Aggregation-induced activation of the epidermal growth factor receptor protein tyrosine kinase

In vitro activation and inhibition of recombinant EGFR tyrosine kinase expressed in Escherichia coli

The present work concerns the heterologous expression of the intracellular domain harbouring the tyrosine kinase activity of the epidermal growth factor receptor (EGFR). Protein expression was improved thanks to the deletion of a 13-amino acid peptide of the juxtamembrane region (JM). The recombinant proteins were produced as a glutathione S-transferase (GST) fusion in Escherichia coli, and the solubilisation was performed by sarkosyl addition during extraction. The produced proteins spontaneously dimerize allowing the activation of the tyrosine kinase domain in the presence of [γ-³²P]ATP. The activity assay has revealed the autophosphorylation of EGFR proteins which was decreased in the presence of genistein. Our system could facilitate the screening of EGFR inhibitors without the need of adding an exogenous substrate.

A highly efficient peptide substrate for EGFR activates the kinase by inducing aggregation

Formation of an asymmetric dimer by the EGFR (epidermal growth factor receptor) kinase domains results in allosteric activation. Since this dimer does not readily form in solution, the EGFR kinase domain phosphorylates most peptide substrates with a relatively low catalytic efficiency. Peptide C is a synthetic peptide substrate of EGFR developed by others that is phosphorylated with a significantly higher catalytic efficiency, and we sought to understand the basis for this. Peptide C was found to increase EGFR kinase activity by promoting formation of the EGFR kinase domain asymmetric dimer. Activation of the kinase domain by Peptide C also enhances phosphorylation of other substrates. Aggregation of the EGFR kinase domain by Peptide C probably underlies activation, and Peptide C precipitates several other proteins. Peptide C was found to form fibrils independent of the presence of EGFR, and these fibrils may facilitate aggregation and activation of the kinase domain. These results establish that a peptide substrate of EGFR may increase catalytic activity by promoting kinase domain dimerization by an aggregation-mediated mechanism.

Mechanism for activation of mutated epidermal growth factor receptors in lung cancer

The initiation of epidermal growth factor receptor (EGFR) kinase activity proceeds via an asymmetric dimerization mechanism in which a "donor" tyrosine kinase domain (TKD) contacts an "acceptor" TKD, leading to its activation. In the context of a ligand-induced dimer, identical wild-type EGFR TKDs are thought to assume the donor or acceptor roles in a random manner. Here, we present biochemical reconstitution data demonstrating that activated EGFR mutants found in lung cancer preferentially assume the acceptor role when coexpressed with WT EGFR. Mutated EGFRs show enhanced association with WT EGFR, leading to hyperphosphorylation of the WT counterpart. Mutated EGFRs also hyperphosphorylate the related erythroblastic leukemia viral oncogene (ErbB) family member, ErbB-2, in a similar manner. This directional "superacceptor activity" is particularly pronounced in the drug-resistant L834R/T766M mutant. A 4-Å crystal structure of this mutant in the active conformation reveals an asymmetric dimer interface that is essentially the same as that in WT EGFR. Asymmetric dimer formation induces an allosteric conformational change in the acceptor subunit. Thus, superacceptor activity likely arises simply from a lower energetic cost associated with this conformational change in the mutant EGFR compared with WT, rather than from any structural alteration that impairs the donor role of the mutant. Collectively, these findings define a previously unrecognized mode of mutant-specific intermolecular regulation for ErbB receptors, knowledge of which could potentially be exploited for therapeutic benefit.

Mapping molecular diffusion in the plasma membrane by Multiple-Target Tracing (MTT)

Our goal is to obtain a comprehensive description of molecular processes occurring at cellular membranes in different biological functions. We aim at characterizing the complex organization and dynamics of the plasma membrane at single-molecule level, by developing analytic tools dedicated to Single-Particle Tracking (SPT) at high density: Multiple-Target Tracing (MTT). Single-molecule videomicroscopy, offering millisecond and nanometric resolution, allows a detailed representation of membrane organization by accurately mapping descriptors such as cell receptors localization, mobility, confinement or interactions. We revisited SPT, both experimentally and algorithmically. Experimental aspects included optimizing setup and cell labeling, with a particular emphasis on reaching the highest possible labeling density, in order to provide a dynamic snapshot of molecular dynamics as it occurs within the membrane. Algorithmic issues concerned each step used for rebuilding trajectories: peaks detection, estimation and reconnection, addressed by specific tools from image analysis. Implementing deflation after detection allows rescuing peaks initially hidden by neighboring, stronger peaks. Of note, improving detection directly impacts reconnection, by reducing gaps within trajectories. Performances have been evaluated using Monte-Carlo simulations for various labeling density and noise values, which typically represent the two major limitations for parallel measurements at high spatiotemporal resolution. The nanometric accuracy obtained for single molecules, using either successive on/off photoswitching or non-linear optics, can deliver exhaustive observations. This is the basis of nanoscopy methods such as STORM, PALM, RESOLFT or STED, which may often require imaging fixed samples. The central task is the detection and estimation of diffraction-limited peaks emanating from single-molecules. Hence, providing adequate assumptions such as handling a constant positional accuracy instead of Brownian motion, MTT is straightforwardly suited for nanoscopic analyses. Furthermore, MTT can fundamentally be used at any scale: not only for molecules, but also for cells or animals, for instance. Hence, MTT is a powerful tracking algorithm that finds applications at molecular and cellular scales.

NOK/STYK1 has a strong tendency towards forming aggregates and colocalises with epidermal growth factor receptor in endosomes

Our previous studies showed that the overexpression of Novel Oncogene with Kinase-domain (NOK)/STYK1 led to cellular transformation, tumorigenesis and metastasis. This report characterises the subcellular distribution of NOK in HeLa cells and its localisation in early endosomes. Confocal immunolocalisation studies indicated that NOK had structural subtypes and was distributed into two distinct expression patterns: a dot pattern (DP) and an aggregation pattern (AP). The results of an immunohistochemistry (IHC) analysis of pathological tissues also showed that high expression level of endogenous NOK was expressed in an aggregate-like structure in vivo. Importantly, we found that NOK was localised in endosomes and colocalised with epidermal growth factor receptor (EGFR) in activated endosomal vesicles. However, as the stimulation time increased, NOK and EGFR began to progress through different pathways. EGFR was gradually degraded after treatment with EGF for approximately 20 min, whereas NOK levels were not reduced. This result suggests that NOK mainly plays a role in facilitating the trafficking of EGFR from early endosomes to later endosomes/lysosomes. Taken together, NOK has a strong tendency towards forming aggregates, which may have physiological implications and provide the first evidence that this novel receptor kinase is colocalised with EGFR in endosomes to participate in a post-internalisation step of EGFR.

Transmembrane helix of novel oncogene with kinase-domain (NOK) influences its oligomerization and limits the activation of RAS/MAPK signaling

Ligand-dependent or independent oligomerization of receptor protein tyrosine kinase (RPTK) is often an essential step for receptor activation and intracellular signaling. The novel oncogene with kinase-domain (NOK) is a unique RPTK that almost completely lacks an ectodomain, expresses intracellularly and activates constitutively. However, it is unknown whether NOK can form oligomer or what function oligomerization would have. In this study, two NOK deletion mutants were generated by either removing the ectodomain (NOKDeltaECD) or including the endodomain (NOK-ICD). Co-immunoprecipitation demonstrated that the transmembrane (TM) domain of NOK was essential for its intermolecular interaction. The results further showed that NOK aggregated more closely as lower order oligomers (the dimer- and trimer-sized) than either deletion mutant did since NOK could be cross-linked by both Sulfo-EGS and formaldehyde, whereas either deletion mutant was only sensitive to Sulfo-EGS. Removing the NOK TM domain (NOK-ICD) not only markedly promoted higher order oligomerization, but also altered the subcellular localization of NOK and dramatically elevated the NOK-mediated constitutive activation of extracellular signal-regulated kinase (ERK). Moreover, NOK-ICD but not NOK or NOKDeltaECD was co-localized with the upstream signaling molecule RAS on cell membrane. Thus, TM-mediated intermolecular contacting may be mainly responsible for the constitutive activation of NOK and contribute to the autoinhibitory effect on RAS/MAPK signaling.

The ErbB kinase domain: structural perspectives into kinase activation and inhibition

Epidermal growth factor receptor (EGFR) and its family members, ErbB2, ErbB3 and ErbB4, are receptor tyrosine kinases which send signals into the cell to regulate many critical processes including development, tissue homeostasis, and tumorigenesis. Central to the signaling of these receptors is their intracellular kinase domain, which is activated by ligand-induced dimerization of the receptor and phosphorylates several tyrosine residues in the C-terminal tail. The phosphorylated tail then recruits other signaling molecules and relays the signal to downstream pathways. A model of the autoinhibition, activation and feedback inhibition mechanisms for the ErbB kinase domain has emerged from a number of recent structural studies. Meanwhile, recent clinical studies have revealed the relationship between specific ErbB kinase mutations and the responsiveness to kinase inhibitor drugs. We will review these regulation mechanisms of the ErbB kinase domain, and discuss the binding specificity of kinase inhibitors and the effects of kinase domain mutations found in cancer patients from a structural perspective.

Binding and clustering of glycosaminoglycans: a common property of mono- and multivalent cell-penetrating compounds

Recent observations in cell culture provide evidence that negatively charged glycosaminoglycans (GAGs) at the surface of biological cells bind cationic cell-penetrating compounds (CPCs) and cluster during CPC binding, thereby contributing to their endocytotic uptake. The GAG binding and clustering occur in the low-micromolar concentration range and suggest a tight interaction between GAGs and CPCs, although the relation between binding affinity and specificity of this interaction remains to be investigated. We therefore measured the GAG binding and clustering of various mono- and multivalent CPCs such as DNA transfection vectors (polyethylenimine; 1,2-dioleoyl-3-trimethylammonium-propane), amino acid homopolymers (oligoarginine; oligolysine), and cell-penetrating peptides (Penetratin; HIV-1 Tat) by means of isothermal titration calorimetry and dynamic light scattering. We find that these structurally diverse CPCs share the property of GAG binding and clustering. The binding is very tight (microscopic dissociation constants between 0.34 and 1.34 microM) and thus biologically relevant. The hydrodynamic radius of the resulting aggregates ranges from 78 nm to 586 nm, suggesting that they consist of numerous GAG chains cross-linked by CPCs. Likewise, the membrane-permeant monovalent cation acridine orange leads to GAG binding and clustering, in contrast to its membrane-impermeant structural analogs propidium iodide and ethidium bromide. Because the binding and clustering of GAGs were found to be a common denominator of all CPCs tested, these properties might be helpful to identify further CPCs.

Mutational activation of ErbB2 reveals a new protein kinase autoinhibition mechanism

Autoinhibition plays a key role in the control of protein kinase activity. ErbB2 is a unique receptor-tyrosine kinase that does not bind ligand but possesses an extracellular domain poised to engage other ErbBs. Little is known about the molecular mechanism for ErbB2 catalytic regulation. Here we show that ErbB2 kinase is strongly autoinhibited, and a loop connecting the alphaC helix and beta4 sheet within the kinase domain plays a major role in the control of kinase activity. Mutations of two Gly residues at positions 776 and 778 in this loop dramatically increase ErbB2 catalytic activity. Kinetic analysis demonstrates that mutational activation is due to approximately 10- and approximately 7-fold increases in ATP binding affinity and turnover number, respectively. Expression of the activated ErbB2 mutants in cells resulted in elevated ligand-independent ErbB2 autophosphorylation, ErbB3 phosphorylation, and stimulation of mitogen-activated protein kinase. Molecular modeling suggests that the ErbB2 kinase domain is stabilized in an inactive state via a hydrophobic interaction between the alphaC-beta4 and activation loops. Importantly, many ErbB2 human cancer mutations have been identified in the alphaC-beta4 loop, including the activating G776S mutation studied here. Our findings reveal a new kinase regulatory mechanism in which the alphaC-beta4 loop functions as an intramolecular switch that controls ErbB2 activity and suggests that loss of alphaC-beta4 loop-mediated autoinhibition is involved in oncogenic activation of ErbB2.

Signal integration during development: mechanisms of EGFR and Notch pathway function and cross-talk

Metazoan development relies on a highly regulated network of interactions between conserved signal transduction pathways to coordinate all aspects of cell fate specification, differentiation, and growth. In this review, we discuss the intricate interplay between the epidermal growth factor receptor (EGFR; Drosophila EGFR/DER) and the Notch signaling pathways as a paradigm for signal integration during development. First, we describe the current state of understanding of the molecular architecture of the EGFR and Notch signaling pathways that has resulted from synergistic studies in vertebrate, invertebrate, and cultured cell model systems. Then, focusing specifically on the Drosophila eye, we discuss how cooperative, sequential, and antagonistic relationships between these pathways mediate the spatially and temporally regulated processes that generate this sensory organ. The common themes underlying the coordination of the EGFR and Notch pathways appear to be broadly conserved and should, therefore, be directly applicable to elucidating mechanisms of information integration and signaling specificity in vertebrate systems.

Protein-protein interactions in the allosteric regulation of protein kinases

Protein-protein interactions involving the catalytic domain of protein kinases are likely to be generally important in the regulation of signal transduction pathways, but are rather sparsely represented in crystal structures. Recently determined structures of the kinase domains of the mitogen-activated protein kinase Fus3, the RNA-dependent kinase PKR, the epidermal growth factor receptor and Ca(2+)/calmodulin-dependent protein kinase II have revealed unexpected and distinct mechanisms by which interactions with the catalytic domain can modulate kinase activity.

An Allosteric Mechanism for Activation of the Kinase Domain of Epidermal Growth Factor Receptor

The mechanism by which the epidermal growth factor receptor (EGFR) is activated upon dimerization has eluded definition. We find that the EGFR kinase domain can be activated by increasing its local concentration or by mutating a leucine (L834R) in the activation loop, the phosphorylation of which is not required for activation. This suggests that the kinase domain is intrinsically autoinhibited, and an intermolecular interaction promotes its activation. Using further mutational analysis and crystallography we demonstrate that the autoinhibited conformation of the EGFR kinase domain resembles that of Src and cyclin-dependent kinases (CDKs). EGFR activation results from the formation of an asymmetric dimer in which the C-terminal lobe of one kinase domain plays a role analogous to that of cyclin in activated CDK/cyclin complexes. The CDK/cyclin-like complex formed by two kinase domains thus explains the activation of EGFR-family receptors by homo- or heterodimerization.

Src-dependent ERK5 and Src/EGFR-dependent ERK1/2 activation is required for cell proliferation by asbestos

Crocidolite asbestos elicits oxidative stress and cell proliferation, but the signaling cascades linked to these outcomes are unclear. To determine the role of mitogen-activated protein kinases (MAPK) in asbestos-induced cell signaling, we evaluated the effects of crocidolite asbestos, EGF and H2O2, on MAPK activation in murine lung epithelial cells (C10 line). In contrast to rapid and transient activation of extracellular signal-regulated kinase 5 (ERK5) by EGF or H2O2, asbestos caused protracted oxidant-dependent ERK5 activation that was inhibited by an Src kinase inhibitor (PP2), but not by an inhibitor of epidermal growth factor receptor (EGFR) phosphorylation (AG1478). ERK1/2 activation by asbestos was inhibited by either PP2 or AG1478. To confirm the involvement of Src in ERK1/2 and ERK5 activation, a dominant-negative Src construct was used. These experiments showed that Src was essential for ERK1/2 and also ERK5 phosphorylation by asbestos. Time frame studies indicated immediate activation of Src by asbestos fibers, whereas EGFR phosphorylation occurred subsequently. Data suggest that asbestos causes activation of ERK5 through an EGFR-independent pathway, whereas ERK1/2 activation is dependent on Src through a mechanism involving phosphorylation of the EGFR. Furthermore, Src, ERK1/2 and ERK5 activation are essential for cell proliferation by asbestos. The use of a dominant-negative ERK5 construct caused selective downregulation of c-jun expression, whereas inhibition of Src by PP2 or MEK1 by PD98059 caused decreases in c-fos, fra-1 and c-jun expression in asbestos-exposed C10 cells. These observations may have broad relevance to cell proliferation by carcinogenic mineral fibers and oxidants.

EGFR activation coupled to inhibition of tyrosine phosphatases causes lateral signal propagation

The epidermal growth factor receptor (EGFR) belongs to the receptor tyrosine kinase (RTK) superfamily and is involved in regulating cell proliferation, differentiation and motility. Growth factor binding induces receptor oligomerization at the plasma membrane, which leads to activation of the intrinsic RTK activity and trans-phosphorylation of tyrosine residues in the intracellular part of the receptor. These residues are docking sites for proteins containing Src homology domain 2 and phosphotyrosine-binding domains that relay the signal inside the cell. In response to EGF attached to beads, lateral propagation of EGFR phosphorylation occurs at the plasma membrane, representing an early amplification step in EGFR signalling. Here we have investigated an underlying reaction network that couples RTK activity to protein tyrosine phosphatase (PTP) inhibition by reactive oxygen species. Mathematical analysis of the chemical kinetic equations of the minimal reaction network detects general properties of this system that can be observed experimentally by imaging EGFR phosphorylation in cells. The existence of a bistable state in this reaction network explains a threshold response and how a high proportion of phosphorylated receptors can be maintained in plasma membrane regions that are not exposed to ligand.

Persistent localization of activated extracellular signal-regulated kinases (ERK1/2) is epithelial cell-specific in an inhalation model of asbestosis

Asbestos fibers up-regulate the extracellular signal-regulated kinase (ERK1/2) pathway in mesothelial and pulmonary epithelial cells in vitro, but the cell-type expression patterns and intracellular localization of activated, ie, phosphorylated, ERK in the lung after inhalation of asbestos are unclear. C57/BL6 mice were exposed to 7-mg/m(3) air of crocidolite asbestos for 5 and 30 days, the times required for the development of epithelial cell hyperplasia and fibrotic lesions, respectively. Exposure to asbestos caused striking increases in both unphosphorylated and phosphorylated ERK (p-ERK), which were most marked at 30 days and co-localized in bronchiolar and alveolar epithelial cells using an antibody to cytokeratin. Alveolar macrophages, detected with an anti-macrophage antibody, did not express p-ERK. p-ERK was localized at the apical cell surface of bronchiolar and alveolar type II epithelial cells exposed to asbestos fibers, and was most marked in areas of epithelial hyperplasia in association with fibrotic lesions. Because translocation of p-ERK to the nucleus is associated with activation of early response genes and transcription factors, laser scanning cytometry was used to determine the kinetics of activation and nuclear translocation of p-ERK in an alveolar type II epithelial cell line in vitro after exposure to asbestos or the ERK stimuli, epidermal growth factor, or H(2)O(2). Results showed that cytoplasmic to nuclear translocation of p-ERK occurred in a protracted manner in cells exposed to asbestos. The immunolocalization of p-ERK at the membrane surface, a site of initial exposure to asbestos fibers, and the chronic activation of p-ERK in epithelial cells at sites of fibrogenesis are consistent with the concept that epithelial cell signaling through the ERK pathway contributes to remodeling of the lung during the development of pulmonary fibrosis.

Autoantibodies to the epidermal growth factor receptor in systemic sclerosis, lupus, and autoimmune mice

Autoantibodies to the recombinant extracellular domain of epidermal growth factor receptor (exEGFR) were detected by ELISA in the serum of Fas-defective old MRL/MpJ/lpr and C3H/HeJ/gld mice, but not young mice from these strains, or nonautoimmune young and old BALB/c, MRL/MpJ/++, and C3H/HeJ/MMTV mice. Compared with control human subjects without autoimmune disease, the frequency of exEGFR-binding autoantibodies was increased in scleroderma (systemic sclerosis) patients and to a lesser extent in lupus patients. Phage autoantibodies (Fv fragments) isolated from a lupus library by selection on a linear epitope of EGFR (residues 294-310) displayed the ability to bind exEGFR. Treatment of EGFR-expressing A431 cells with autoantibodies purified by affinity chromatography on immobilized exEGFR resulted in specific staining of the cells. Short-lived but strong inhibition of cellular DNA synthesis was observed in the presence of the autoantibodies. We concluded that autoantibody responses to EGFR hold the potential of fulfilling a pathogenic role in autoimmune disease.

Biochemical characterization of the Arabidopsis protein kinase SOS2 that functions in salt tolerance

The Arabidopsis Salt Overly Sensitive 2 (SOS2) gene encodes a serine/threonine (Thr) protein kinase that has been shown to be a critical component of the salt stress signaling pathway. SOS2 contains a sucrose-non-fermenting protein kinase 1/AMP-activated protein kinase-like N-terminal catalytic domain with an activation loop and a unique C-terminal regulatory domain with an FISL motif that binds to the calcium sensor Salt Overly Sensitive 3. In this study, we examined some of the biochemical properties of the SOS2 in vitro. To determine its biochemical properties, we expressed and isolated a number of active and inactive SOS2 mutants as glutathione S-transferase fusion proteins in Escherichia coli. Three constitutively active mutants, SOS2T168D, SOS2T168D Delta F, and SOS2T168D Delta 308, were obtained previously, which contain either the Thr-168 to aspartic acid (Asp) mutation in the activation loop or combine the activation loop mutation with removal of the FISL motif or the entire regulatory domain. These active mutants exhibited a preference for Mn(2+) relative to Mg(2+) and could not use GTP as phosphate donor for either substrate phosphorylation or autophosphorylation. The three enzymes had similar peptide substrate specificity and catalytic efficiency. Salt overly sensitive 3 had little effect on the activity of the activation loop mutant SOS2T168D, either in the presence or absence of calcium. The active mutant SOS2T168D Delta 308 could not transphosphorylate an inactive protein (SOS2K40N), which indicates an intramolecular reaction mechanism of SOS2 autophosphorylation. Interestingly, SOS2 could be activated not only by the Thr-168 to Asp mutation but also by a serine-156 or tyrosine-175 to Asp mutation within the activation loop. Our results provide insights into the regulation and biochemical properties of SOS2 and the SOS2 subfamily of protein kinases.

Comparison of the biochemical and kinetic properties of the type 1 receptor tyrosine kinase intracellular domains. Demonstration of differential sensitivity to kinase inhibitors

Epidermal growth factor receptor (EGFR), ErbB-2, and ErbB-4 are members of the type 1 receptor tyrosine kinase family. Overexpression of these receptors, especially ErbB-2 and EGFR, has been implicated in multiple forms of cancer. Inhibitors of EGFR tyrosine kinase activity are being evaluated clinically for cancer therapy. The potency and selectivity of these inhibitors may affect the efficacy and toxicity of therapy. Here we describe the expression, purification, and biochemical comparison of EGFR, ErbB-2, and ErbB-4 intracellular domains. Despite their high degree of sequence homology, the three enzymes have significantly different catalytic properties and substrate kinetics. For example, the catalytic activity of ErbB-2 is less stable than that of EGFR. ErbB-2 uses ATP-Mg as a substrate inefficiently compared with EGFR and ErbB-4. The three enzymes have very similar substrate preferences for three optimized peptide substrates, but differences in substrate synergies were observed. We have used the biochemical and kinetic parameters determined from these studies to develop an assay system that accurately measures inhibitor potency and selectivity between the type 1 receptor family. We report that the selectivity profile of molecules in the 4-anilinoquinazoline series can be modified through specific aniline substitutions. Moreover, these compounds have activity in whole cells that reflect the potency and selectivity of target inhibition determined with this assay system.

Effect of membrane fluidity on tyrosine kinase activity of reconstituted epidermal growth factor receptor

Epidermal growth factor receptor (EGFR) was functionally reconstituted into liposome membrane. Triton X-100 was removed by Bio-beads SM-2. More than 80% of the reconstituted EGFR possessed right-side-out orientation with the EGF binding side facing the medium. The tyrosine kinase assay of the EGFR was carried out in the presence of the antibiotic alamethicin. The reconstituted EGFR tyrosine kinase was well activated by EGF. The influence of lipid composition on tyrosine kinase activity was investigated. Introduction of cholesterol into the dioleoylphophatidylcholine (DOPC) liposome membrane resulted in the decrease of tyrosine kinase activity. The tyrosine kinase activity of EGFR in distearylphosphatidylcholine liposome was much lower than that of EGFR-DOPC proteoliposome. Results indicated the importance of membrane fluidity on the apparent tyrosine kinase activity of reconstituted EGFR.

Protein tyrosine kinase structure and function

Tyrosine phosphorylation is one of the key covalent modifications that occurs in multicellular organisms as a result of intercellular communication during embryogenesis and maintenance of adult tissues. The enzymes that carry out this modification are the protein tyrosine kinases (PTKs), which catalyze the transfer of the phosphate of ATP to tyrosine residues on protein substrates. Phosphorylation of tyrosine residues modulates enzymatic activity and creates binding sites for the recruitment of downstream signaling proteins. Two classes of PTKs are present in cells: the transmembrane receptor PTKs and the nonreceptor PTKs. Because PTKs are critical components of cellular signaling pathways, their catalytic activity is strictly regulated. Over the past several years, high-resolution structural studies of PTKs have provided a molecular basis for understanding the mechanisms by which receptor and nonreceptor PTKs are regulated. This review will highlight the important results that have emerged from these structural studies.

Quantitative imaging of lateral ErbB1 receptor signal propagation in the plasma membrane

Evidence for a new signaling mechanism consisting of ligand-independent lateral propagation of receptor activation in the plasma membrane is presented. We visualized the phosphorylation of green fluorescent protein (GFP)-tagged ErbB1 (ErbB1-GFP) receptors in cells focally stimulated with epidermal growth factor (EGF) covalently attached to beads. This was achieved by quantitative imaging of protein reaction states in cells by fluorescence resonance energy transfer (FRET) with global analysis of fluorescence lifetime imaging microscopy (FLIM) data. The rapid and extensive propagation of receptor phosphorylation over the entire cell after focal stimulation demonstrates a signaling wave at the plasma membrane resulting in full activation of all receptors.

Novel kinetics, behaviour and cell-type specificity of CD157-mediated tyrosine kinase signalling

CD157, a recently characterized leukocyte surface antigen, has recently been shown to induce tyrosine phosphorylation of a 130-kDa protein (p130) when cross-linked with its antibody (ligand). We have further investigated the detailed kinetics, behaviour and cell-type specificity of this CD157-stimulated p130 phosphorylation. We demonstrate that CD157-mediated p130 phosphorylation is ligand independent in recombinant CD157-expressing CHO, MCA102 and COS-7 cells but is ligand dependent in HL-60-differentiated monocytes (mHL-60) having enhanced CD157 expression. This p130 phosphorylation is activated only at lower temperatures (0-4 degrees C) in MCA102, COS-7 and mHL-60 cells but is temperature insensitive in CHO cells. We further demonstrate that the CHO/CD157 cell clones have approximately 22-28% slower rates of proliferation than that of a CHO/mock clone. But the MCA102 cell proliferation remains unaffected by CD157 expression. We postulate that the difference in the temperature sensitivity of p130 phosphorylation can be responsible for the discrepancy in the rates of MCA102/CD157 and CHO/CD157 cell proliferation.

Fluorescence lifetime imaging of receptor tyrosine kinase activity in cells

We report a highly specific fluorescence lifetime imaging microscopy (FLIM) method for monitoring epidermal growth factor receptor (EGFR) phosphorylation in cells based on fluorescence resonance energy transfer (FRET). EGFR phosphorylation was monitored using a green fluorescent protein (GFP)-tagged EGFR and Cy3-conjugated anti-phosphotyrosine antibodies. In this FRET-based imaging method, the information about phosphorylation is contained only in the (donor) GFP fluorescence lifetime and is independent of the antibody-derived (acceptor) fluorescence signal. A pixel-by-pixel reference lifetime of the donor GFP in the absence of FRET was acquired from the same cell after photobleaching of the acceptor. We show that this calibration, by acceptor photobleaching, works for the GFP-Cy3 donor-acceptor pair and allows the full quantitation of FRET efficiencies, and therefore the degree of exposed phosphotyrosines, at each pixel. The hallmark of EGFR stimulation is receptor dimerisation [1] [2] [3] [4] and concomitant activation of its intracellular tyrosine kinase domain [5] [6] [7]. Trans-autophosphorylation of the receptor [8] [9] on specific tyrosine residues couples the activated dimer to the intracellular signal transduction machinery as these phosphorylated residues serve as docking sites for adaptor and effector molecules containing Src homology 2 (SH2; reviewed in [10]) and phosphotyrosine-binding (PTB) [11] domains. The time-course and extent of EGFR phosphorylation are therefore important determinants of the underlying pathway and resulting cellular response. Our results strongly suggest that secondary proteins are recruited by activated receptors in endosomes, indicating that these are active compartments in signal transduction.

Asbestos-induced phosphorylation of epidermal growth factor receptor is linked to c-fos and apoptosis

We examined the mechanisms of interaction of crocidolite asbestos fibers with the epidermal growth factor (EGF) receptor (EGFR) and the role of the EGFR-extracellular signal-regulated kinase (ERK) signaling pathway in early-response protooncogene (c-fos/c-jun) expression and apoptosis induced by asbestos in rat pleural mesothelial (RPM) cells. Asbestos fibers, but not the nonfibrous analog riebeckite, abolished binding of EGF to the EGFR. This was not due to a direct interaction of fibers with ligand, inasmuch as binding studies using fibers and EGF in the absence of membranes showed that EGF did not adsorb to the surface of asbestos fibers. Exposure of RPM cells to asbestos caused a greater than twofold increase in steady-state message and protein levels of EGFR (P < 0.05). The tyrphostin AG-1478, which inhibits the tyrosine kinase activity of the EGFR, but not the tyrphostin A-10, which does not affect EGFR activity, significantly ameliorated asbestos-induced increases in mRNA levels of c-fos but not of c-jun. Pretreatment of RPM cells with AG-1478 significantly reduced apoptosis in cells exposed to asbestos. Our findings suggest that asbestos-induced binding to EGFR initiates signaling pathways responsible for increased expression of the protooncogene c-fos and the development of apoptosis. The ability to block asbestos-induced elevations in c-fos mRNA levels and apoptosis by small-molecule inhibitors of EGFR phosphorylation may have therapeutic implications in asbestos-related diseases.

4-hydroxynonenal triggers an epidermal growth factor receptor-linked signal pathway for growth inhibition

Lipid peroxidation has been implicated in the pathogenesis of various diseases. As a major product of membrane lipid peroxidation, 4-hydroxynonenal (HNE) appears after various kinds of oxidative stress, and is known to induce cell growth inhibition. We here analysed the HNE-mediated signal transduction cascade for the growth inhibition of human epidermoid carcinoma A431 cells. HNE dose-dependently induced phosphorylation of multiple cellular proteins including epidermal growth factor receptor (EGFR) in A431 cells, and rapidly upregulated the catalytic actions of EGFR for autophosphorylation and for phosphorylation of casein as an exogenous substrate. Immunoblot analysis by use of HNE-specific antibody demonstrated the binding of HNE to EGFR along with its activation. This binding, which did not induce cross-linking of EGFR, caused a capping of the receptor on the cell surface which mimicked the capping induced by EGF. Phosphorylation and activation of EGFR were followed by phosphorylation of adaptor protein Shc and activation of MAP kinase. Both genistein as a wide spectrum protein tyrosine kinase inhibitor and AG1478 as a specific EGFR tyrosine phosphorylation blocker inhibited activation of EGFR and MAP kinase by HNE. The same inhibitors prevented HNE-mediated growth inhibition, suggesting a close linkage between EGFR/MAP kinase activation and growth inhibition after exposure to HNE. Our results suggest that EGFR may be one of the primary targets of HNE for an oxidative stress-linked cell growth inhibition.

Collagen type IV promotes the differentiation of neuronal progenitors and inhibits astroglial differentiation in cortical cell cultures

In an effort to elucidate the interactions between cells in the developing cortex and their microenvironment, we have employed dissociated cell cultures and immunocytochemistry to analyze the effect of collagen type IV (COL) on the proliferation and differentiation of rat cortical progenitor cells during the period of corticogenesis. COL, present in the proliferative zones throughout the period of neurogenesis, belongs to a group of macromolecular proteins that make up a considerable portion of the extracellular matrix (ECM). We have shown that this ECM molecule inhibits cell proliferation and glial cell differentiation while promoting neuronal differentiation. We have also demonstrated that COL, when applied to the cultures with basic fibroblast growth factor (bFGF), induces glial cell differentiation while continuing to promote neuronal differentiation. These results indicate that cortical progenitor cells respond differentially to local environmental signals, and that components of the ECM are involved in the regulation of corticogenesis.

Decorin suppresses tumor cell growth by activating the epidermal growth factor receptor

Decorin, a small leucine-rich proteoglycan, is capable of suppressing the growth of various tumor cell lines when expressed ectopically. In this report, we investigated the biochemical mechanism by which decorin inhibits cell cycle progression. In A431 squamous carcinoma cells, decorin proteoglycan or protein core induced a marked growth suppression, when either exogenously added or endogenously produced by a transgene. Decorin caused rapid phosphorylation of the EGF receptor and a concurrent activation of mitogen-activated protein (MAP) kinase signal pathway. This led to a protracted induction of endogenous p21, a potent inhibitor of cyclin-dependent kinases, and ultimate cell cycle arrest. Biglycan, a related proteoglycan, had no effect. Moreover, decorin activated the EGF receptor/MAP kinase/ p21 axis in cell lines of various histogenetic backgrounds. These results provide the first evidence that EGF and decorin converge functionally to regulate the cell cycle through activation of a common pathway which ultimately leads to growth suppression.

Unliganded epidermal growth factor receptor dimerization induced by direct interaction of quinazolines with the ATP binding site

Receptor dimerization is critical for signaling by the epidermal growth factor receptor (EGFR) tyrosine kinase. This occurs after binding of the receptor's extracellular domain by ligand or bivalent antibodies. The role of other receptor domains in dimerization is less clear, and there are no examples of dimers induced by direct perturbation of the EGFR kinase domain. Submicromolar concentrations of AG-1478 and AG-1517, quinazolines specific for inhibition of the EGFR kinase, induced reversible receptor dimerization in vitro and in intact A431 cells. Consistent with the inhibitory effect of quinazolines on receptor kinase activity, the dimers formed lacked a detectable Tyr(P) signal. Quinazoline-induced EGFR dimerization was abrogated in vitro by ATP and the ATP analog adenyl-5'-yl imidodiphosphate. Receptors with a single-point mutation in the ATP binding site as well as wild-type EGFR with a covalent modification of the ATP site failed to dimerize in response to AG-1478 and AG-1517. These data suggest that EGFR dimerization can be induced by the interaction of quinazolines at the ATP site in the absence of receptor ligand binding. In SKBR-3 cells, the quinazolines induced the formation of inactive EGFR/ErbB-2 heterodimers, potentially sequestering ErbB-2 from interacting with other coreceptors of the ErbB family. Structural studies of the quinazoline interaction with the EGFR tyrosine kinase domain should allow for an analysis of receptor-specific chemical features required for binding to the ATP site and disruption of signaling, a strategy that can be perhaps applied to other tumor cell receptor systems.

Stimulation of growth factor receptor signal transduction by activation of voltage-sensitive calcium channels

To understand the mechanisms by which electrical activity may generate long-term responses in the nervous system, we examined how activation of voltage-sensitive calcium channels (VSCCs) can stimulate the Ras/mitogen-activated protein kinase (MAPK) signaling pathway. Calcium influx through L-type VSCCs leads to tyrosine phosphorylation of the adaptor protein Shc and its association with the adaptor protein Grb2, which is bound to the guanine nucleotide exchange factor Sos1. In response to calcium influx, Shc, Grb2, and Sos1 inducibly associate with a 180-kDa tyrosine-phosphorylated protein, which was determined to be the epidermal growth factor receptor (EGFR). Calcium influx induces tyrosine phosphorylation of the EGFR to levels that can activate the MAPK signaling pathway. Thus, ion channel activation stimulates growth factor receptor signal transduction.

Role of the regulatory domain of the EGF-receptor cytoplasmic tail in selective binding of the clathrin-associated complex AP-2

BACKGROUND

After stimulation of a cell by the mitogenic epidermal growth factor (EGF), the EGF receptor (EGF-R) is cleared from the cell surface in order to turn off receptor signaling. This internalization is mediated via clathrin-coated pits and coated vesicles, and ultimately the receptors are delivered to the lysosome and destroyed. It is believed that clathrin-associated protein complexes or adaptors (APs) link the entrapment of EGF-R and other nutrient and growth-factor receptors to the formation of the clathrin-coated pit. Two classes of APs are known--AP-2, found at the plasma membrane, and AP-1, found in the trans-Golgi network. Activated EGF-R associates with AP-2s at the plasma membrane, but the mechanism responsible for this association is not known. Here, we investigate, in vivo and in vitro, three aspects of the interaction between APs and EGF-R: firstly, we ask whether EGF-R at the plasma membrane distinguishes between AP-1 and AP-2; secondly, we ask which part of the receptor's cytoplasmic tail is responsible for binding; finally, we ask whether autophosphorylation by EGF-R is essential for the interaction.

RESULTS

We demonstrate that EGF-R displays a selective association for AP-2 over AP-1 in vivo, and that this preferential interaction can also be detected using surface plasmon resonance in vitro. Using a truncated mutant and a kinase-dead mutant of EGF-R, we show that the regulatory domain of the cytoplasmic tail is essential for the recruitment of AP-2 in vivo and that this domain is required for association between purified AP-2 and EGF-R in vitro. Finally, we demonstrate, in vivo and in vitro, that tyrosine auto-phosphorylation by the receptor is not an essential pre-condition for the recruitment of AP-2.

CONCLUSIONS

EGF-R binds selectively to AP-2s, and the regulatory domain of its cytoplasmic tail is required for this interaction. The lack of correlation between receptor autophosphorylation and AP-2 recruitment suggests that activation of the EGF-R kinase stimulates endocytosis by the phosphorylation of a factor distinct from EGF-R itself, as also proposed by others based on experiments measuring receptor traffic and entrapment.

Tyrosine phosphorylation of the c-cbl proto-oncogene protein product and association with epidermal growth factor (EGF) receptor upon EGF stimulation

The murine retroviral oncogene v-cbl induces pre-B cell lymphomas and myelogenous leukemias. The protein product of the mammalian c-cbl proto-oncogene is a widely expressed cytoplasmic 120-kDa protein (p120cbl) whose normal cellular function has not been determined. Here we show that upon stimulation of human epidermal growth factor (EGF) receptor, p12ocbl becomes strongly tyrosine-phosphorylated and associates with activated EGF receptor in vivo. A GST fusion protein containing amino acids 1-486 of p120cbl, including a region highly conserved in nematodes, binds directly to the autophosphorylated carboxyl-terminal tail of the EGF receptor. Platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), or nerve growth factor (NGF) stimulation also results in tyrosine phosphorylation of p120cbl. Recent genetic studies in Caenorhabditis elegans indicate that Sli-1, a p120cbl homologue, plays a negative regulatory role in control of the Ras signaling pathway initiated by the C. elegans EGF receptor homologue. Our results indicate that p120cbl is involved in an early step in the EGF signaling pathway that is conserved from nematodes to mammals.

Oligomerization of epidermal growth factor receptors on A431 cells studied by time-resolved fluorescence imaging microscopy. A stereochemical model for tyrosine kinase receptor activation

The aggregation states of the epidermal growth factor receptor (EGFR) on single A431 human epidermoid carcinoma cells were assessed with two new techniques for determining fluorescence resonance energy transfer: donor photobleaching fluorescence resonance energy transfer (pbFRET) microscopy and fluorescence lifetime imaging microscopy (FLIM). Fluorescein-(donor) and rhodamine-(acceptor) labeled EGF were bound to the cells and the extent of oligomerization was monitored by the spatially resolved FRET efficiency as a function of the donor/acceptor ratio and treatment conditions. An average FRET efficiency of 5% was determined after a low temperature (4 degrees C) incubation with the fluorescent EGF analogs for 40 min. A subsequent elevation of the temperature for 5 min caused a substantial increase of the average FRET efficiency to 14% at 20 degrees C and 31% at 37 degrees C. In the context of a two-state (monomer/dimer) model for the EGFR, these FRET efficiencies were consistent with minimal average receptor dimerizations of 13, 36, and 69% at 4, 20, and 37 degrees C, respectively. A431 cells were pretreated with the monoclonal antibody mAb 2E9 that specifically blocks EGF binding to the predominant population of low affinity EGFR (15). The average FRET efficiency increased dramatically to 28% at 4 degrees C, indicative of a minimal receptor dimerization of 65% for the subpopulation of high affinity receptors. These results are in accordance with prior studies indicating that binding of EGF leads to a fast and temperature-dependent microclustering of EGFR, but suggest in addition that the high affinity functional subclass of receptors on quiescent A431 cells are present in a predimerized or oligomerized state. We propose that the transmission of the external ligand-binding signal to the cytoplasmic domain is effected by a concerted relative rotational rearrangement of the monomeric units comprising the dimeric receptor, thereby potentiating a mutual activation of the tyrosine kinase domains.

Association of epidermal growth factor receptors with coated pit adaptins via a tyrosine phosphorylation-regulated mechanism

We investigated the mechanism by which ligand-activated epidermal growth factor receptors (EGFR) associate with coated pit adaptor protein (AP) complexes. In vivo association, assayed by coimmunoprecipitation of AP with mutant EGFR, required tyrosine kinase activity, intact autophosphorylation sites, and the regulatory carboxyl terminus of EGFR. The role of autophosphorylation of EGFR in interaction with AP was examined in vitro using a BIAcore instrument. Purified active EGFR, immobilized on the biosensor surface, was reversibly autophosphorylated or dephosphorylated by treatment with ATP or phosphatase. Autophosphorylation of EGFR significantly increased AP binding. Once formed, EGFR AP complexes were resistant to disassembly by dephosphorylation of EGFR or competition with phosphotyrosine, indicating that phosphorylated tyrosine residues do not directly participate in AP binding. Induction of conformational changes in EGFR by treatment with urea increased AP binding up to 10-fold in the absence of EGFR autophosphorylation. A recombinant EGFR carboxyl terminus specifically bound the AP complex and each of the isolated alpha- and beta-subunits of AP2. We conclude that tyrosine autophosphorylation of EGFR exposes structural motif(s) in the carboxyl terminus of EGFR that interact specifically with AP2.

Physicochemical characterization of the cytoplasmic domain of the epidermal growth factor receptor and evidence for conformational changes associated with its activation by ammonium sulphate

The physiochemical properties of the purified cytoplasmic domain of the epidermal growth factor (EGF) receptor, its self-phosphorylation and peptide phosphorylation activities, and its activation by ammonium sulphate have been studied. Highly efficient purification procedures for the isolation of the recombinant cytoplasmic domain (Met644-Ala1186) of the EGF receptor, expressed in the baculovirus/insect cell system, are described. Physicochemical characterization of the protein included investigation of its isoelectric and hydrodynamic properties, stability, oligomeric status, and secondary structure using far-u.v. circular dichroism. The recombinant protein was not recognized by anti-phosphotyrosine antibodies, unless first self-phosphorylated in vitro. Tryptic phosphopeptide maps of self-phosphorylated recombinant cytoplasmic domain and the EGF-stimulated A431-membrane receptor were very similar, suggesting that the recombinant had similar self-phosphorylation capacity and specificity. The preparations were characterized by high specific activity towards peptide tyrosine phosphorylation. Although the cytoplasmic domain was isolated as a homogeneously monomeric protein, storage at 4 degrees C led to slow, spontaneous aggregation with reduction in specific activity. Both high activity and monomeric state were maintained by storage below 0 degree C. The dependence of the initial rate of self-phosphorylation on protein concentration was consistent with cross-phosphorylation but not with the known oligomerization-induced activation of holoreceptor. The peptide phosphorylation activity was stimulated by Mn2+, Mg2+ and (NH4)2SO4 at high concentrations. The substrate specificity of (NH4)2SO4 activation was studied using synthetic peptides. Self-phosphorylation was inhibited by (NH4)2SO4 in the range 0-0.25 M but activated at 1.0-1.5 M, possibly as a result of ionic and hydrophobic protein interactions respectively. Phosphopeptide maps of cytoplasmic domain phosphorylated in the presence of high (NH4)2SO4 showed that the protein was more extensively phosphorylated than in the absence of salt, or than the native receptor. Far-u.v. circular-dichroism spectra of the cytoplasmic domain changed dramatically at 1 M (NH4)2SO4, raising the possibility that (NH4)2SO4 activates the kinase catalytic domain by inducing conformational changes.

Activation of phospholipase C gamma in Schizosaccharomyces pombe by coexpression of receptor or nonreceptor tyrosine kinases

The fission yeast Schizosaccharomyces pombe has no detectable endogenous receptor tyrosine kinases or associated signalling apparatus, and we have used this cell system to reconstitute mammalian platelet-derived growth factor beta (PDGF beta) receptor-linked activation of phospholipase C gamma 2 (PLC gamma 2). The PDGF beta receptor migrates as a glycosylated protein of 165 kDa associated exclusively with membrane fractions. No tyrosine autophosphorylation was detected when PDGF beta was expressed alone. PLC gamma 2 appears as a 140-kDa protein distributed between particulate and soluble fractions which exhibits characteristic selectivity for phosphatidylinositol 4,5-bisphosphate and is sensitive to powerful activation by Ca2+. When coexpressed, both PDGF beta and PLC gamma 2 undergo tyrosine phosphorylation, and this is accompanied by a > 26-fold increase in [3H]inositol 4,5-biphosphate ([3H]IP2) and [3H]inositol 1,4,5-triphosphate [3H]IP3 production. Treatment with the tyrosine phosphatase inhibitor pervanadate further increased PLC gamma 2 tyrosine phosphorylation as well as [3H]IP2 and [3H]IP3 generation. Phosphorylated PLC gamma 2 was found predominantly in membrane fractions. To test a nonreceptor tyrosine kinase, we then expressed the human proto-oncogene c-src together with its negative regulator Csk. These were immunodetectable as bands at 60 kDa (c-Src) and 50 kDa (Csk) and distributed between membrane and cytosolic fractions. When yeast coexpressing c-Src, Csk, and PLC gamma 2 was incubated with pervanadate, PLC gamma 2 was tyrosine phosphorylated and [3H]IP2 and [3H]IP3 production increased 11.0- and 7.0-fold, respectively. Csk expressed alone with PLC gamma 2 was ineffective. Similar PLC gamma 2 activation was observed upon in vitro mixing with the extracts expressing either c-Src or the PDGF beta receptor. In summary, this is the first report of a reconstitution of mammalian tyrosine kinase-linked effector activation in yeast cells and also the first demonstration of direct PLC gamma 2 activation by the proto-oncogene c-src. These observations indicate that S. pombe provides a powerful cell system in which to study critical molecular interactions and activities underlying receptor and nonreceptor tyrosine kinase-dependent cell signaling.

Specificity and promiscuity in membrane helix interactions

The membrane-spanning portions of many integral membrane proteins consist of one or a number of transmembrane α-helices, which are expected to be independently stable on thermodynamic grounds. Side-by-side interactions between these transmembrane α-helices are important in the folding and assembly of such integral membrane proteins and their complexes. In considering the contribution of these helix–helix interactions to membrane protein folding and oligomerization, a distinction between the energetics and specificity should be recognized. A number of contributions to the energetics of transmembrane helix association within the lipid bilayer will be relatively non-specific, including those resulting from charge–charge interactions and lipid–packing effects. Specificity (and part of the energy) in transmembrane α-helix association, however, appears to rely mainly upon a detailed stereochemical fit between sets of dynamically accessible states of particular helices. In some cases, these interactions are mediated in part by prosthetic groups.