Analysis of deletions of the carboxyl terminus of the epidermal growth factor receptor reveals self-phosphorylation at tyrosine 992 and enhanced in vivo tyrosine phosphorylation of cell substrates

The calpain cleavage sites in the epidermal growth factor receptor kinase domain

The proteolysis of the human epidermal growth factor receptor cytoplasmic domain by calpain has been studied in vitro using purified recombinant cytoplasmic domain expressed in insect cells. Limited proteolysis produced kinase that was truncated at either N- or C-termini, as well as in the hinge region. We identified seven sites of calpain proteolysis by N-terminal sequencing of purified fragments. Calpain cleaved between the catalytic and autophosphorylation domains at two sites in the sequence Gln996-Asp1059, in the hinge region. Three new sites were also found in the autophosphorylation domain, preceding each of the major autophosphorylation sites. A fourth new site was located in the juxta-membrane domain, C-terminal to the regulatory Thr654. We purified an active 42-kDa fragment generated by calpain proteolysis between Leu659-Gln660 in the juxta-membrane domain, and in the hinge region. A fifth new site of calpain cleavage was found between the nucleotide binding motif Gly-Xaa-Gly-Xaa-Xaa-Gly and the essential Lys721 in the catalytic core of the kinase. Since both of these features are required for catalysis, calpain cleavage at this site may potentially provide a mechanism for down-regulation of kinase activity in vivo, under conditions of calpain activation. Thus the distribution of calpain cleavage sites along the kinase domain is consistent with a role for calpain both as a processing and as a degradative protease in epidermal growth factor receptor signalling.

SH2 domain proteins as high-affinity receptor tyrosine kinase substrates

Activation of a growth factor receptor tyrosine kinase (RTK) is accompanied by a rapid autophosphorylation of the receptor on tyrosine residues. Receptor activation has been shown to promote the association of signal-transducing proteins containing SH2 domains (second domain of src homology). These receptor-associated proteins can, in turn, be phosphorylated by the RTK, an event which presumably regulates their activities. It has been suggested that SH2 domains in signal-transducing proteins target these proteins as substrates of the activated RTK. To test this hypothesis, recombinant proteins were generated that contained tyrosine phosphorylation sites of the erbB3 receptor and/or the SH2 domain of c-src. Incorporation of the SH2 domain led to a decrease in KM and an increase in Vmax for the substrate. The KM determined for one chimeric SH2/erbB3 substrate was among the lowest reported for epidermal growth factor RTK substrates. Experiments with a truncated kinase lacking C-terminal autophosphorylation sites indicated that the reduction in KM for these substrates was mediated by interactions between the substrate SH2 domain and phosphotyrosine residues of the RTK. These interactions could also inhibit RTK activity. These results demonstrate that the SH2 domain can effectively target substrates to a RTK and that SH2 domain proteins can regulate RTK activity.

EGF receptor in neoplasia and metastasis

EGFR is a member of the tyrosine kinase family of cell surface receptors with a wide range of expression throughout development and in a variety of different cell types. The receptor can transmit signals to cells: i) upon interaction with ligands such as EGF, TGF alpha, amphiregulin or heparin binding EGF, ii) upon truncation or mutation of extracellular and/or intracellular domains, iii) upon amplification of a basal receptor activity (in the absence of ligand) through cooperation with other cellular signaling pathways or nuclear events (e.g. expression of v-erbA). The activated EGFR can exert pleiotropic functions on cells, depending on their tissue origin and state of differentiation. Under certain conditions it can also contribute to neoplasia and development of metastases. Such conditions can exist upon aberrant receptor/ligand expression and activation (e.g. in the wrong cell; at the wrong time; in the wrong amounts). Aberrant signalling can also occur through constitutive EGFR activation. Oncogenic potential of EGFR has been demonstrated in a wide range of experimental animals. EGFR is also implicated in human cancer, where it may contribute both to the initiation (glioblastoma) and progression (epithelial tumors) of the disease. EGFR may influence key steps in the processes of tumor invasion and dissemination. Involvement of EGFR in tumor spread may indicate a potential use of this receptor as a target for antimetastatic therapy.

Growth factors and wound healing: biochemical properties of growth factors and their receptors

Wound healing is a complex biologic process that involves chemotaxis and division of cells, neovascularization, synthesis of extracellular matrix proteins, and remodeling of scar. Peptide growth factors have been shown to regulate many of these processes in vitro, leading to the hypothesis that peptide growth factors also regulate important phases of wound healing in vivo. Part I of this two-part series presents an overview of the biochemical properties of five families of peptide growth factors that are thought to be involved in wound healing: epidermal growth factor (EGF), transforming growth factor-beta (TGF-beta), platelet-derived growth factor (PDGF), insulin-like growth factor (IGF), and fibroblast growth factor (FGF).

Antibodies against highly conserved sites in the epidermal growth factor receptor tyrosine kinase domain as probes for structure and function

We generated anti-peptide antibodies against four highly conserved sequences in the kinase domain and against two nonconserved sequences surrounding autophosphorylation sites in the carboxyl-terminal domain of the epidermal growth factor receptor (EGFR). These antibodies were used to examine topology and function in catalysis of specific sequences. Two of the highly conserved sites, HRD (residues 811-818) and DFG (residues 827-838), appeared to participate in catalysis since alpha HRD and alpha DFG but not the other anti-peptide antibodies inhibited EGFR kinase activity. Examination of the topology of the six sites revealed that epitopes in all except the HRD site appeared to be exposed to antibody binding in the EGFR. The conditions that caused increased exposure of the HRD site to interaction with antibody included autophosphorylation, addition of the ionic detergent sodium dodecyl sulfate (SDS), and elevation in temperature from 4 to 34 degrees C.

Detergent solubilization is a prerequisite for aggregation-induced stimulation of epidermal growth factor (EGF) receptor kinase

Unlike EGF, concanavalin A and wheat germ agglutinin do not increase EGF receptor-kinase activity in intact A-431 membranes. However, they increase both autophosphorylation and phosphorylation of exogenous substrates about twice as much as EGF if the membranes are solubilized in detergent. Following solubilization, autophosphorylation due to the combined presence of a lectin and EGF is additive suggesting that each increases kinase activity by a different mechanism. These different mechanisms were studied by autophosphorylating membranes at increasing detergent concentrations after they had been permeabilized to [gamma-32P]ATP with alamethicin. As the detergent concentration increased, EGF stimulated autophosphorylation decreased 3-fold and 6-fold for the native 170 kDa receptor and for a protease-generated 150 kDa receptor form, respectively. However, in the presence of either lectin the same increase in detergent concentration only slightly altered the autophosphorylation rates which never exceeded the rate measured in the absence of EGF and detergent. Hence, the lectins increase kinase activity in solubilized membranes by preventing the adverse effects of detergent on the receptor-kinase and may not be useful models for how EGF activates its receptor.

Nucleotide sequence of the C-terminal region of the mouse epidermal growth factor receptor and expression in teratoma-derived cell lines with increased tumorigenic properties

The isolation of a cDNA corresponding to a portion (amino acid 943 to 1073) of the cytoplasmic domain of the mouse EGF receptor surrounding the auto phosphorylation sites was obtained by using the reverse transcriptase polymerase chain reaction (RT-PCR) approach. Deduced amino acid sequence of mouse EGF receptor (EGFr) shows a 92% and 76% homology to corresponding regions in the human and the chicken EGFr, respectively. This cDNA was used to develop a sensitive RNase protection assay to investigate EGF receptor mRNA expression in mouse C3H teratoma derived cell lines with increased tumorigenic properties which display a progressive decrease of EGF binding and response. The results show that increased tumorigenicity was not accompanied by a change in EGF receptor mRNA expression. Moreover, they indicate that the RNase protection assay developed using the probe described here is a sensitive approach to investigate EGF receptor expression in murine cells.

The type 1 (EGFR-related) family of growth factor receptors and their ligands

This review considers the biology of the type 1 growth factor receptor family which is increasingly recognised as important in the control of normal cell proliferation and in the pathogenesis of human cancer. The family currently comprises three closely related members: the epidermal growth factor (EGF) receptor, c-erbB-2 and c-erbB-3, all of which show abnormalities of expression in various human tumours. The family of factors related to EGF has also expanded recently and now includes transforming growth factor alpha, heparin-binding EGF, amphiregulin, cripto and heregulin, as well as several other potential ligands for the c-erbB2-2 receptor. The involvement of these receptors and growth factors in human cancer has implications for the design of novel forms of therapy for cancer, and we review recent advances and future avenues for investigation.

A highly conserved tyrosine residue at codon 845 within the kinase domain is not required for the transforming activity of human epidermal growth factor receptor

Epidermal growth factor receptor (EGF-R) is a widely expressed ligand-dependent tyrosine kinase. The tyrosine residue at 845 in EGF-R corresponds to Y416 of v/c-src kinase, which is highly conserved and functionally important in many tyrosine kinases. To clarify the functional role of Y845, we constructed a mutant human EGF-R in which this tyrosine was replaced with phenylalanine and transfected it to NIH3T3 cells. EGF-R F845 induced EGF-dependent cellular transformation and revealed tyrosine-autophosphorylation of a 170 kDa protein, and initiated DNA synthesis similar to the wild-type EGF-R. We conclude here that Y845 is dispensable in the above mentioned functions of EGF-R tyrosine kinase.

Epidermal growth factor receptor: elements of intracellular communication

While EGF has an important function in cell growth regulation, the molecular mechanisms by which intracellular signal connect the EGF: receptor complex on the plasma membrane with the initiation of DNA synthesis and mitogenesis is not well understood. The discovery that rasGAP, PI-3 kinase and PLC-gamma 1 are substrates for the EGF receptor tyrosine kinase has provided a beginning in understanding the biochemistry underlying growth factor receptor transduction.

Potentiation of epidermal growth factor receptor protein-tyrosine kinase activity by sulfate

The protein-tyrosine kinase activity of the epidermal growth factor (EGF) receptor is critical for EGF-stimulated cell growth, although little is known about the molecular details of its enzymatic activity. Previous studies have found that EGF receptor kinase activity can be stimulated by factors such as ammonium sulfate ((NH4)2SO4), but the manner in which (NH4)2SO4 induces this effect is unclear. Therefore, we have explored the processes by which (NH4)2SO4 potentiated tyrosine kinase activity to better understand not only the molecular events involved in (NH4)2SO4 activation, but also the kinetic properties and mechanism of the EGF receptor. In this study, the addition of an optimum concentration of (NH4)2SO4 (250 mM) resulted in a 5-fold stimulation of kinase activity toward the peptide substrate, angiotensin II. The sulfate group is primarily involved in this action, since other salts containing SO4(2-) increased kinase activity similarly, whereas salts containing Cl- and F- had less of an effect, and divalent salts such as HPO4(2-) and NaVO4(2-) were inhibitory at doses of 1 mM or more. In addition, EGF receptor kinase activation by (NH4)2SO4 did not strictly correlate with changes in the ionic strength or conductivity of the solution. However, several lines of evidence suggest that SO4(2-) directly alters the kinetic properties of the EGF receptor kinase: (1) the maximum velocity (Vmax) and Km (ATP) for EGF receptor phosphorylation of angiotensin II were substantially higher in the presence of (NH4)2SO4. (2) EGF receptor kinase activity in the absence of (NH4)2SO4 required either Mn2+ or Mg2+, yet in the presence of (NH4)2SO4, only Mn2+ supported the increase in kinase activity. (3) Ammonium sulfate addition altered the product inhibition pattern of ADP versus angiotensin II, suggesting that an enzyme-angiotensin II-ADP complex can form in the presence of (NH4)2SO4 but not in its absence. (4) The near-maximal rate of self-phosphorylation was not affected by (NH4)2SO4 but the apparent Km (ATP) was greatly increased. From these results, we propose a model for (NH4)2SO4 stimulation of EGF receptor kinase activity in which SO4(2-) interacts directly with the receptor or receptor-Mn(2+)-ATP complex and alters reactant binding and the catalytic efficiency of the tyrosine kinase.

Effect of carboxyl terminal truncation on the tyrosine kinase activity of the epidermal growth factor receptor

The carboxyl terminal domain of the epidermal growth factor receptor (EGFR) is an important regulatory region in mediating the tyrosine kinase-dependent biological effects of EGF. The effect of a 164-amino-acid carboxyl deletion of the EGFR or the EGFR cytoplasmic kinase domain on in vitro tyrosine kinase activity was assessed. C'-terminal truncation of the EGFR resulted in dependence on Mn2+ for full activity. The EGFR kinase domain (kd EGFR) and the C'-terminally truncated kinase domain (kd c'1022 EGFR) also exhibited a strong preference for Mn2+ compared to Mg2+, with kd c'1022 EGFR being completely inactive in the presence of Mg2+ alone. Sphingosine or ammonium sulfate specifically activated both kd EGFR and kd c'1022 EGFR. EGFR and c'1022 EGFR displayed similar EGF-stimulated in vitro tyrosine kinase activities; however, kd EGFR was 5- to 10-fold more active in vitro than kd c'1022 EGFR. Thus, the regulatory contribution of the C'-terminus is most evident when the EGFR ligand binding domain is removed. These results indicate that an intact EGFR C'-terminus is necessary for the protein to assume a fully active conformation.