermal growth factor receptor (EGFR). Overexpression of erbB-2 occurs in approximately 20% of human breast tumours, where increased expression correlates with poor patient prognosis. The EGFR is coupled to the Ras signalling pathway by interaction with the adaptor protein Grb2, and Sos, a Ras GDP-GTP exchange factor. In this study, activation of the erbB-2 receptor and its association with Grb2 and Sos was investigated in breast cancer cell lines which overexpress erbB-2. The receptor was found to be tyrosine phosphorylated in all cell lines in which it is overexpressed. Western blotting of Grb2 and Sos immuneprecipitates from such cells revealed co-precipitation of erbB-2, demonstrating association of the Grb2/Sos complex with erbB-2 in vivo. Furthermore, a fusion protein containing only the SH2 domain of Grb2 bound to erbB-2 immobilized on nitrocellulose, indicating that association with Grb2 is direct and mediated by the SH2 domain of Grb2. The degree of association between the erbB-2 receptor and Grb2 in vivo was related to erbB-2 overexpression, and MAP kinase, which functions downstream from Ras, displayed markedly increased activity in cell lines overexpressing erbB-2. These results demonstrate that erbB-2 is coupled to Ras signalling via the Grb2/Sos complex, and that overexpression of this receptor in breast cancer cells leads to amplification of the Ras signalling pathway.
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PMID:Activation of the Ras signalling pathway in human breast cancer cells overexpressing erbB-2. 797 Jul 20


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