Epidermal growth factor receptor cytoplasmic domain mutations trigger ligand-independent transformation

Extracellular Juxtamembrane Motif Critical for TrkB Preformed Dimer and Activation

Receptor tyrosine kinases are believed to be activated through ligand-induced dimerization. We now demonstrate that in cultured neurons, a substantial amount of endogenous TrkB, the receptor for brain-derived neurotrophic factor (BDNF), exists as an inactive preformed dimer, and the application of BDNF activates the pre-existing dimer. Deletion of the extracellular juxtamembrane motif (EJM) of TrkB increased the amount of preformed dimer, suggesting an inhibitory role of EJM on dimer formation. Further, binding of an agonistic antibody (MM12) specific to human TrkB-EJM activated the full-length TrkB and unexpectedly also truncated TrkB lacking ECD (TrkBdelECD365), suggesting that TrkB is activated by attenuating the inhibitory effect of EJM through MM12 binding-induced conformational changes. Finally, in cells co-expressing rat and human TrkB, MM12 could only activate TrkB human-human dimer but not TrkB human-rat TrkB dimer, indicating that MM12 binding to two TrkB monomers is required for activation. Our results support a model that TrkB preforms as an inactive dimer and BDNF induces TrkB conformation changes leading to its activation.

A putative mechanism for downregulation of the catalytic activity of the EGF receptor via direct contact between its kinase and C-terminal domains

Tyrosine kinase receptors of the EGFR family play a significant role in vital cellular processes and in various cancers. EGFR members are unique among kinases, as the regulatory elements of their kinase domains are constitutively ready for catalysis. Nevertheless, the receptors are not constantly active. This apparent paradox has prompted us to seek mechanisms of regulation in EGFR's cytoplasmic domain that do not involve conformational changes of the kinase domain. Our computational analyses, based on the three-dimensional structure of EGFR's kinase domain suggest that direct contact between the kinase and a segment from the C-terminal regulatory domains inhibits enzymatic activity. EGFR activation would then involve temporal dissociation of this stable complex, for example, via ligand-induced contact formation between the extracellular domains, leading to the reorientation of the transmembrane and intracellular domains. The model provides an explanation at the molecular level for the effects of several cancer-causing EGFR mutations.

Epidermal growth factor receptor glycosylation is required for ganglioside GM3 binding and GM3-mediated suppression [correction of suppresion] of activation

Gangliosides are able to bind to the epidermal growth factor receptor and inhibit its activation, but the mechanism of this inhibition is unknown. To address the role of receptor carbohydrates in facilitating interaction with gangliosides, we examined the ability of GM3 to bind the deglycosylated receptor and inhibit its autophosphorylation. Flow cytometry studies demonstrated that deglycosylation of the receptor did not affect its ability to be transported to the cell membrane. In contrast with the native (fully glycosylated) receptor, GM3 did not coimmunoprecipitate with the deglycosylated receptor. Using a novel colorimetric bead binding assay, GM3 was shown to bind well to the immunoprecipitated native receptor but not at all to the deglycosylated receptor. Finally, the addition of GM3 to cells with deglycosylated epidermal growth factor receptors did not result in significant further inhibition of autophosphorylation of the receptor, despite a 10-fold decrease in phosphorylation of the native epidermal growth factor receptor by 200 microM GM3. These studies suggest that ganglioside affects epidermal growth factor receptor activity through a direct interaction that requires receptor glycosylation, and contribute to our understanding of the role of gangliosides in cell membrane function.

Ca2+/calmodulin-dependent kinase II phosphorylates the epidermal growth factor receptor on multiple sites in the cytoplasmic tail and serine 744 within the kinase domain to regulate signal generation

Down-regulation of receptor tyrosine kinase activity plays an essential role in coordinating and controlling cellular growth/differentiation. Ca2+/calmodulin-dependent kinase II (CaM kinase II)-mediated phosphorylation of threonine 1172 in the cytoplasmic tail of HER2/c-erbB2 can modulate tyrosine kinase activity and consensus phosphorylation sites are also found at serines 1046/1047 in the structurally related epidermal growth factor receptor (EGFR). We show that serines 1046/1047 are sites for CaM kinase II phosphorylation, although there is a preference for serine 1047, which resides within the consensus -R-X-X-S-. In addition, we have identified major phosphorylation sites at serine 1142 and serine 1057, which lie within a novel -S-X-D- consensus. Mutation of serines 1046/1047 in full-length EGFR enhanced both fibroblast transformation and tyrosine autokinase activity that was significantly potentiated by additional mutation of serines 1057 and 1142. A single CaM kinase II site was also identified at serine 744 within sub-kinase domain III, and autokinase activity was significantly affected by mutation of this serine to an aspartic acid making this site appear constitutively phosphorylated. We have addressed the mechanism by which CaM kinase II phosphorylation of the EGFR might regulate receptor autokinase activity and show that this modification can hinder association of the cytoplasmic tail with the kinase domain to prevent an enzyme-substrate interaction. We postulate that the location and greater number of CaM kinase II phosphorylation sites in the EGFR compared with HER-2/c-erbB2, leading to differential regulation of autokinase activity, contributes to differences in the strength of downstream signaling events and may explain the higher relative transforming potential of HER-2/cerbB2.

Mitosis-specific negative regulation of epidermal growth factor receptor, triggered by a decrease in ligand binding and dimerization, can be overcome by overexpression of receptor

The function of epidermal growth factor receptor (EGFR) was found to be negatively regulated in M phase in which it showed less phosphotyrosine content and reduced intrinsic kinase activity accompanied by retarded electrophoretic mobility owing to total hyperphosphorylation. Ligand-induced autophosphorylation and downstream signaling of EGFR were tightly suppressed in M phase due to a decrease in ligand binding affinity and the inability of epidermal growth factor (EGF) to induce receptor dimerization. There was no change in the number of surface-exposed EGF receptors between G0/G1 and M phases of the cell cycle. Hyperphosphorylation (due to serine and/or threonine phosphorylation) correlates with the unresponsiveness of cells to EGF-mediated stimulation of tyrosine phosphorylation in cells that express the normal or basal level of EGFR. This M phase-specific negative regulation was overcome by overexpression of EGFR, which was responsive to ligand throughout the cell cycle and revealed ligand-induced signaling in the M phase. These findings indicate that EGFR does not respond to ligand stimulation in M phase and suggest that a negative regulation of ligand-receptor interactions in M phase may control the normal function of receptor tyrosine kinase and that receptor overexpression will disrupt this cell cycle-dependent regulation of receptor tyrosine kinases.

Dephosphorylation of receptor tyrosine kinases as target of regulation by radiation, oxidants or alkylating agents

Several non-physiologic agents such as radiation, oxidants and alkylating agents induce ligand-independent activation of numerous receptor tyrosine kinases (RTKs) and of protein tyrosine kinases at the inner side of the plasma membrane (e.g. Dévary et al., 1992; Sachsenmaier et al., 1994; Schieven et al., 1994; Coffer et al., 1995). Here we show additional evidence for the activation of epidermal growth factor receptor (EGFR), and we show activation of v-ErbB, ErbB2 and platelet-derived growth factor receptor. As a common principle of action the inducing agents such as UVC, UVB, UVA, hydrogen peroxide and iodoacetamide inhibit receptor tyrosine dephosphorylation in a thiol-sensitive and, with the exception of the SH-alkylating agent, reversible manner. EGFR dephosphorylation can also be modulated by these non-physiologic agents in isolated plasma membranes in the presence of Triton X-100. Further, substrate (EGFR) and phosphatase have been separated: a membrane preparation of cells that have been treated with epidermal growth factor (EGF) and whose dephosphorylating enzymes have been permanently destroyed by iodoacetamide can be mixed with a membrane preparation from untreated cells which re-establishes EGFR dephosphorylation. This dephosphorylation can be modulated in vitro by UV and thiol agents. We conclude that RTKs exhibit significant spontaneous protein kinase activity; several adverse agents target (an) essential SH-group(s) carried by (a) membrane-bound protein tyrosine phosphatase(s).

Proliferation and motility responses of primary and recurrent gliomas related to changes in epidermal growth factor receptor expression

Astrocytic neoplasms show a high incidence of elevated or mutated epidermal growth factor receptor (EGFR) expression. Although proliferative effects from EGFR activation are well described, the role that changes in this receptor play in glioma growth and migration remain poorly addressed. This report characterizes changes in the levels of EGFR expression in three glial tumors at initial presentation (resection) and at the time of recurrence. By quantitative flow cytometry the mean level of EGFR expression increased, decreased, or remained the same in different recurrent astrocytomas relative to their primary tumor cells. Immunocytochemistry for EGFR on monolayer cells corroborated the level of expression in the recurrent tumors relative to their matched primary specimen. Immunoprecipitation indicated that 170 kd EGFR was expressed in each of the tumors, and showed normal down regulation following treatment with EGF. Proliferation response to EGF was seen in only 1/6 instances, but was concentration-dependent when observed. Stimulated migration of the cells was frequently seen and was also concentration-dependent on EGF; the magnitude of response was related to the relative level of 170 kd EGFR expression in the cells. EGFR immunostaining of tissue sections from the tumors confirmed the levels of EGFR expressed in primary and recurrent astrocytomas as was seen in the cultured cells. These results indicate that the relative levels of EGFR in early passage cell cultures from glioma specimens concurs with the measured tissue levels of expression. Human glioma cells are more responsive to migration induction than proliferation induction by EGF.

Oncogenes, Protein Tyrosine Kinases, and Signal Transduction

Many oncogenes encode protein tyrosine kinases (PTKs). Oncogenic mutations of these genes invariably result in constitutive activation of these PTKs. Autophosphorylation of the PTKs and tyrosine phosphorylation of their cellular substrates are essential events for transmission of the mitogenic signal into cells. The recent discovery of the characteristic amino acid sequences, of the src homology domains 2 and 3 (SH2 and SH3), and extensive studies on proteins containing the SH2 and SH3 domains have revealed that protein tyrosine-phosphorylation of PTKs provides phosphotyrosine sites for SH2 binding and allows extracellular signals to be relayed into the nucleus through a chain of protein-protein interactions mediated by the SH2 and SH3 domains. Studies on oncogenes, PTKs and SH2/SH3-containing proteins have made a tremendous contribution to our understanding of the mechanisms for the control of cell growth, oncogenesis, and signal transduction. This review is intended to provide an outline of the most recent progress in the study of signal transduction by PTKs. Copyright 1994 S. Karger AG, Basel

Oncogenic activation of tyrosine kinases

Tyrosine kinases comprise the largest group of oncoproteins, a fact that underscores the importance of reversible tyrosine phosphorylation in the regulation of essential cellular functions. Oncogenic activation of tyrosine kinases results in the constitutive activation of what is normally a conditionally regulated enzyme activity. Studies of tyrosine kinase oncoproteins, and a comparison with their corresponding proto-oncogene products, have identified important functional and regulatory domains within these proteins, positive and negative regulators of their enzyme activities and signalling cascades that control cell growth and differentiation.

Growth factor second messenger systems: oncogenes and the heterotrimeric GTP-binding protein connection

We feel that there is now compelling evidence that the GTP-binding proteins play more than just a coordinating role in the actions of both tyrosine kinase and nontyrosine kinase receptor signal transduction. These similarities appear to represent just a small component of the convergence in the signaling pathways for structurally dissimilar receptor subsets. Future years will see further understanding of the intricacies of these G-protein-proto-oncogene interactions, and the extension into the potential role in growth factor action played by the expanding number of known members of this G-protein family.

Epidermal growth factor receptors in the oesophagus

The quantity and distribution of epidermal growth factor receptors (EGF-R) in oesophageal mucosa was studied in the oesophagus in order to determine its role in oesophageal disease. Fifty five biopsies were taken from different levels of the oesophagus in 25 consecutive patients undergoing endoscopy. Another group of eight patients with histologically proven Barrett's oesophagitis had a biopsy taken from the area of columnar lined oesophagus. A peripheral, membranous pattern was seen predominantly confined to the basal and immediately suprabasal cells in all of the first group of patients. In the superficial cells a few granular cytoplasmic structures were positive. All patients with Barrett's oesophagitis showed EGF-R staining of the surface epithelium. A computerised planimeter was used to determine the proportion of stained areas of squamous cells which were expressed as a percentage of the total area of squamous cells. The difference in the area of cells stained for EGF-R between normal and inflamed oesophageal mucosa (29.5% and 43.1% respectively) was significant (p less than 0.001).