Crystal structure of the complex of human epidermal growth factor and receptor extracellular domains

The epidermal growth factor receptor family: biology driving targeted therapeutics

The epidermal growth factor family of receptor tyrosine kinases (ErbBs) plays essential roles in regulating cell proliferation, survival, differentiation and migration. The ErbB receptors carry out both redundant and restricted functions in mammalian development and in the maintenance of tissues in the adult mammal. Loss of regulation of the ErbB receptors underlies many human diseases, most notably cancer. Our understanding of the function and complex regulation of these receptors has fueled the development of targeted therapeutic agents for human malignancies in the last 15 years. Here we review the biology of ErbB receptors, including their structure, signaling, regulation, and roles in development and disease, then briefly touch on their increasing roles as targets for cancer therapy.

The antibody zalutumumab inhibits epidermal growth factor receptor signaling by limiting intra- and intermolecular flexibility

The epidermal growth factor receptor (EGFR) activates cellular pathways controlling cell proliferation, differentiation, migration, and survival. It thus represents a valid therapeutic target for treating solid cancers. Here, we used an electron microscopy-based technique (Protein Tomography) to study the structural rearrangement accompanying activation and inhibition of native, individual, EGFR molecules. Reconstructed tomograms (3D density maps) showed a level of detail that allowed individual domains to be discerned. Monomeric, resting EGFR ectodomains demonstrated large flexibility, and a number of distinct conformations were observed. In contrast, ligand-activated EGFR complexes were detected only as receptor dimers with ring-like conformations. Zalutumumab, a therapeutic inhibitory EGFR antibody directed against domain III, locked EGFR molecules into a very compact, inactive conformation. Biochemical analyses showed bivalent binding of zalutumumab to provide potent inhibition of EGFR signaling. The structure of EGFR-zalutumumab complexes on the cell surface visualized by Protein Tomography indicates that the cross-linking spatially separates the EGFR molecules' intracellular kinase domains to an extent that appears incompatible with the induction of signaling. These insights into the mechanisms of action of receptor inhibition may also apply to other cell-surface tyrosine kinase receptors of the ErbB family.

Synergy between cetuximab and chemotherapy in tumors of the gastrointestinal tract

Cetuximab is a recently approved monoclonal antibody that targets the epidermal growth factor receptor, a receptor tyrosine kinase involved in the development and progression of colorectal cancer (CRC) and other solid tumors. Cetuximab, as a single agent or in combination with chemotherapy, has demonstrated significant clinical efficacy against CRC. Combinations of cetuximab with chemotherapy have proven to be well tolerated, with minimal overlap of toxicities between agents; and the anticancer synergy between cetuximab and traditional chemotherapy agents has made cetuximab a vital treatment for patients who are no longer responsive to chemotherapy alone. The U.S. Food and Drug Administration approved cetuximab in combination with irinotecan for the treatment of irinotecan-refractory metastatic CRC or as monotherapy for treating patients intolerant to irinotecan. Combination chemotherapies involving cetuximab as well as combinations involving cetuximab and other targeted agents, such as bevacizumab, an anti-vascular endothelial growth factor monoclonal antibody, constitute powerful new treatment options for the management of CRC. This review discusses recent clinical studies that have further defined this synergy, focusing primarily on tumors of the gastrointestinal tract.

Structural basis for EGF receptor inhibition by the therapeutic antibody IMC-11F8

Therapeutic anticancer strategies that target and inactivate the epidermal growth factor receptor (EGFR) are under intense study in the clinic. Here we describe the mechanism of EGFR inhibition by an antibody drug IMC-11F8. IMC-11F8 is a fully human antibody that has similar antitumor potency as the chimeric cetuximab/Erbitux and might represent a safer therapeutic alternative. We report the X-ray crystal structure of the Fab fragment of IMC-11F8 (Fab11F8) in complex with the entire extracellular region and with isolated domain III of EGFR. We compare this to our previous study of the cetuximab/EGFR interaction. Fab11F8 interacts with a remarkably similar epitope, but through a completely different set of interactions. Both the similarities and differences in binding of these two antibodies have important implications for the development of inhibitors that could exploit this same mechanism of EGFR inhibition.

A structural model of the extracellular domain of the oncogenic EGFR variant Xmrk

The epidermal growth factor receptor (EGFR) variant Xmrk of the Mexican fish Xiphophorus is one of the first oncogenes described. Its overexpression in pigment cells of the skin occurs after certain repeated cross-breedings between the platyfish Xiphophorus maculatus and the swordtail Xiphophorus hellerii. Xmrk overexpression results in melanoma with a high malignant potential. The Xiphophorus melanoma model provides a valuable tool for the understanding of melanoma development in general. The xmrk gene is the duplicated version of the proto-oncogene egfr-b. Compared to this, it contains 14 nonsynonymous mutations. Two of these mutations lead to a permanent protein dimerization which results in constitutive signaling. For a better understanding of the structural conditions responsible for this effect, the authors modeled the extracellular domain of Egfr-b, based on the crystal structure of the closely related human EGFR, and introduced the mutations G336R or C555S. The resulting protein models show that in either mutation the formation of an intramolecular disulfide bridge is very likely prevented, leading to an intermolecular disulfide bridge between released cysteine residues. In the G336R model, the distance between the membrane-proximal domains from both chains is smaller than in the C555S model, confirming the previous observation than Egfr-b-G336R is more tumorigenic than Egfr-b-C555S in vivo.

A betacellulin mutant promotes differentiation of pancreatic acinar AR42J cells into insulin-producing cells with low affinity of binding to ErbB1

Betacellulin (BTC) is one of the members of the epidermal growth factor (EGF) ligand family of ErbB receptor tyrosine kinases. It is a differentiation factor as well as a potent mitogen. BTC promotes the differentiation of pancreatic acinar-derived AR42J cells into insulin-producing cells. It independently and preferentially binds to two type I tyrosine kinase receptors, the EGF receptor (ErbB1) and ErbB4. However, the physiochemical characteristics of BTC that are responsible for its preferential binding to these two receptors have not been fully defined. In this study, to investigate the essential amino acid residues of BTC for binding to the two receptors, we introduced point mutations into the EGF domain of BTC employing error-prone PCR. The receptor binding abilities of 190 mutants expressed in Escherichia coli were assessed by enzyme immunoassay. Replacement of the glutamic acid residue at position 88 with a lysine residue in BTC was found to produce a significant loss of affinity for binding to ErbB1, while the affinity of binding to ErbB4 was unchanged. In addition, the mutant of BTC-E/88/K showed less growth-promoting activity on BALB/c 3T3 cells compared with that of the wild-type BTC protein. Interestingly, the BTC mutant protein promoted differentiation of pancreatic acinar AR42J cells at a high frequency into insulin-producing cells compared with AR42J cells that were treated with wild-type BTC protein. These results indicate the possibility of designing BTC mutants, which have an activity of inducing differentiation only, without facilitating growth promotion.

Matuzumab binding to EGFR prevents the conformational rearrangement required for dimerization

An increasing number of therapeutic antibodies targeting tumors that express the epidermal growth factor receptor (EGFR) are in clinical use or late stages of clinical development. Here we investigate the molecular basis for inhibition of EGFR activation by the therapeutic antibody matuzumab (EMD72000). We describe the X-ray crystal structure of the Fab fragment of matuzumab (Fab72000) in complex with isolated domain III from the extracellular region of EGFR. Fab72000 interacts with an epitope on EGFR that is distinct from the ligand-binding region on domain III and from the cetuximab/Erbitux epitope. Matuzumab blocks ligand-induced receptor activation indirectly by sterically preventing the domain rearrangement and local conformational changes that must occur for high-affinity ligand binding and receptor dimerization.

Insight into the recognition patterns of the ErbB receptor family transmembrane domains: heterodimerization models through molecular dynamics search

ErbB receptors undergo a complex interaction network defining hierarchical and competition relationships. Dimerization is driven entirely by receptor-receptor interactions and the transmembrane domains play a role in modulating the specificity and the selection of the partners during signal transduction. To shed light on the role of the GxxxG-like dimerization motifs in the formation of ErbB transmembrane heterodimers, we propose structural models resulting from conformational search method combined with molecular dynamics simulations. Left-handed structures of the transmembrane heterodimers are found preponderant over right-handed structures. All heterotypic heterodimers undergo two modes of association either via the N-terminal motif or the C-terminal motif. The transmembrane domain of ErbB3 impairs this C-terminal motif but also associates with the other partners owing to the presence of Gly residues. The two dimerization modes involve different orientations of the two helices. Thus, a molecular-switch model allowing the transition between the two dimerizing states may apply to the heterodimers and could help interpret receptor competition for the formation of homodimers and heterodimers. The comparison between experimental and theoretical results on the dimerization hierarchy of the transmembrane domains is not straightforward. However, we demonstrate that the intrinsic properties of the transmembrane sequences are an important component in heterodimer formation and that the ErbB2 and ErbB3 transmembrane domains have a strong power for heterodimerization as observed experimentally.

Epidermal growth factor receptor (EGFR) is transcriptionally induced by the Y-box binding protein-1 (YB-1) and can be inhibited with Iressa in basal-like breast cancer, providing a potential target for therapy

Introduction

Basal-like breast cancers (BLBCs) are very aggressive, and present serious clinical challenges as there are currently no targeted therapies available. We determined the regulatory role of Y-box binding protein-1 (YB-1) on epidermal growth factor receptor (EGFR) overexpression in BLBC, and the therapeutic potential of inhibiting EGFR. We pursued this in light of our recent work showing that YB-1 induces the expression of EGFR, a new BLBC marker.

Methods

Primary tumour tissues were evaluated for YB1 protein expression by immunostaining tissue microarrays, while copy number changes were assessed by comparative genomic hybridization (CGH). The ability of YB-1 to regulate EGFR was evaluated using luciferase reporter, chromatin immunoprecipitation (ChIP) and gel shift assays. The impact of Iressa on monolayer cell growth was measured using an ArrayScan VTI high-throughput analyser to count cell number, and colony formation in soft agar was used to measure anchorage-independent growth.

Results

YB-1 (27/37 or 73% of cases, P = 3.899 × 10^-4) and EGFR (20/37 or 57.1% of cases, P = 9.206 × 10^-12) are expressed in most cases of BLBC. However, they are not typically amplified in primary BLBC, suggesting overexpression owing to transcriptional activation. In support of this, we demonstrate that YB-1 promotes EGFR reporter activity. YB-1 specifically binds the EGFR promoter at two different YB-1-responsive elements (YREs) located at -940 and -968 using ChIP and gel shift assays in a manner that is dependent on the phosphorylation of S102 on YB-1. Inhibiting EGFR with Iressa suppressed the growth of SUM149 cells by ~40% in monolayer, independent of mutations in the receptor. More importantly anchorage-independent growth of BLBC cell lines was inhibited with combinations of Iressa and YB-1 suppression.

Conclusion

We have identified for the first time a causal link for the expression of EGFR in BLBC through the induction by YB-1 where it binds specifically to two distinguished YREs. Finally, inhibition of EGFR in combination with suppression of YB-1 presents a potential opportunity for therapy in BLBC.

Biochemical basis for the functional switch that regulates hepatocyte growth factor receptor tyrosine kinase activation

Ligand-induced dimerization of receptor tyrosine kinases (RTKs) modulates a system of linked biochemical reactions, sharply switching the RTK from a quiescent state to an active state that becomes phosphorylated and triggers intracellular signaling pathways. To improve our understanding of this molecular switch, we developed a quantitative model for hepatocyte growth factor receptor (c-MET) activation using parameters derived in large part from c-MET kinetic and thermodynamic experiments. Our model accurately produces the qualitative and quantitative dynamic features of c-MET phosphorylation observed in cells following ligand binding, including a rapid transient buildup of phosphorylated c-MET at high ligand concentrations. In addition, our model predicts a slow buildup of phosphorylated c-MET under conditions of reduced phosphatase activity and no extracellular agonist. Significantly, this predicted response is observed in cells treated with phosphatase inhibitors, further validating our model. Parameter sensitivity studies clearly show that synergistic oligomerization-dependent changes in c-MET kinetic, thermodynamic, and dephosphorylation properties result in the selective activation of the dimeric receptor, confirming that this model can be used to accurately evaluate the relative importance of linked biochemical reactions important for c-MET activation. Our model suggests that the functional differences observed between c-MET monomers and dimers may have incrementally evolved to optimize cell surface signaling responses.

ErbB receptors, their ligands, and the consequences of their activation and inhibition in the myocardium

The epidermal growth factor (EGF) receptor (or ErbB1) and the related ErbB4 are transmembrane receptor protein tyrosine kinases which bind extracellular ligands of the EGF family. ErbB2 and ErbB3 are "co-receptors" structurally related to ErbB1/ErbB4, but ErbB2 is an "orphan" receptor and ErbB3 lacks tyrosine kinase activity. However, both are important in transmembrane signalling. All ErbB receptors/ligands are intimately involved in the regulation of cell growth, differentiation and survival, and their dysregulation contributes to some human malignancies. After extracellular ligand binding, receptor dimerisation and transautophosphorylation of intracellular C-terminal tyrosine residues, they bind signalling proteins which recognise specific tyrosine-phosphorylated motifs. This leads to activation of multiple signalling pathways, notably the extracellular signal-regulated kinase 1/2 (ERK1/2) cascade and the phosphoinositide 3-kinase (PI3K)/protein kinase B [PKB/(Akt)] pathway. In heart, targeted deletion of ErbB2, ErbB3, ErbB4 and some ErbB receptor extracellular ligands leads to embryonic lethality resulting from cardiovascular defects. ErbB receptor ligands improve cardiac myocyte viability and are hypertrophic, partly because of activation of ERK1/2 and/or PI3K/PKB(Akt). Furthermore, ErbB transactivation by Gq protein-coupled receptor (GqPCR) signalling may mediate the hypertrophic effects of GqPCR agonists. The utility of anthracyclines in cancer chemotherapy can be limited by their cardiotoxic side effects and these may be counteracted by ErbB receptor ligands. ErbB2 is the target of anti-cancer monoclonal antibody trastuzumab (Herceptin), and its myocardial downregulation may account for the occasional cardiotoxicity of this therapy. Here, we review the basic biochemistry of ErbB receptors/ligands, and emphasise their particular roles in the myocardium.

Endocytic downregulation of ErbB receptors: mechanisms and relevance in cancer

ErbB receptors (EGFR (ErbB1), ErbB2, ErbB3, and ErbB4) are important regulators of normal growth and differentiation, and they are involved in the pathogenesis of cancer. Following ligand binding and receptor activation, EGFR is endocytosed and transported to lysosomes where the receptor is degraded. This downregulation of EGFR is a complex and tightly regulated process. The functions of ErbB2, ErbB3, and ErbB4 are also regulated by endocytosis to some extent, although the current knowledge of these processes is sparse. Impaired endocytic downregulation of signaling receptors is frequently associated with cancer, since it can lead to increased and uncontrolled receptor signaling. In this review we describe the current knowledge of ErbB receptor endocytic downregulation. In addition, we outline how ErbB receptors can escape endocytic downregulation in cancer, and we discuss how targeted anti-cancer therapy may induce endocytic downregulation of ErbB receptors.

Impact of common epidermal growth factor receptor and HER2 variants on receptor activity and inhibition by lapatinib

The goal of this study was to characterize the effects of non-small cell lung carcinoma (NSCLC)-associated mutations in epidermal growth factor receptor (EGFR/ErbB1) and HER2 (ErbB2) on interactions with the dual tyrosine kinase inhibitor lapatinib. Biochemical studies show that commonly observed variants of EGFR [G719C, G719S, L858R, L861Q, and Delta746-750 (del15)] are enzyme activating, increasing the tyrosine kinase V(max) and increasing the K(m)((app)) for ATP. The point mutations G719C and L861Q had minor effects on lapatinib K(i)s, whereas EGFR mutations L858R and del15 had a higher K(i) for lapatinib than wild-type EGFR. Structural analysis of wild-type EGFR-lapatinib complexes and modeling of the EGFR mutants were consistent with these data, suggesting that loss of structural flexibility and possible stabilization of the active-like conformation could interfere with lapatinib binding, particularly to the EGFR deletion mutants. Furthermore, EGFR deletion mutants were relatively resistant to lapatinib-mediated inhibition of receptor autophosphorylation in recombinant cells expressing the variants, whereas EGFR point mutations had a modest or no effect. Of note, EGFR T790M, a receptor variant found in patients with gefitinib-resistant NSCLC, was also resistant to lapatinib-mediated inhibition of receptor autophosphorylation. Two HER2 insertional variants found in NSCLC were less sensitive to lapatinib inhibition than two HER2 point mutants. The effects of lapatinib on the proliferation of human NSCLC tumor cell lines expressing wild-type or variant EGFR and HER2 cannot be explained solely on the basis of the biochemical activity or receptor autophosphorylation in recombinant cells. These data suggest that cell line genetic heterogeneity and/or multiple determinants modulate the role played by EGFR/HER2 in regulating cell proliferation.

Differential binding patterns of monoclonal antibody 2C4 to the ErbB3-p185her2/neu and the EGFR-p185her2/neu complexes

2C4 (Pertuzumab, Omnitarg) is a monoclonal antibody targeting p185(her2/neu), which is overexpressed in 30% of invasive breast cancer. 2C4 is currently in phase II clinical trials for several types of cancers. This antibody has been reported to disrupt the association between p185(her2/neu) and ErbB3. In our studies of epidermal growth factor receptor (EGFR)-p185(her2/neu) heterodimerization, we noted that 2C4 formed associations with the EGFR-p185(her2/neu) receptor complex. Our data argue against 2C4 as a universal heterodimerization blocker for p185(her2/neu), but indicate that cocktails of monoclonal antibodies binding distinct interaction surfaces of p185(her2/neu) will emerge as the most potent targeted therapy.

Enhancement of cell type specificity by quantitative modulation of a chimeric ligand

Evolution modulates the quantitative characteristics of protein interactions and often uses combinations of weak interactions to achieve a particular specificity. We addressed how quantitative optimization might be used in the design of multidomain proteins, using a chimera containing epidermal growth factor (EGF) as a cell targeting element and interferon-alpha-2a (IFNalpha-2a) to initiate signal transduction. We first connected EGF and IFNalpha-2a via a linker that allows both ligands to bind to their receptors on a cell surface and then incorporated a series of mutations into the IFNalpha-2a portion that progressively decrease both the on rate and the dissociation constant of the IFNalpha-2a-IFNalpha receptor 2 (IFNAR2) interaction. Using this strategy, we designed chimeric proteins in which the activation of the IFNalpha receptor in HeLa, A431, and engineered Daudi cells depends on the presence of EGF receptor on the same cell. The mutant chimeric proteins also inhibited proliferation of IFNalpha-sensitive cells in an EGF receptor-dependent manner. These results provide insights into the quantitative requirements for specific binding to multisubunit receptors and illustrate the value of a quantitative approach in the design of synthetic-biological constructs.

The origin of protein interactions and allostery in colocalization

Two fundamental principles can account for how regulated networks of interacting proteins originated in cells. These are the law of mass action, which holds that the binding of one molecule to another increases with concentration, and the fact that the colocalization of molecules vastly increases their local concentrations. It follows that colocalization can amplify the effect on one protein of random mutations in another protein and can therefore, through natural selection, lead to interactions between proteins and to a startling variety of complex allosteric controls. It also follows that allostery is common and that homologous proteins can have different allosteric mechanisms. Thus, the regulated protein networks of organisms seem to be the inevitable consequence of natural selection operating under physical laws.

The kinetics of the hydrogen/deuterium exchange of epidermal growth factor receptor ligands

Five highly homologous epidermal growth factor receptor ligands were studied by mass spectral analysis, hydrogen/deuterium (H/D) exchange via attenuated total reflectance Fourier transform-infrared spectroscopy, and two-dimensional correlation analysis. These studies were performed to determine the order of events during the exchange process, the extent of H/D exchange, and associated kinetics of exchange for a comparative analysis of these ligands. Furthermore, the secondary structure composition of amphiregulin (AR) and heparin-binding-epidermal growth factor (HB-EGF) was determined. All ligands were found to have similar contributions of 3(10)-helix and random coil with varying contributions of beta-sheets and beta-turns. The extent of exchange was 40%, 65%, 55%, 65%, and 98% for EGF, transforming growth factor-alpha (TGF-alpha), AR, HB-EGF, and epiregulin (ER), respectively. The rate constants were determined and classified as fast, intermediate, and slow: for EGF the 0.20 min(-1) (Tyr), 0.09 min(-1) (Arg, beta-turns), and 1.88 x 10(-3) min(-1) (beta-sheets and 3(10)-helix); and for TGF-alpha 0.91 min(-1) (Tyr), 0.27 min(-1) (Arg, beta-turns), and 1.41 x 10(-4) min(-1) (beta-sheets). The time constants for AR 0.47 min(-1) (Tyr), 0.04 min(-1) (Arg), and 1.00 x 10(-4) min(-1) (buried 3(10)-helix, beta-turns, and beta-sheets); for HB-EGF 0.89 min(-1) (Tyr), 0.14 min(-1) (Arg and 3(10)-helix), and 1.00 x 10(-3) min(-1) (buried 3(10)-helix, beta-sheets, and beta-turns); and for epiregulin 0.16 min(-1) (Tyr), 0.03 min(-1) (Arg), and 1.00 x 10(-4) min(-1) (3(10)-helix and beta-sheets). These results provide essential information toward understanding secondary structure, H/D exchange kinetics, and solvation of these epidermal growth factor receptor ligands in their unbound state.

Heterogeneity in EGF-binding affinities arises from negative cooperativity in an aggregating system

Scatchard analysis of the binding of EGF to its receptor yields concave up plots that indicate the presence of two classes of binding sites. However, how two independent classes of sites arise from the expression of a single EGF receptor protein has never been adequately explained. Using a new analytical approach involving the simultaneous fitting of binding isotherms from cells expressing increasing levels of EGF receptors, we show that (125)I-EGF-binding data can be completely explained by a model involving negative cooperativity in an aggregating system. This approach provides an experimentally determined value for the monomer-dimer equilibrium constant, which, for wild-type EGF receptors, corresponds to approximately 50,000 receptors per cell. Therefore, changes in receptor expression within the physiological range can modulate the outcome of a signaling stimulus. Analysis of the L680N-EGF receptor mutant, in which the formation of asymmetric kinase domain dimers is blocked, indicates that the kinase dimers make a substantial energetic contribution to the ligand-independent association of EGF receptor monomers, but are not necessary for negative cooperativity. The model accurately predicts the behavior of receptor mutants, such as the dimerization-defective Y246D-EGF receptor, which exhibit a single class of binding sites and provides a framework for understanding secondary dimer formation and lateral signaling in the EGF receptor family.

Keratinocyte growth factor receptors

Modulation of the number of functional growth factor receptors on the epithelial cell surface that is exposed to the action of cognate ligands represents a key strategy in cellular physiology to regulate the proliferation rate and the differentiation process. The keratinocyte growth factor receptor (KGFR) and the epidermal growth factor receptor (EGFR), among the growth factor receptors expressed on keratinocytes, are believed to play a unique crucial role in controlling epithelial proliferation. KGFR and EGFR appear to also contribute to the cell differentiation process. Modulation of KGFR and EGFR on the proliferation rate and differentiation process has been reported either in in vivo or in vitro conditions. This article reviews the architecture, the ligand binding activated-signaling pathways, and the biologic effects of KGFR and EGFR on keratinocytes.

Evolvable signaling networks of receptor tyrosine kinases: relevance of robustness to malignancy and to cancer therapy

Robust biological signaling networks evolved, through gene duplications, from simple, relatively fragile cascades. Architectural features such as layered configuration, branching and modularity, as well as functional characteristics (e.g., feedback control circuits), enable fail-safe performance in the face of internal and external perturbations. These universal features are exemplified here using the receptor tyrosine kinase (RTK) family. The RTK module is richly mutated and overexpressed in human malignancies, and pharmaceutical interception of its signaling effectively retards growth of specific tumors. Therapy-induced interception of RTK-signaling pathways and the common evolvement of drug resistance are respectively considered here as manifestations of fragility and plasticity of robust networks. The systems perspective we present views pathologies as hijackers of biological robustness and offers ways for identifying fragile hubs, as well as strategies to overcome drug resistance.

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.

Reaching out for signals: filopodia sense EGF and respond by directed retrograde transport of activated receptors

ErbB1 receptors situated on cellular filopodia undergo systematic retrograde transport after binding of the epidermal growth factor (EGF) and activation of the receptor tyrosine kinase. Specific inhibitors of the erbB1 receptor tyrosine kinase as well as cytochalasin D, a disruptor of the actin cytoskeleton, abolish transport but not free diffusion of the receptor–ligand complex. Diffusion constants and transport rates were determined with single molecule sensitivity by tracking receptors labeled with EGF conjugated to fluorescent quantum dots. Retrograde transport precedes receptor endocytosis, which occurs at the base of the filopodia. Initiation of transport requires the interaction and concerted activation of at least two liganded receptors and proceeds at a constant rate mediated by association with actin. These findings suggest a mechanism by which filopodia detect the presence and concentration of effector molecules far from the cell body and mediate cellular responses via directed transport of activated receptors.

Aplysia temptin - the 'glue' in the water-borne attractin pheromone complex

Temptin, a component of the complex of water-borne protein pheromones that stimulate attraction and mating behavior in the marine mollusk Aplysia, has sequence homology to the epidermal growth factor (EGF)-like domains of higher organisms that mediate protein-cell surface contact during fertilization and blood coagulation. In this work, recombinant temptin for structural and functional studies was produced in Escherichia coli using a cold shock promoter and purified by RP-HPLC. CD spectra confirmed a predominantly beta-sheet structure. Two disulfide bonds were determined via limited proteolysis and MS. One internal disulfide (Cys57-Cys77) was predicted from initial alignments with class I EGF-like domains; the second, between Cys18 and Cys103, could protect temptin against proteolysis in seawater and stabilize its interacting surface. A three-dimensional model of temptin was prepared with our MPACK suite, based on the Ca(2+)-binding, EGF-like domain of the extracellular matrix protein fibrillin. Two temptin residues, Trp52 and Trp79, which align with cysteine residues conserved in fibrillins, lie adjacent to and could stabilize the disulfide bonds and a proposed metal-binding loop. The water-borne pheromone attractin in egg cordon eluates is complexed with other proteins. Docking results with our model and the NMR structure of attractin suggest that one face of temptin interacts with the pheromone, perhaps controlling its access to the cellular receptors. Gel shifts confirmed that temptin complexes with wild-type attractin. These results indicate that temptin, analogous to the role of fibrillin in controlling transforming growth factor-beta concentration, modulates pheromone signaling by direct binding to attractin.

Single-molecule imaging and fluorescence lifetime imaging microscopy show different structures for high- and low-affinity epidermal growth factor receptors in A431 cells

Epidermal growth factor (EGF) receptor (EGFR) modulates mitosis and apoptosis through signaling by its high-affinity (HA) and low-affinity (LA) EGF-binding states. The prevailing model of EGFR activation-derived from x-ray crystallography-involves the transition from tethered ectodomain monomers to extended back-to-back dimers and cannot explain these EGFR affinities or their different functions. Here, we use single-molecule Förster resonant energy transfer analysis in combination with ensemble fluorescence lifetime imaging microscopy to investigate the three-dimensional architecture of HA and LA EGFR-EGF complexes in cells by measuring the inter-EGF distances within discrete EGF pairs and the vertical distance from EGF to the plasma membrane. Our results show that EGFR ectodomains form interfaces resulting in two inter-EGF distances ( approximately 8 nm and < 5.5 nm), different from the back-to-back EGFR ectodomain interface ( approximately 11 nm). Distance measurements from EGF to the plasma membrane show that HA EGFR ectodomains are oriented flat on the membrane, whereas LA ectodomains stand proud from it. Their flat orientation confers on HA EGFR ectodomains the exclusive ability to interact via asymmetric interfaces, head-to-head with respect to the EGF-binding site, whereas LA EGFRs must interact only side-by-side. Our results support a structural model in which asymmetric EGFR head-to-head interfaces may be relevant for HA EGFR oligomerization.

A mechanistic basis for converting a receptor tyrosine kinase agonist to an antagonist

Hepatocyte growth factor (HGF) activates the Met receptor tyrosine kinase by binding and promoting receptor dimerization. Here we describe a mechanistic basis for designing Met antagonists based on NK1, a natural variant of HGF containing the N-terminal and the first kringle domain. Through detailed biochemical and structural analyses, we demonstrate that both mouse and human NK1 induce Met dimerization via a conserved NK1 dimer interface. Mutations designed to alter the NK1 dimer interface abolish its ability to promote Met dimerization but retain full Met-binding activity. Importantly, these NK1 mutants act as Met antagonists by inhibiting HGF-mediated cell scattering, proliferation, branching, and invasion. The ability to separate the Met-binding activity of NK1 from its Met dimerization activity thus provides a rational basis for designing Met antagonists. This strategy of antagonist design may be applicable for other growth factor receptors by selectively abolishing the receptor activation ability but not the receptor binding of the growth factors.

Peptide vaccines of the HER-2/neu dimerization loop are effective in inhibiting mammary tumor growth in vivo

Human epidermal growth factor receptor-2 (HER-2)/neu (ErbB2), a member of the epidermal growth factor family of receptors, is overexpressed in 20-30% of breast cancers. It is an attractive target for receptor-directed antitumor therapy using mAbs. Unlike other epidermal growth factor receptor family members, HER-2/neu does not bind a high-affinity ligand, but rather functions as the preferred dimerization partner. Pertuzumab (Omnitarg) is a humanized mAb directed against the HER-2/neu dimerization domain that inhibits receptor signaling. The recent definition of the crystal structure of the HER-2/neu-pertuzumab complex demonstrated that the receptor dimerization region encompassed residues 266-333. Based on the three-dimensional structure of the complex, we have designed three conformational peptide constructs (sequences 266-296, 298-333, and 315-333) to mimic regions of the dimerization loop of the receptor and to characterize their in vitro and in vivo antitumor efficacy. All the constructs elicited high-affinity peptide Abs that inhibited multiple signaling pathways including HER-2/neu-specific inhibition of cellular proliferation and cytoplasmic receptor domain phosphorylation. All the peptide Abs showed Ab-dependent cellular cytotoxicity to varying degrees with the 266-296 constructs being equally effective as compared with Herceptin. The 266-296 peptide vaccine had statistically reduced tumor onset in both transplantable tumor models (FVB/n and BALB/c) and significant reduction in tumor development in two transgenic mouse tumor models (BALB-neuT and VEGF(+/-)Neu2-5(+/-)). The 266-296 construct represents the most promising candidate for antitumor vaccination and could also be used to treat a variety of cancers with either normal or elevated expression of HER-2 including breast, lung, ovarian, and prostate.

Oligomerization of the EGF receptor investigated by live cell fluorescence intensity distribution analysis

Recent evidence suggests that the EGF receptor oligomerizes or clusters in cells even in the absence of agonist ligand. To assess the status of EGF receptors in live cells, an EGF receptor fused to eGFP was stably expressed in CHO cells and studied using fluorescence correlation spectroscopy and fluorescent brightness analysis. By modifying FIDA for use in a two-dimensional system with quantal brightnesses, a method was developed to quantify the degree of clustering of the receptors on the cell surface. The analysis demonstrates that under physiological conditions, the EGF receptor exists in a complex equilibrium involving single molecules and clusters of two or more receptors. Acute depletion of cellular cholesterol enhanced EGF receptor clustering whereas cholesterol loading decreased receptor clustering, indicating that receptor aggregation is sensitive to the lipid composition of the membrane.

ErbB receptors: from oncogenes to targeted cancer therapies

Understanding the genetic origin of cancer at the molecular level has facilitated the development of novel targeted therapies. Aberrant activation of the ErbB family of receptors is implicated in many human cancers and is already the target of several anticancer therapeutics. The use of mAbs specific for the extracellular domain of ErbB receptors was the first implementation of rational targeted therapy. The cytoplasmic tyrosine kinase domain is also a preferred target for small compounds that inhibit the kinase activity of these receptors. However, current therapy has not yet been optimized, allowing for opportunities for optimization of the next generation of targeted therapy, particularly with regards to inhibiting heteromeric ErbB family receptor complexes.

Ligand-Induced Structural Transitions in ErbB Receptor Extracellular Domains

Crystallographic studies showed that epidermal growth factor (EGF) receptor activation involves major domain rearrangements. Without bound ligand, the extracellular region of the receptor (sEGFR) adopts a "tethered" configuration with its dimerization site occluded by apparently autoinhibitory intramolecular interactions. Ligand binding causes the receptor to become "extended," breaking the tether and exposing the dimerization site. Using small-angle X-ray scattering (SAXS), we confirm that the tethered and extended conformations are also adopted in solution, and we describe low-resolution molecular envelopes for an intact sEGFR dimer. We also use SAXS to monitor directly the transition from a tethered to extended configuration in the monomeric extracellular regions of ErbB3 and a dimerization-defective EGFR mutant. Finally, we show that mutating every intramolecular tether interaction in sEGFR does not greatly alter its conformation. These findings explain why tether mutants fail to activate EGF receptor and provide new insight into regulation of ErbB receptor conformation.

Mechanisms for oncogenic activation of the epidermal growth factor receptor

The Epidermal growth factor receptor (EGFR) is a membrane spanning glycoprotein, which frequently has been implicated in various cancer types. The mechanisms by which EGFR becomes oncogenic are numerous and are often specific for each cancer type. In some tumors, EGFR is activated by autocrine/paracrine growth factor loops, whereas in others activating mutations promote EGFR signaling. Overexpression and/or amplification of the EGFR gene are prevalent in many cancer types leading to aberrant EGFR signaling. In addition, failure to attenuate receptor signaling by receptor downregulation can also lead to cellular transformation. Heterodimerization of EGFR with ErbB2 inhibits downregulation of EGFR and thereby prolongs growth factor signaling. This also indicates that cross-talk between EGFR and heterologous receptor systems serves as another mechanism for oncogenic activation of EGFR. Because of its role in tumor promotion, the EGFR has been intensely studied as a therapeutic target. There are currently two major mechanisms by which the EGFR is targeted: antibodies binding to the extracellular domain of EGFR and small-molecule tyrosine-kinase inhibitors. However, tumorigenesis is a multi-step process involving several mutations, which might explain why EGFR therapeutics has only been partially successful. This highlights the importance of pinpointing the mechanisms by which EGFR becomes oncogenic in a particular cancer. In this review, each of the above mentioned mechanisms will be discussed, as a detailed molecular and genetic understanding of how EGFR contributes to the malignant phenotype might offer new promise for the design, development and clinical evaluation of future tumor-specific anticancer approaches.

Her signaling in pancreatic cancer

Pancreatic cancer remains a treatment-refractory cancer. Standard therapy for metastatic cancer is gemcitabine chemotherapy, with a median survival of 5-6 months. The epidermal growth factor receptor (EGFR) is commonly expressed in pancreatic cancer and has been evaluated as a therapeutic target. A Phase III trial of gemcitabine with or without the EGFR inhibitor, erlotinib, demonstrated a modest but significant prolongation of survival with the addition of erlotinib. A Phase II trial of gemcitabine with the anti-EGFR antibody cetuximab resulted in a 1-year survival of 32%. A Phase III trial of gemcitabine with or without cetuximab and a randomized Phase II trial of the Murren regimen with or without cetuximab are completed; results for both are anticipated in 2007. A Phase I trial of gemcitabine with the EGFR/Her-2 inhibitor, lapatinib, is completed. Improved patient selection and rational combination of targeted therapies will be necessary to optimize the management of patients with this tragic disease.

Tenascin cytotactin epidermal growth factor-like repeat binds epidermal growth factor receptor with low affinity

Select epidermal growth factor (EGF)-like (EGFL) repeats of human tenascin cytotactin (tenascin C) can stimulate EGF receptor (EGFR) signaling, but activation requires micromolar concentrations of soluble EGFL repeats in contrast to subnanomolar concentrations of classical growth factors such as EGF. Using in silico homology modeling techniques, we generated a structure for one such repeat, the 14th EGFL repeat (Ten14). Ten14 assumes a tight EGF-like fold with truncated loops, consistent with circular dichroism studies. We generated bound structures for Ten14 with EGFR using two different approaches, resulting in two distinctly different conformations. Normal mode analysis of both structures indicated that the binding pocket of EGFR exhibits a significantly higher mobility in Ten14-EGFR complex compared to that of the EGF-EGFR complex; we hypothesized this may be attributed to loss of key high-affinity interactions within the Ten14-EGFR complex. We proved the efficacy of our in silico models by in vitro experiments. Surface plasmon resonance measurements yielded equilibrium constant K(D) of 74 microM for Ten14, approximately three orders of magnitude weaker than that of EGF. In accordance with our predicted bound models, Ten14 in monomeric form does not bind EGFR with sufficient stability so as to induce degradation of receptor, or undergo EGFR-mediated internalization over either the short (20 min) or long (48 h) term. This transient interaction with the receptor on the cell surface is in marked contrast to other EGFR ligands which cause EGFR transit through, and signaling from intracellular locales in addition to cell surface signaling.

Cancer cells assemble and align gold nanorods conjugated to antibodies to produce highly enhanced, sharp, and polarized surface Raman spectra: a potential cancer diagnostic marker

Human oral cancer cells are found to assemble and align gold nanorods conjugated to anti-epidermal growth factor receptor (anti-EGFR) antibodies. Immnoconjugated gold nanorods and nanospheres were shown previously to exhibit strong Rayleigh (Mie) scattering useful for imaging. In the present letter, molecules near the nanorods on the cancer cells are found to give a Raman spectrum that is greatly enhanced (due to the high surface plasmon field of the nanorod assembly in which their extended surface plasmon fields overlap), sharp (due to a homogeneous environment), and polarized (due to anisotropic alignments). These observed properties can be used as diagnostic signatures for cancer cells.

HER2 therapy. HER2 (ERBB2): functional diversity from structurally conserved building blocks

EGFR-type receptor tyrosine kinases achieve a broad spectrum of cellular responses by utilizing a set of structurally conserved building blocks. Based on available crystal structures and biochemical information, significant new insights have emerged into modes of receptor control, its deregulation in cancer, and the nuances that differentiate the four human receptors. This review gives an overview of current models of the control of receptor activity with a special emphasis on HER2 and HER3.

Complementation Analysis Demonstrates That Insulin Cross-links Both α Subunits in a Truncated Insulin Receptor Dimer

The insulin receptor is a homodimer composed of two alphabeta half receptors. Scanning mutagenesis studies have identified key residues important for insulin binding in the L1 domain (amino acids 1-150) and C-terminal region (amino acids 704-719) of the alpha subunit. However, it has not been shown whether insulin interacts with these two sites within the same alpha chain or whether it cross-links a site from each alpha subunit in the dimer to achieve high affinity binding. Here we have tested the contralateral binding mechanism by analyzing truncated insulin receptor dimers (midi-hIRs) that contain complementary mutations in each alpha subunit. Midi-hIRs containing Ala(14), Ala(64), or Gly(714) mutations were fused with Myc or FLAG epitopes at the C terminus and were expressed separately by transient transfection. Immunoblots showed that R14A+FLAG, F64A+FLAG, and F714G+Myc mutant midi-hIRs were expressed in the medium but insulin binding activity was not detected. However, after co-transfection with R14A+FLAG/F714G+Myc or F64A+FLAG/F714G+Myc, hybrid dimers were obtained with a marked increase in insulin binding activity. Competitive displacement assays revealed that the hybrid mutant receptors bound insulin with the same affinity as wild type and also displayed curvilinear Scatchard plots. In addition, when hybrid mutant midi-hIR was covalently cross-linked with (125)I(A14)-insulin and reduced, radiolabeled monomer was immunoprecipitated only with anti-FLAG, demonstrating that insulin was bound asymmetrically. These results demonstrate that a single insulin molecule can contact both alpha subunits in the insulin receptor dimer during high affinity binding and this property may be an important feature for receptor signaling.

Investigation of the dimerization of proteins from the epidermal growth factor receptor family by single wavelength fluorescence cross-correlation spectroscopy

Single wavelength fluorescence cross-correlation spectroscopy (SW-FCCS), introduced to study biomolecular interactions, has recently been reported to monitor enzyme activity by using a newly developed fluorescent protein variant together with cyan fluorescent protein. Here, for the first time to our knowledge, SW-FCCS is applied to detect interactions between membrane receptors in vivo by using the widely used enhanced green fluorescent protein and monomeric red fluorescent protein. The biological system studied here is the epidermal growth factor/ErbB receptor family, which plays pivotal roles in the development of organisms ranging from worms to humans. It is widely thought that a ligand binds to the monomeric form of the receptor and induces its dimeric form for activation. By using SW-FCCS and Förster resonance energy transfer, we show that the epidermal growth factor receptor and ErbB2 have preformed homo- and heterodimeric structures on the cell surface and quantitation of dimer fractions is performed by SW-FCCS. These receptors are major targets of anti-cancer drug development, and the receptors' homo- and heterodimeric structures are relevant for such developments.

The Asn418-linked N-glycan of ErbB3 plays a crucial role in preventing spontaneous heterodimerization and tumor promotion

ErbB2 and ErbB3, two members of the ErbB family, form a high-affinity heregulin coreceptor that elicits potent mitogenic and transforming signals, and clinical studies indicate that these receptors play an important role in tumor incidence and progression. To determine whether N-glycosylation is involved in the function of ErbB3, a series of human ErbB3 molecules devoid of N-glycans were prepared and transfected to Flp-In-CHO cells for stable expression. A cross-linking study showed that the Asn(418) to Gln mutant (N418Q) of ErbB3 underwent autodimerization without its ligand, heregulin. The wild-type or N418Q mutant of ErbB3 was next coexpressed with ErbB2 in Flp-In-CHO cells, and the effect of N-glycan on heterodimerization was examined. The N418Q mutant of ErbB3 was autodimerized with ErbB2 without ligand stimulation, and receptor tyrosine phosphorylation and subsequent extracellular signal-regulated kinase (ERK) and Akt phosphorylation were promoted in the absence of heregulin. A cell proliferation assay and a soft agar colony formation assay showed that the N418Q mutant of ErbB3 coexpressed with ErbB2 promoted cell proliferation and colony formation in soft agar in an ERK- and Akt-dependent manner. The mutation also promoted the growth of tumors in athymic mice when injected s.c. These findings suggest that the Asn(418)-linked N-glycan in ErbB3 plays an essential role in regulating receptor heterodimerization with ErbB2 and might have an effect on transforming activity.

Differential constitutive activation of the epidermal growth factor receptor in non-small cell lung cancer cells bearing EGFR gene mutation and amplification

The identification of somatic mutations in the tyrosine kinase domain of the epidermal growth factor receptor (EGFR) in patients with non-small cell lung cancer (NSCLC) and the association of such mutations with the clinical response to EGFR tyrosine kinase inhibitors (TKI), such as gefitinib and erlotinib, have had a substantial effect on the treatment of this disease. EGFR gene amplification has also been associated with an increased therapeutic response to EGFR-TKIs. The effects of these two types of EGFR alteration on EGFR function have remained unclear, however. We have now examined 16 NSCLC cell lines, including eight newly established lines from Japanese NSCLC patients, for the presence of EGFR mutations and amplification. Four of the six cell lines that harbor EGFR mutations were found to be positive for EGFR amplification, whereas none of the 10 cell lines negative for EGFR mutation manifested EGFR amplification, suggesting that these two types of EGFR alteration are closely associated. Endogenous EGFRs expressed in NSCLC cell lines positive for both EGFR mutation and amplification were found to be constitutively activated as a result of ligand-independent dimerization. Furthermore, the patterns of both EGFR amplification and EGFR autophosphorylation were shown to differ between cell lines harboring the two most common types of EGFR mutation (exon 19 deletion and L858R point mutation in exon 21). These results reveal distinct biochemical properties of endogenous mutant forms of EGFR expressed in NSCLC cell lines and may have implications for treatment of this condition.

The EGF receptor family: spearheading a merger of signaling and therapeutics

The ErbB receptor tyrosine kinases evolved as key regulatory entities enabling the extracellular milieu to communicate with the intracellular machinery to bring forth the appropriate biological response in an ever-changing environment. Since its discovery, many aspects of the ErbB family have been deciphered, with emphasis on aberration of signaling in human diseases. However, only now, with the availability of the atomic coordinates of these receptors, can we construct a comprehensive model of the mechanisms underlying ligand-induced receptor dimerization and subsequent tyrosine kinase activation. Furthermore, the recent introduction of new high-throughput screening methodologies, combined with the materialization of a systems biology perspective, reveals an overwhelming network complexity, enabling robust signaling and evolvability. This knowledge is likely to impact our view of diseases as system perturbations and resistance to ErbB-targeted therapeutics as manifestations of robustness.

Receptor tyrosine kinases: mechanisms of activation and signaling

Receptor tyrosine kinases (RTKs) are essential components of signal transduction pathways that mediate cell-to-cell communication. These single-pass transmembrane receptors, which bind polypeptide ligands - mainly growth factors - play key roles in processes such as cellular growth, differentiation, metabolism and motility. Recent progress has been achieved towards an understanding of the precise (and varied) mechanisms by which RTKs are activated by ligand binding and by which signals are propagated from the activated receptors to downstream targets in the cell.