Epidermal growth factor receptor tyrosine kinase mediates Ras activation by gonadotropin-releasing hormone

Gonadotropin releasing hormone (GnRH) contributes to the maintenance of gonadotrope function by increasing extracellular signal-regulated kinase (ERK) activity subsequent to binding to its cognate G-protein-coupled receptor. As the GnRH receptor exclusively interacts with G(q/11) proteins and as receptor expression is regulated in a beta-arrestin-independent fashion, it represents a good model to systematically dissect underlying signaling pathways. In alphaT3-1 gonadotropes endogenously expressing the GnRH receptor, GnRH challenge resulted in a rapid increase in ERK activity which was attenuated by the epidermal growth factor receptor (EGFR)-specific tyrosine kinase inhibitor AG1478. In COS-7 cells transiently expressing the human GnRH receptor, agonist-induced ERK activation was independent of free Gbetagamma subunits but could be mimicked by short-term phorbol ester treatment. Most notably, G(q/11)-induced ERK activation was sensitive to N17-Ras and to expression of the C-terminal Src kinase but also to other dominant negative mutants of signaling components localized upstream of Ras, like Shc and the EGFR. GnRH as well as phorbol esters led to Ras activation in COS-7 and alphaT3-1 cells, which was dependent on Src and EGFR tyrosine kinases, indicating that both tyrosine kinases act downstream of protein kinase C (PKC) and upstream of Ras. However, Src did not contribute to Shc tyrosine phosphorylation. GnRH or phorbol ester challenge resulted in PKC-dependent EGFR autophosphorylation. Furthermore, a 5-min phorbol ester treatment was sufficient to trigger tyrosine phosphorylation of the platelet-derived growth factor-beta receptor in L cells. Thus, in several cell systems PKC is able to stimulate Ras via activation of receptor tyrosine kinases.     

Epidermal growth factor receptor: mechanisms of activation and signalling

The epidermal growth factor (EGF) receptor (EGFR) is one of four homologous transmembrane proteins that mediate the actions of a family of growth factors including EGF, transforming growth factor-alpha, and the neuregulins. We review the structure and function of the EGFR, from ligand binding to the initiation of intracellular signalling pathways that lead to changes in the biochemical state of the cell. The recent crystal structures of different domains from several members of the EGFR family have challenged our concepts of these processes.     

Epidermal growth factor receptor threonine and serine residues phosphorylated in vivo

The epidermal growth factor (EGF) receptor is regulated by EGF-stimulated autophosphorylation and by phorbol ester-stimulated, protein kinase C (Ca2+/phospholipid-dependent enzyme) mediated phosphorylation at identified sites. The EGF receptor contains additional phosphorylation sites including a prominent phosphothreonine and several phosphoserines which account for the majority of phosphate covalently bound to the receptor in vivo. We have identified three of these sites in EGF receptor purified from 32P-labeled A431 cells. The major phosphothreonine was identified as threonine 669 in the EGF receptor sequence. Phosphoserine residues were identified as serines 671 and 1046/1047 of the EGF receptor. Two other phosphoserine residues were localized to tryptic peptides containing multiple serine residues located carboxyl-terminal to the conserved protein kinase domain. The amino acid sequences surrounding the three identified phosphorylation sites are highly conserved in the EGF receptor and the protein products of the v-erb B and neu oncogenes. Analysis of predicted secondary structure of the EGF receptor reveals that all of the phosphorylation sites are located near beta turns. In A431 cells phosphorylation of the serine residues was dependent upon serum. In mouse B82 L cells transfected with a wild type human EGF receptor. EGF increased the 32P content in all tryptic phosphopeptides. A mutant EGF receptor lacking protein tyrosine kinase activity was phosphorylated only at threonine 669. Regulated phosphorylation of the EGF receptor at these threonine and serine residues may influence aspects of receptor function.     

Epidermal growth factor receptor signal transactivation

Cross-communication between heterologous signaling systems and the epidermal growth factor receptor (EGFR) has been shown to be critical for a variety of biological responses: EGFR transactivation when G-protein-coupled receptors (GPCRs) are stimulated represents the paradigm of an interreceptor network that is dependent on G-proteins, kinases, metalloproteases, and growth factor precursors. Investigating the mechanism of this process will help expand our knowledge of physiological regulatory mechanisms and diverse pathophysiological disorders.     

Ratiometric assay of epidermal growth factor receptor tyrosine kinase activation

Activation of cells is frequently followed by tyrosine phosphorylation of proteins. To quantify this process, we developed a ratiometric enzyme-linked immunosorbent assay (ELISA) using epidermal growth factor receptors (EGFR) as a model. Microtiter dishes were coated with anti-EGFR monoclonal antibodies to capture the receptor followed by parallel detection of receptor and phosphotyrosine content with secondary antibodies. The ratio of these two parameters was found to directly reflect EGFR activation and was insensitive to the effect of receptor downregulation. Our assay could resolve differences in EGFR activation due to small changes (less than 1 ng/ml) in ligand. We found that phosphotyrosine detection by ELISA was 8- to 32-fold more sensitive than Western blot detection and could be reliably detected using as little as 4 ng of cellular lysate. Detection of EGFR levels by ELISA was 30 times more sensitive than Western blot analysis and was reliable for as low as 8 ng of cellular lysate per well. Because of the wide linear range of the ELISA, we could directly compare receptor activation in cell types with different EGFR expression levels. Our assay provides a rapid and sensitive method of determining EGFR activation status and could be easily modified to evaluate any tyrosine-phosphorylated protein.     

Epidermal growth factor receptor tyrosine kinase phosphorylation of glucose-6-phosphate dehydrogenase in vitro

The specific tyrosine phosphorylation of glucose-6-phosphate dehydrogenase (G6PDH) by the epidermal growth factor (EGF) receptor in vitro is demonstrated. The Km values of the substrate G6PDH and of ATP for the receptor tyrosine kinase were ca. 1 and 10 microM, respectively. The rate of phosphorylation was EGF dependent, with a four-fold increase in Vmax in the presence of EGF. The phosphorylation was stimulated maximally by 0.2 microM or greater EGF, with an ED50 of ca. 20 nM which is consistent with the affinity of the solubilized receptor for EGF. Using conditions of 5 microM G6PDH, 100 microM ATP, 5 mM Mg2+, and 1 mM Mn2+, up to 0.3 mol phosphate was incorporated into 1 mol of the 55-kDa subunit of Baker's yeast G6PDH. Tryptic peptide mapping revealed several unique phosphopeptides for both Baker's yeast and bovine adrenal G6PDH. The patterns of phosphopeptides for a given enzyme were identical for basal and EGF-stimulated phosphorylation.     

Epidermal growth factor receptor dimerization and activation require ligand-induced conformational changes in the dimer interface

Structural studies have shown that ligand-induced epidermal growth factor receptor (EGFR) dimerization involves major domain rearrangements that expose a critical dimerization arm. However, simply exposing this arm is not sufficient for receptor dimerization, suggesting that additional ligand-induced dimer contacts are required. To map these contributions to the dimer interface, we individually mutated each contact suggested by crystallographic studies and analyzed the effects on receptor dimerization, activation, and ligand binding. We find that domain II contributes >90% of the driving energy for dimerization of the extracellular region, with domain IV adding little. Within domain II, the dimerization arm forms much of the dimer interface, as expected. However, a loop from the sixth disulfide-bonded module (immediately C-terminal to the dimerization arm) also makes a critical contribution. Specific ligand-induced conformational changes in domain II are required for this loop to contribute to receptor dimerization, and we identify a set of ligand-induced intramolecular interactions that appear to be important in driving these changes, effectively "buttressing" the dimer interface. Our data also suggest that similar conformational changes may determine the specificity of ErbB receptor homo- versus heterodimerization.     

Epidermal growth factor receptor expression in human gliomas

Summary The expression of epidermal growth factor receptor (EGFR) was determined in cryosections of 42 human gliomas using biotinylated epidermal growth factor (B-EGF) and two monoclonal antibodies (mAb) against EGFR. All gliomas were found to express EGFR when examined with B-EGF, whereas 33 expressed EGFR when examined with the two mAbs. The highly malignant gliomas (glioblastomas and anaplastic astrocytomas) had a more heterogeneous staining pattern and a larger proportion of tumour cells staining strongly with B-EGF than did the low-grade gliomas (astrocytomas, oligodendrogliomas, mixed gliomas, and ependymomas). This indicates that high-grade gliomas contain more tumour cells rich in EGFR than do the low-grade gliomas. Reactive astrocytes, ependymal cells, and many types of nerve cells (cerebral cortical pyramidal cells, pyramidal and granular hippocampal cells, Purkinje cells, cerebellar granular cells and neurons in the molecular layer of the cerebellum) expressed EGFR, whereas small neurons and normal glial cells were not found to express EGFR.     

表皮生长因子受体及其变异性质

EGFR主要由三个结构区域组成,即EGF结合区域、跨膜区域和依赖EGF蛋白激酶区域。EGFR被激活后,不仅能使许多底物蛋白磷酸化,自身也发生磷酸化,这些磷酸化作用在信息传导中有重要作用。EGFR与EGF结合后,其复合体以胞饮方式内容形成受体小体,除部分被降解外,大部分在高尔基GERL区加工后到达细胞核调节基因表达。本文通过对受体激活模型和用体外遗传变异方法对变异EGFR的研究阐述了EGFR的结构与功能的关系。     

Epidermal growth factor stimulates phosphatidylinositol turnover in human foreskin fibroblasts without activation of protein kinase C

Epidermal growth factor stimulates phosphatidylinositol turnover in human foreskin fibroblasts. This is a primary cell culture with normal numbers of epidermal growth factor receptors that is stimulated to divide by epidermal growth factor. Increases are seen in the inositol phospholipids and inositol phosphates. Despite this activation of phosphatidylinositol turnover, there is no detectable activation of protein kinase C.     

Epidermal growth factor receptor activation under oxidative stress fails to promote c-Cbl mediated down-regulation

Activation of the epidermal growth factor (EGF) receptor by its ligand, EGF, rapidly enhances receptor internalization and degradation, which desensitizes receptor signaling. In contrast, we have shown previously that exposure to oxidative stress in the form of hydrogen peroxide (H(2)O(2)) activated the EGF receptor but that the levels of activated receptors did not decline, which resulted in prolonged receptor signaling. This study provides mechanistic insights into these different modes of EGF receptor activation. Here we demonstrate that the pattern of receptor tyrosine phosphorylation induced by H(2)O(2) differs from that induced by its ligand, EGF. Importantly, H(2)O(2) generates a receptor with negligible phosphorylation at tyrosine 1045, the major docking site for the ubiquitin ligase c-Cbl. As a result, H(2)O(2)-activated receptors fail to recruit c-Cbl and do not undergo ubiquitination and endocytosis. In summary, H(2)O(2) stimulation results in an activated receptor uncoupled from normal down-regulation, a process that may contribute to oxidant-mediated tumorigenesis.     

Epidermal growth factor receptor down-regulation induced by UVA in human keratinocytes does not require the receptor kinase activity

Activation of the epidermal growth factor (EGF) receptor by EGF, its ligand, results in receptor internalization and down-regulation, which requires receptor kinase activity, phosphorylation, and ubiquitination. In contrast, we have found here in human HaCaT keratinocytes that exposure to UVA induces EGF receptor internalization and down-regulation without receptor phosphorylation and ubiquitination. The presence of the receptor kinase activity inhibitor AG1478 increased UVA-induced receptor down-regulation, whereas it inhibited EGF-induced receptor down-regulation. These observations demonstrate that, in contrast to EGF, receptor kinase activity is not required for receptor down-regulation by UVA. Concurrent with receptor down-regulation, caspases were activated by UVA exposure. The presence of caspase inhibitors blocked receptor down-regulation in a pattern similar to poly(ADP)-ribose polymerase cleavage. Much more receptor down-regulation was observed after UVA exposure in apoptotic detached cells in which caspase is activated completely. These results indicate that UVA-induced receptor down-regulation is dependent on caspase activation. Similar to UVA, both UVB and UVC induced receptor down-regulation, in which receptor kinase activity is not required, whereas caspase activation is involved. Inhibition of EGF receptor down-regulation increased receptor activation and activation of its downstream survival signaling ERK and AKT after UVA exposure. Preventing the activation of each of these pathways enhanced apoptosis induced by UVA. These findings suggest that EGF receptor down-regulation by UVA may play an important role in the execution of the cell suicide program by attenuating its anti-apoptotic function and thereby preventing cell transformation and tumorigenesis in vivo.     

Epidermal growth factor and transforming growth factor alpha bind differently to the epidermal growth factor receptor

Epidermal growth factor (EGF) and transforming growth factor alpha (TGF alpha) compete with each other for binding to the EGF receptor. These two growth factors have similar actions, but there are distinguishable differences in their biological activities. It has never been clear how this one receptor can mediate different responses. A monoclonal antibody to the EGF receptor (13A9) has been identified which has only small effects on the binding of EGF to the EGF receptor, but which has very large effects on the binding of TGF alpha to the EGF receptor; 5 micrograms/mL antibody has been shown to totally block 0.87 microM TGF alpha from binding to purified EGF receptor and to lower both the high- and low-affinity binding constants of TGF alpha binding to EGF receptor on A431 cells by about 10-fold. The 13A9 antibody causes a 2.5-fold stimulation of the tyrosine kinase activity of partially purified EGF receptor, compared to a 4.0-fold stimulation of the tyrosine kinase activity by EGF under the same conditions. The data suggest either that the antibody stabilizes a conformation of the EGF receptor which is not favorable for TGF alpha binding or that it blocks a part of the surface of the receptor which is necessary for TGF alpha binding but not EGF binding.     

Epidermal growth factor receptor function in early mammalian development

We review here the data indicating a role for epidermal growth factor receptor (EGF receptor) signalling in early mouse development. Embryonic development of the metazoan embryo generally begins with the formation of a cystic structure and epithelial layers that subsequently form anlagen of the definitive body parts and organs. For the mammalian embryo, this cystic structure is a blastocyst whose wall consists of trophectoderm, the first epithelium to develop during mammalian embryogenesis. The onset of expression and function of EGF receptors is coincident with the onset of trophectoderm development. Modulating EGF receptor expression and function modulates trophectoderm differentiation, leading to the hypothesis that functional EGF receptors participate in the induction of trophectoderm development and perhaps of other embryonic epithelial derivatives such as nervous tissues.     

Epidermal growth factor receptor activation of calpain is required for fibroblast motility and occurs via an ERK/MAP kinase signaling pathway

To become migratory, cells must reorganize their connections to the substratum, and during locomotion they must break rear attachments. The molecular and biochemical mechanisms underlying these biophysical processes are unknown. Recent studies have implicated both extracellular signal-regulated kinase/mitogen-activated protein (ERK/MAP) kinase and calpain (EC 3.4.22.17) in these processes, but it is uncertain whether these are two distinct pathways acting on different modes of motility. We report that cell deadhesion involved in epidermal growth factor (EGF) receptor-mediated fibroblast motility requires activation of M-calpain downstream of ERK/MAP kinase signaling. NR6 fibroblasts expressing full-length wild type epidermal growth factor receptor required both calpain and ERK activation, as demonstrated by pharmacological inhibitors (calpeptin and calpain inhibitor I and PD98059, respectively) for EGF-induced deadhesion and motility. EGF induced rapid activation of calpain that was preventable by molecular inhibition of the Ras-Raf-MEK but not phospholipase Cgamma signaling pathway, and calpain was stimulated by transfection of constitutively active MEK. Enhanced calpain activity was not mirrored by increased calpain protein levels or decreased levels of its endogenous inhibitor calpastatin. The link between ERK/MAP kinase signaling and cell motility required the M-isoform of calpain (calpain II), as determined by specific antisense-mediated down-regulation. These data promote a previously undescribed signaling pathway of ERK/MAP kinases activating calpain to destabilize cell-substratum adhesions in response to EGF stimulation.