An Open-and-Shut Case? Recent Insights into the Activation of EGF/ErbB Receptors

Signaling through ERBB receptors: Multiple layers of diversity and control

The four known ERBB receptors in humans are involved in a broad range of cellular responses, and their deregulation is a significant aspect in a large number of disease states. However, their mechanism of action and modes of control are still poorly understood. This is largely due to the fact that the control of ERBB activity is a multilayered process with significant differences between the various ERBB members. In contrast to other receptor tyrosine kinases, the kinase domain of EGFR (ERBB1) does not require phosphorylation for activation. Consequently, the overall activation state of the receptor is controlled by constant balancing of activity favoring and activity suppressing actions within the receptor molecule. Influences of the membrane microenvironment and context dependent interactions with varying sets of signaling partners are superimposed on this system of intramolecular checks and balances. We will discuss current models of the control of ERBB signaling with an emphasis on the multilayered nature of activation control and aspects that give rise to diversity between ERBB receptors.

Biosynthesis of tumorigenic HER2 C-terminal fragments by alternative initiation of translation

The overactivation of the HERs, a family of tyrosine kinase receptors, leads to the development of cancer. Although the canonical view contemplates HER receptors restricted to the secretory and endocytic pathways, full-length HER1, HER2 and HER3 have been detected in the nucleoplasm. Furthermore, limited proteolysis of HER4 generates nuclear C-terminal fragments (CTFs). Using cells expressing a panel of deletion and point mutants, here we show that HER2 CTFs are generated by alternative initiation of translation from methionines located near the transmembrane domain of the full-length molecule. In vitro and in vivo, HER2 CTFs are found in the cytoplasm and nucleus. Expression of HER2 CTFs to levels similar to those found in human tumors induces the growth of breast cancer xenografts in nude mice. Tumors dependent on CTFs are sensitive to inhibitors of the kinase activity but do not respond to therapeutic antibodies against HER2. Thus, the kinase domain seems necessary for the activity of HER2 CTFs and the presence of these HER2 fragments could account for the resistance to treatment with antibodies.

Humanization of a recombinant monoclonal antibody to produce a therapeutic HER dimerization inhibitor, pertuzumab

Dimerization is essential for activity of human epidermal growth factor receptors (HER1/EGFR, HER2/ErbB2, HER3/ErbB3, and ErbB4) and mediates intracellular signaling events leading to cancer cell proliferation, survival, and resistance to therapy. HER2 is the preferred dimerization partner. Activation of HER signaling pathways may be blocked by inhibition of dimer formation using a monoclonal antibody (MAb) directed against the dimerization domain of HER2. The murine MAb 2C4 that specifically binds the HER2 dimerization domain was cloned as a chimeric antibody, humanized using a computer-generated model to guide framework substitutions, and variants were tested as Fabs. Pharmacokinetics and toxicology were evaluated in rodents and cynomolgus monkeys. Cloning the variable domains of MAb 2C4 into a vector containing human kappa and CH1 domains allowed construction of a mouse-human chimeric Fab. DNA sequencing of the chimeric clone permitted identification of CDR residues. The full-length IgG1 of variant F-10 was equivalent in binding to chimeric IgG1 and was designated pertuzumab (rhuMAb 2C4; Omnitarg). Pertuzumab pharmacokinetics was best described by a two-compartment model with a distribution phase of <1 day, terminal half-life of approximately 10 days, and volume of distribution of approximately 40 mL/kg that approximates serum volume. With the exception of diarrhea, pertuzumab was generally well tolerated in cynomolgus monkeys. Pertuzumab, a recombinant humanized IgG1 MAb, is the first of a new class of agents known as HER dimerization inhibitors. Inhibition of HER dimerization may be an effective anticancer strategy in tumors with either normal or elevated expression of HER2.

Control of ErbB signaling through metalloprotease mediated ectodomain shedding of EGF-like factors

Epidermal growth factor (EGF)-like proteins comprise a group of structurally similar growth factors, which contain a conserved six-cysteine residue motif called the EGF-domain. EGF-like factors are synthesized as transmembrane precursors, which can undergo proteolytic cleavage at the cell surface to release a mature soluble ectodomain; a process often referred to as "ectodomain shedding". Ectodomain shedding of EGF-like factors has been linked to multiple zinc-binding metalloproteases of the matrix metalloprotease (MMP) and a disintegrin and metalloprotease (ADAM) families. Shedding can be activated by a variety of pharmacological and physiological stimuli and these activation events have been linked to the enhancement of metalloprotease activity, possibly via the action of intracellular signaling modules. Once shed from the cell surface, EGF-like factors bind to a family of four cell surface receptors named ErbB-1, -2, -3 and -4. Heterodimerization or homodimerization of these receptors following ligand binding drives intracellular signal transduction cascades, which eventuate in diverse cell fates including proliferation, differentiation, migration and inhibition of apoptosis. In addition to its role in driving normal developmental processes, a wealth of evidence now exists showing that de-regulated ErbB signaling is associated with the formation of tumors in a variety of tissues and that ectodomain shedding of EGF-like factors plays a critical event in this process. Thus, knowledge of the molecular mechanisms by which EGF-like factors are shed from the cell surface and the nature of the proteases and cellular signals that govern this process is crucial to understanding ErbB receptor signaling and potentially also in the development of novel cancer therapeutics targeting the ErbB pathway. This review focuses on the structure and function of EGF-like factors, and the mechanisms that govern the shedding of these transmembrane molecules from the cell surface.

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.

Full-length ADAMTS-1 and the ADAMTS-1 fragments display pro- and antimetastatic activity, respectively

The exact role of a disintegrin and metalloproteinase with thrombospondin motifs-1 (ADAMTS-1) and the underlying mechanism of its involvement in tumor metastasis have not been established. We have now demonstrated that overexpression of ADAMTS-1 promotes pulmonary metastasis of TA3 mammary carcinoma and Lewis lung carcinoma cells and that a proteinase-dead mutant of ADAMTS-1 (ADAMTS-1E/Q) inhibits their metastasis, indicating that the prometastatic activity of ADAMTS-1 requires its metalloproteinase activity. Overexpression of ADAMTS-1 in these cells promoted tumor angiogenesis and invasion, shedding of the transmembrane precursors of heparin-binding epidermal growth factor (EGF) and amphiregulin (AR), and activation of the EGF receptor and ErbB-2, while overexpression of ADAMTS-1E/Q inhibited these events. Furthermore, we found that ADAMTS-1 undergoes auto-proteolytic cleavage to generate the NH(2)- and COOH-terminal cleavage fragments containing at least one thrombospondin-type-I-like motif and that overexpression of the NH(2)-terminal ADAMTS-1 fragment and the COOH-terminal ADAMTS-1 fragment can inhibit pulmonary tumor metastasis. These fragments also inhibited Erk1/2 kinase activation induced by soluble heparin-binding EGF and AR. Taken together, our results suggest that the proteolytic status of ADAMTS-1 determines its effect on tumor metastasis, and that the ADAMTS-1E/Q and the ADAMTS-1 fragments likely inhibit tumor metastasis by negatively regulating the availability and activity of soluble heparin-binding EGF and AR.

Role of receptor tyrosine kinase transmembrane domains in cell signaling and human pathologies

Receptor tyrosine kinases (RTKs) conduct biochemical signals via lateral dimerization in the plasma membrane, and their transmembrane (TM) domains play an important role in the dimerization process. Here we present two models of RTK-mediated signaling, and we discuss the role of the TM domains within the framework of these two models. We summarize findings of single-amino acid mutations in RTK TM domains that induce unregulated signaling and, as a consequence, pathological phenotypes. We review the current knowledge of pathology induction mechanisms due to these mutations, focusing on the structural and thermodynamic basis of pathogenic dimer stabilization.

Structure and dynamics of the epidermal growth factor receptor C-terminal phosphorylation domain

The C-terminal phosphorylation domain of the epidermal growth factor receptor is believed to regulate protein kinase activity as well as mediate the assembly of signal transduction complexes. The structure and dynamics of this proposed autoregulatory domain were examined by labeling the extreme C terminus of the EGFR intracellular domain (ICD) with an extrinsic fluorophore. Fluorescence anisotropy decay analysis of the nonphosphorylated EGFR-ICD yielded two rotational correlation times: a longer time, consistent with the global rotational motion of a 60- to 70-kDa protein with an elongated globular conformation, and a shorter time, presumably contributed by segmental motion near the fluorophore. A C-terminally truncated form of EGFR-ICD yielded a slow component consistent with the rotational motion of the 38-kDa kinase core. These findings suggested a structural arrangement of the EGFR-ICD in which the C-terminal phosphorylation domain interacts with the kinase core to move as an extended structure. A marked reduction in the larger correlation time of EGFR-ICD was observed upon its autophosphorylation. This dynamic component was faster than predicted for the globular motion of the 62-kDa EGFR-ICD, suggesting an increase in the mobility of the C-terminal domain and a likely displacement of this domain from the kinase core. The interaction between the SH2 domain of c-Src and the phosphorylated EGFR C-terminal domain was shown to impede its mobility. Circular dichroism spectroscopy indicated that the EGFR C-terminal domain possessed a significant level of secondary structure in the form of alpha-helices and beta-sheets, with a marginal change in beta-sheet content occurring upon phosphorylation.

On the nature of low- and high-affinity EGF receptors on living cells

The small subpopulation of high-affinity EGF receptors (EGFRs) on living cells revealed by Scatchard analysis of (125)I-EGF binding results was discovered nearly three decades ago, yet not much is known about the underlying mechanism. After the determination of the structure of different forms of EGFR extracellular domain it was proposed that the monomeric tethered configuration corresponds to the majority of low-affinity receptors, whereas the extended dimeric configuration corresponds to the minority of the high-affinity class of EGFRs. Mathematical modeling of EGF-binding experiments to different conformational mutants of EGFR has shown that the high-affinity class of EGFR on living cells does not correspond to the extended configuration of EGFR and can only be accounted for by including in the mathematical model an additional binding event that is attributed to the dynamic nature of EGFR on living cells. To circumvent this problem we have performed similar experiments in the background of an EGFR mutant that does not form high-affinity sites. Quantitative analysis and mathematical modeling of these data show that release of the intramolecular tether causes a 2-fold increase in EGF-binding affinity, whereas elimination of the dimerization arm reduces EGF-binding affinity by approximately 6-fold. These experiments confirm the salient features of the structural model for EGFR regulation and argue further that the intramolecular tether provides only limited autoinhibitory control of EGFR activity and that the low-affinity class of EGF-binding sites on living cells reflects interconverting, tethered, and extended receptor configurations.

Activity of the dual kinase inhibitor lapatinib (GW572016) against HER-2-overexpressing and trastuzumab-treated breast cancer cells

Lapatinib (GW572016) is a selective inhibitor of both epidermal growth factor receptor (EGFR) and HER-2 tyrosine kinases. Here, we explore the therapeutic potential of lapatinib by testing its effect on tumor cell growth in a panel of 31 characterized human breast cancer cell lines, including trastuzumab-conditioned HER-2-positive cell lines. We further characterize its activity in combination with trastuzumab and analyze whether EGFR and HER-2 expression or changes induced in the activation of EGFR, HER-2, Raf, AKT, or extracellular signal-regulated kinase (ERK) are markers of drug activity. We report that concentration-dependent antiproliferative effects of lapatinib were seen in all breast cancer cell lines tested but varied significantly between individual cell lines with up to 1,000-fold difference in the IC(50)s (range, 0.010-18.6 micromol/L). Response to lapatinib was significantly correlated with HER-2 expression and its ability to inhibit HER-2, Raf, AKT, and ERK phosphorylation. Long-term in vivo lapatinib studies were conducted with human breast cancer xenografts in athymic mice. Treatment over 77 days resulted in a sustained and significant reduction in xenograft volume compared with untreated controls. For the combination of lapatinib plus trastuzumab, synergistic drug interactions were observed in four different HER-2-overexpressing cell lines. Moreover, lapatinib retained significant in vitro activity against cell lines selected for long-term outgrowth (>9 months) in trastuzumab-containing (100 microg/mL) culture medium. These observations provide a clear biological rationale to test lapatinib as a single agent or in combination with trastuzumab in HER-2-overexpressing breast cancer and in patients with clinical resistance to trastuzumab.

Regulation of early Xenopus development by ErbB signaling

ErbB signaling has long been implicated in cancer formation and progression and is shown to regulate cell division, migration, and death during tumorigenesis. The functions of the ErbB pathway during early vertebrate embryogenesis, however, are not well understood. Here we report characterization of ErbB activities during early frog development. Gain-of-function analyses show that EGFR, ErbB2, and ErbB4 induce ectopic tumor-like cell mass that contains increased numbers of mitotic cells. Both the muscle and the neural markers are expressed in these ectopic protrusions. ErbBs also induce mesodermal markers in ectodermal explants. Loss-of-function studies using carboxyl terminal-truncated dominant-negative ErbB receptors demonstrate that blocking ErbB signals leads to defective gastrulation movements and malformation of the embryonic axis with a reduction in the head structures in early frog embryos. These data, together with the observation that ErbBs are expressed early during frog embryogenesis, suggest that ErbBs regulate cell proliferation, movements, and embryonic patterning during early Xenopus development.

Preclinical analysis of the analinoquinazoline AG1478, a specific small molecule inhibitor of EGF receptor tyrosine kinase

The tyrphostin 4-(3-chloroanilino)-6,7-dimethoxyquinazoline (AG1478) is a potent and specific inhibitor of EGFR tyrosine kinase whose favourable preclinical profile supports progression towards clinical trials. Microphysiometric evaluation revealed a short (<24 min) effective inhibition of cellular receptor response to EGF challenge in BaF/ERX cells indicating a need to maintain sustained levels of inhibitor. Initial pharmacokinetic evaluation in mice of novel AG1478 formulations in a beta-cyclodextrin (Captisol) showed monoexponential elimination from plasma (half-life 30 min) following subcutaneous administration. A two-fold dose escalation gave a 2.4-fold increase in the total AUC. Bolus i.v. and 6 h continuous infusion were investigated in rats to mimic a more clinically relevant administration regimen. Drug elimination following bolus i.v. administration was biphasic (terminal elimination half-life 30-48 min). The linear relationship between dose and AUC(0-->infinity) (r2=0.979) enabled the prediction of infusion rates and doses for sustained delivery using continuous 6 h infusions, where steady state was reached in 120 min. Plasma levels of AG1478>10 microM were achieved over the duration of the infusion. At the lowest dose, plasma drug levels after the cessation of infusion declined with a half-life of approximately 43 min. EGFR activity, measured both by autophosphorylation and downstream signalling, was inhibited in a dose-dependent manner by injection of AG1478 in mice bearing xenografts of the human glioblastoma cell line U87MG.delta2-7, which expresses a constitutively active variant of the EGF receptor. Taken together, these experiments provide essential data to assess the anti-tumour efficacy of AG1478 and will assist in the rational design of dose regimens for clinical studies.

Essential role for Rac in heregulin beta1 mitogenic signaling: a mechanism that involves epidermal growth factor receptor and is independent of ErbB4

Heregulins are a family of ligands for the ErbB3/ErbB4 receptors that play important roles in breast cancer cell proliferation and tumorigenesis. Limited information is available on the contribution of Rho GTPases to heregulin-mediated signaling. In breast cancer cells, heregulin beta1 (HRG) causes a strong activation of Rac; however, it does so with striking differences in kinetics compared to epidermal growth factor, which signals through ErbB1 (epidermal growth factor receptor [EGFR]). Using specific ErbB receptor inhibitors and depletion of receptors by RNA interference (RNAi), we established that, surprisingly, activation of Rac by HRG is mediated not only by ErbB3 and ErbB2 but also by transactivation of EGFR, and it is independent of ErbB4. Similar receptor requirements are observed for HRG-induced actin cytoskeleton reorganization and mitogenic activity via extracellular signal-regulated kinase (ERK). HRG-induced Rac activation was phosphatidylinositol 3-kinase dependent and Src independent. Furthermore, inactivation of Rac by expression of the Rac GTPase-activating protein beta2-chimerin inhibited HRG-induced ERK activation, mitogenicity, and migration in breast cancer cells. HRG mitogenic activity was also impaired by depletion of Rac1 using RNAi. Our studies established that Rac is a critical mediator of HRG mitogenic signaling in breast cancer cells and highlight additional levels of complexity for ErbB receptor coupling to downstream effectors that control aberrant proliferation and transformation.

The ErbB-4 s80 intracellular domain is a constitutively active tyrosine kinase

The ErbB-4 receptor tyrosine kinase homo- and heterodimerizes following heregulin binding, which provokes increased levels of tyrosine autophosphorylation. Unique to the ErbB family, ErbB-4 is then proteolytically cleaved by alpha- and gamma-secretase to produce an 80 kDa intracellular domain (s80 ICD) fragment. This fragment is found in both the cytoplasm and nucleus of many normal and cancer cells and can interact with transcription factors in the cytoplasm and nucleus. Since the s80 ICD lacks ectodomain sequences known to play a major role in dimerization of ErbB family members, we asked whether the s80 ICD is an active tyrosine kinase. Here, we demonstrate that the s80 ICD is a constitutively active tyrosine kinase and can form homodimers. The s80 ICD is autophosphorylated in cells and can phosphorylate an exogenous substrate in vitro. Also, the s80 ICD can coassociate and dimers are detected by chemical crosslinking. This is the first example of constitutive kinase activation and dimerization totally within the cytoplasmic domain of an ErbB receptor and suggests that the s80 ICD may function to phosphorylate substrates in the cytoplasm or nucleus.

Lapatinib: a novel dual tyrosine kinase inhibitor with activity in solid tumors

OBJECTIVE

To review the pharmacology, pharmacokinetics, clinical trials, adverse effects, and drug interactions of lapatinib.

DATA SOURCES

A PubMed search was conducted (1966-August 2005) using the following terms: lapatinib, GW572016, and dual tyrosine kinase inhibitor. Additional information sources included meeting abstracts, clinical trial data, and bibliographies from articles identified through PubMed.

STUDY SELECTION AND DATA EXTRACTION

Preclinical and clinical trials that evaluated lapatinib in cell culture, animal models, and human subjects were selected from the data sources. Pivotal in vitro data and all in vivo data published regarding lapatinib were included.

DATA SYNTHESIS

The development of tyrosine kinase inhibitors has resulted from a search for targeted cancer therapeutics made possible by recent gains in our understanding of tumor cell biology. Lapatinib is a dual tyrosine kinase inhibitor selective for inhibition of epidermal growth factor receptor and human epidermal growth factor receptor-2 autophosphorylation, leading to suppression of proliferation pathways of solid tumors. Lapatinib has shown clinical activity in solid tumors, with the most notable in advanced or metastatic breast cancer, including tumors refractory to trastuzumab. It has a mild adverse effect profile, with the most common adverse events being diarrhea and rash.

CONCLUSIONS

Lapatinib has novel, dual tyrosine kinase inhibitory properties selective for factors overexpressed in some solid tumors. Results from preclinical and Phase I/II trials indicate activity in the treatment of solid tumors, especially advanced or metastatic breast cancer. Application for approval is anticipated pending results of ongoing Phase III trials.

Structural basis for Gas6-Axl signalling

Receptor tyrosine kinases of the Axl family are activated by the vitamin K-dependent protein Gas6. Axl signalling plays important roles in cancer, spermatogenesis, immunity, and platelet function. The crystal structure at 3.3 A resolution of a minimal human Gas6/Axl complex reveals an assembly of 2:2 stoichiometry, in which the two immunoglobulin-like domains of the Axl ectodomain are crosslinked by the first laminin G-like domain of Gas6, with no direct Axl/Axl or Gas6/Gas6 contacts. There are two distinct Gas6/Axl contacts of very different size, both featuring interactions between edge beta-strands. Structure-based mutagenesis, protein binding assays and receptor activation experiments demonstrate that both the major and minor Gas6 binding sites are required for productive transmembrane signalling. Gas6-mediated Axl dimerisation is likely to occur in two steps, with a high-affinity 1:1 Gas6/Axl complex forming first. Only the minor Gas6 binding site is highly conserved in the other Axl family receptors, Sky/Tyro3 and Mer. Specificity at the major contact is suggested to result from the segregation of charged and apolar residues to opposite faces of the newly formed beta-sheet.

ErbB receptor negative regulatory mechanisms: implications in cancer

Activation of ErbB receptor tyrosine kinases (RTKs) must be precisely regulated to ensure the fidelity of developmental and homeostatic processes mediated by growth factors. Insufficient receptor stimulation will lead to defects in tissue development, while excessive stimulation can lead to hyperplastic events associated with cancer and other diseases. A coordinated balance of the intensity and timing of receptor signaling, achieved through both receptor activation and negative regulatory mechanisms, is required for signaling fidelity. While considerable effort has gone into understanding mechanisms by which ErbB receptors are activated, our understanding of the suppression of growth factor receptor activity remains limited. While ligand-stimulated receptor degradation is the most thoroughly examined mechanism for preventing hyper-signaling by ErbBs, recent studies indicate that several other mechanisms act directly on receptors to suppress receptor levels, or the magnitude or duration of receptor signaling. ErbB receptor overexpression or aberrant activation contributes to the progression of numerous solid tumor types. Hence, tumor cells must overcome these endogenous receptor negative regulatory mechanisms before they can exploit ErbB receptors to achieve uncontrolled growth. Here we will discuss several proteins that directly interact with ErbB receptors to suppress signaling, highlighting the potential impact of their loss on tumor progression.

The ErbB2 signaling network as a target for breast cancer therapy

Overexpression of the ErbB2/Her2 receptor tyrosine kinase in breast cancers is associated with the most aggressive tumors. Experimental studies have revealed that ErbB2 shows many features of a therapeutic target: ErbB2 is able to confer many of the characteristics of a cancerous cell, including uncontrolled proliferation, resistance to apoptosis and increased motility; ErbB2 overexpression is specific to tumor cells; as a cell surface-associated protein, it is easily accessible to drugs and as a kinase it is amenable to targeted inhibition by small molecules. Recent clinical results demonstrate the efficacy of ErbB2-targeting therapy and promise an expanding use of ErbB2-targeting drugs for breast cancer treatment. However, as only a fraction of patients responds successfully to therapy and risks of recurrence are still high, further investigation is required for an improved understanding of the complex network of signaling pathways underlying ErbB2-driven cancer progression.

Directed evolution of the epidermal growth factor receptor extracellular domain for expression in yeast

The extracellular domain of epidermal growth factor receptor (EGFR-ECD) has been engineered through directed evolution and yeast surface display using conformationally-specific monoclonal antibodies (mAbs) as screening probes for proper folding and functional expression in Saccharomyces cerevisiae. An EGFR mutant with four amino acid changes exhibited binding to the conformationally-specific mAbs and human epidermal growth factor, and showed increased soluble secretion efficiency compared with wild-type EGFR. Full-length EGFR containing the mutant EGFR-ECD was functional, as assayed by EGF-dependent autophosphorylation and intracellular MAPK signaling in mammalian cells, and was expressed and localized at the plasma membrane in yeast. This approach should enable engineering of other complex mammalian receptor glycoproteins in yeast for genetic, structural, and biophysical studies.

Sending the right signals: regulating receptor kinase activity

Knowledge of the functions of plant receptor-like-kinases (RLKs) is increasing rapidly, but how their cytoplasmic signalling activity is regulated and how signals are transduced to cytoplasmic or nuclear proteins remain important questions. Recent studies, particularly of the BRASSINOSTEROID INSENSITIVE1 RLK, have begun to shed light on the mechanistic details of RLK activation, including the possible role of ligand binding. Studies of this and other RLKs have also highlighted the potential importance of hetero-oligomerisation and receptor internalisation in RLK signalling. Finally, a range of potential regulatory proteins and putative downstream signalling substrates have been identified for various RLKs. Despite some similarities with animal receptor kinase signalling systems, mechanisms that affect the intracellular behaviour, regulation and interactions of RLKs appear to be very diverse, potentially explaining how signalling specificity is maintained at the cytoplasmic level.

Phe45 of NRG2beta is critical for the affinity of NRG2beta for ErbB4 and for potent stimulation of ErbB4 signaling by NRG2beta*

The Neuregulins (NRGs) are members of the epidermal growth factor (EGF) family of growth factors. EGF family members regulate the signaling of ErbB family receptor tyrosine kinases, including the epidermal growth factor receptor (EGFR/ErbB1), ErbB2/HER2/Neu, ErbB3/HER3 and ErbB4/HER4. We have previously demonstrated that the EGF family hormone NRG2beta is a potent ErbB4 agonist, whereas NRG2alpha is a weak ErbB4 agonist. We have also previously demonstrated that Phe45 of NRG2beta regulates the potency of NRG2beta. Here, we address the hypotheses that Phe45 regulates the potency of NRG2beta by regulating the affinity of NRG2beta for ErbB4. We demonstrate that Phe45 of NRG2beta indeed regulates the affinity of NRG2beta for ErbB4. Furthermore, a hydrophobic or uncharged amino acid side chain at residue 45 contributes to NRG2beta binding to ErbB4. These data indicate that Phe45 of NRG2beta may regulate the affinity of NRG2beta for ErbB4 by interacting with hydrophobic amino acids in ErbB4.

ERBB2 in pediatric cancer: innocent until proven guilty

Adult oncologists and their research colleagues have "led the charge" in the war on cancer. Their efforts have generated effective new chemotherapies that target cancer causing molecular alterations. It is hoped that these successes will be repeated within the pediatric oncology community. Testing whether molecular targeted therapies of adult cancers are also effective against childhood cancers might allow the rapid introduction of these exciting new agents into the pediatric clinic. However, it is imperative that we do not introduce blindly these agents into the pediatric population. We must ensure that molecular targets in adult cancers also fulfill a number of important criteria within the pediatric disease. This review addresses the issues surrounding the identification of molecular targets in pediatric cancers by focusing on studies of the ERBB2 oncogene.

Autocrine, paracrine and juxtacrine signaling by EGFR ligands

Receptor and cytoplasmic protein tyrosine kinases play prominent roles in the control of a range of cellular processes during embryonic development and in the regulation of many metabolic and physiological processes in a variety of tissues and organs. The epidermal growth factor receptor (EGFR) is a well-known and versatile signal transducer that has been highly conserved during evolution. It functions in a wide range of cellular processes, including cell fate determination, proliferation, cell migration and apoptosis. The number of ligands that can activate the EGF receptor has increased during evolution. These ligands are synthesized as membrane-anchored precursor forms that are later shed by metalloproteinase-dependent cleavage to generate soluble ligands. In certain circumstances the membrane anchored isoforms as well as soluble growth factors may also act as biologically active ligands; therefore depending on the circumstances these ligands may induce juxtacrine, autocrine, paracrine and/or endocrine signaling. In this review, we discuss the different ways that EGFR ligands can activate the receptor and the possible biological implications.

A network model of early events in epidermal growth factor receptor signaling that accounts for combinatorial complexity

We consider a model of early events in signaling by the epidermal growth factor (EGF) receptor (EGFR). The model includes EGF, EGFR, the adapter proteins Grb2 and Shc, and the guanine nucleotide exchange factor Sos, which is activated through EGF-induced formation of EGFR-Grb2-Sos and EGFR-Shc-Grb2-Sos assemblies at the plasma membrane. The protein interactions involved in signaling can potentially generate a diversity of protein complexes and phosphoforms; however, this diversity has been largely ignored in models of EGFR signaling. Here, we develop a model that accounts more fully for potential molecular diversity by specifying rules for protein interactions and then using these rules to generate a reaction network that includes all chemical species and reactions implied by the protein interactions. We obtain a model that predicts the dynamics of 356 molecular species, which are connected through 3749 unidirectional reactions. This network model is compared with a previously developed model that includes only 18 chemical species but incorporates the same scope of protein interactions. The predictions of this model are reproduced by the network model, which also yields new predictions. For example, the network model predicts distinct temporal patterns of autophosphorylation for different tyrosine residues of EGFR. A comparison of the two models suggests experiments that could lead to mechanistic insights about competition among adapter proteins for EGFR binding sites and the role of EGFR monomers in signal transduction.

Genetic and biochemical analysis of the role of Egfr in the morphogenetic furrow of the developing Drosophila eye

A key event in patterning the developing Drosophila compound eye is the progressive restriction of the transcription factor Atonal in the morphogenetic furrow. The Atonal pattern evolves from expression in all cells to an over-dispersed pattern of single founder cells (the future R8 photoreceptors). This restriction involves Notch-mediated lateral inhibition. However, there have been inconsistent data on a similar proposed role for the Egf receptor (Egfr). Experiments using a conditional Egfr mutation (Egfr(tsla)) suggested that Egfr does not regulate Atonal restriction, whereas experiments using Egfr-null mosaic Minute+ clones suggested that it does. Here, we have re-examined both approaches. We report that the lesion in Egfr(tsla) is a serine to phenylalanine change in a conserved extracellular ligand-binding domain. We show by biochemical and genetic approaches that the Egfr(tsla) protein is rapidly and completely inactivated upon shift to the non-permissive temperature. We also find that on temperature shift the protein moves from the cell surface into the cell. Finally, we report a flaw in the Egfr-null mosaic Minute+ clone approach. Thus, we demonstrate that Egfr does not play a role in the initial specification or spacing of ommatidial founder cells.

The extracellular region of ErbB4 adopts a tethered conformation in the absence of ligand

The human ErbB family of receptor tyrosine kinases comprises the epidermal growth factor receptor (EGFR/ErbB1/HER1), ErbB2 (HER2/Neu), ErbB3 (HER3), and ErbB4 (HER4). ErbBs play fundamental roles in cell growth and differentiation events in embryonic and adult tissues, and inappropriate ErbB activity has been implicated in several human cancers. We report here the 2.4 A crystal structure of the extracellular region of human ErbB4 in the absence of ligand and show that it adopts a tethered conformation similar to inactive forms of ErbB1 and ErbB3. This structure completes the gallery of unliganded ErbB receptors and demonstrates that all human ligand-binding ErbBs adopt the autoinhibited conformation. We also show that the binding of neuregulin-1beta to ErbB4 and ErbB3 and the binding of betacellulin to both ErbB4 and ErbB1 does not decrease at low pH, unlike the binding of epidermal growth factor and transforming growth factor-alpha to ErbB1. These results indicate an important role for ligand in determining pH-dependent binding and may explain different responses observed when the same ErbB receptor is stimulated by different ligands.

Conformational changes accompany phosphorylation of the epidermal growth factor receptor C-terminal domain

The precise regulation of epidermal growth factor receptor (EGFR) signaling is crucial to its function in cellular growth control. Various studies have suggested that the C-terminal phosphorylation domain, itself a substrate for the EGFR kinase activity, exerts a regulatory influence upon it, although the molecular mechanism for this regulation is unknown. The fluorescence resonance energy transfer (FRET) technique was employed to examine how C-terminal domain conformational changes in the context of receptor activation and autophosphorylation might regulate EGFR enzymatic activity. A novel FRET reporter system was devised in which recombinant purified EGFR intracellular domain (ICD) proteins of varying C-terminal lengths were site-specifically labeled at their extreme C termini with blue fluorescent protein (BFP) and a fluorescent nucleotide analog, 2'(3')-O-(2,4,6-trinitrophenyl)-adenosine 5'-triphosphate (TNP-ATP), binding at their active sites. This novel BFP/TNP-ATP FRET pair demonstrated efficient energy transfer as evidenced by appreciable BFP-donor quenching by bound TNP-ATP. In particular, a marked reduction in energy transfer was observed for the full-length BFP-labeled EGFR-ICD protein upon phosphorylation, likely reflecting its movement away from the active site. The estimated distances from the BFP module to the TNP-ATP-occupied active site for the full-length and C-terminally truncated proteins also reveal the possible folding geometry of this domain with respect to the kinase core. The present studies demonstrate the first use of BFP/TNP-ATP as a FRET reporter system. Furthermore, the results described here provide biophysical evidence for phosphorylation-dependent conformational changes in the C-terminal phosphorylation domain and its likely interaction with the kinase core.

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

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

Synergistic growth inhibition by Iressa and Rapamycin is modulated by VHL mutations in renal cell carcinoma

Epidermal growth factor receptor (EGFR) and tumour growth factor alpha (TGFα) are frequently overexpressed in renal cell carcinoma (RCC) yet responses to single-agent EGFR inhibitors are uncommon. Although von Hippel–Lindau (VHL) mutations are predominant, RCC also develops in individuals with tuberous sclerosis (TSC). Tuberous sclerosis mutations activate mammalian target of rapamycin (mTOR) and biochemically resemble VHL alterations. We found that RCC cell lines expressed EGFR mRNA in the near-absence of other ErbB family members. Combined EGFR and mTOR inhibition synergistically impaired growth in a VHL-dependent manner. Iressa blocked ERK1/2 phosphorylation specifically in wt-VHL cells, whereas rapamycin inhibited phospho-RPS6 and 4E-BP1 irrespective of VHL. In contrast, phospho-AKT was resistant to these agents and MYC translation initiation (polysome binding) was similarly unaffected unless AKT was inhibited. Primary RCCs vs cell lines contained similar amounts of phospho-ERK1/2, much higher levels of ErbB-3, less phospho-AKT, and no evidence of phospho-RPS6, suggesting that mTOR activity was reduced. A subset of tumours and cell lines expressed elevated eIF4E in the absence of upstream activation. Despite similar amounts of EGFR mRNA, cell lines (vs tumours) overexpressed EGFR protein. In the paired cell lines, PRC3 and WT8, EGFR protein was elevated post-transcriptionally in the VHL mutant and EGF-stimulated phosphorylation was prolonged. We propose that combined EGFR and mTOR inhibitors may be useful in the subset of RCCs with wt-VHL. However, apparent differences between primary tumours and cell lines require further investigation.

EGF receptor inhibition: attacks on multiple fronts

The epidermal growth factor receptor (EGFR) drives tumor growth in a subset of human epithelial carcinomas. A crystallographic study by Li et al. in this issue of Cancer Cell provides the molecular basis for inhibition of EGFR by cetuximab (Erbitux), a monoclonal antibody that has been approved by the Food and Drug Administration as a therapeutic for advanced-stage colorectal cancers. Cetuximab targets one of the ligand binding domains of EGFR, thus preventing ligand activation of the receptor.

Roles for neuregulins in human cancer

The human epidermal growth factor (EGF) receptor (HER) family of receptor tyrosine kinases has frequently been implicated in cancer. Apart from overexpression or mutation of these receptors, also the aberrant autocrine or paracrine activation of HERs by EGF-like ligands may be important in cancer progression. Neuregulins constitute a family of EGF-like ligands that bind to HER3 or HER4, preferably forming heterodimers with the orphan receptor HER2. Mesenchymal neuregulin typically serves as a pro-survival and pro-differentiation signal for adjacent epithelia. Disruption of the balance between proliferation and differentiation, because of autocrine production by the epithelial cells, increased sensitivity to paracrine signals or disruption of the spatial organization, may lead to constitutive receptor activation, in the absence of receptor overexpression. Consequently, the analysis of ligand expression and/or activated receptors in tumor samples may broaden the group of patients that can benefit from targeted therapies.

Prospective isolation of late development multipotent precursors whose migration is promoted by EGFR

A simple procedure to isolate neural stem cells would greatly facilitate direct studies of their properties. Here, we exploited the increase in EGF receptor (EGFR) levels, that occurs in late development stem cells or in younger precursors upon exposure to FGF-2, to isolate cells expressing high levels of EGFR (EGFR(high)) from the developing and the adult brain. Independently of age and region of isolation, EGFR(high) cells were highly enriched in multipotent precursors and displayed similar antigenic characteristics, with the exception of GFAP and Lex/SSEA-1 that were mainly expressed in adult EGFR(high) cells. EGFR levels did not correlate with neurogenic potential, indicating that the increase in EGFR expression does not directly affect differentiation. Instead, in the brain, many EGFR(high) precursors showed tangential orientation and, whether isolated from the cortex or striatum, EGFR(high) precursors displayed characteristics of cells originating from the ventral GZ such as expression Dlx and Mash-1 and the ability to generate GABAergic neurons and oligodendrocytes. Moreover, migration of EGFR(high) cells on telencephalic slices required EGFR activity. Thus, the developmentally regulated increase in EGFR levels may affect tangential migration of multipotent precursors. In addition, it can be used as a marker to effectively isolate telencephalic multipotent precursors from embryonic and adult tissue.

Molecular dissection of the semaphorin 4D receptor plexin-B1-stimulated R-Ras GTPase-activating protein activity and neurite remodeling in hippocampal neurons

Plexins serve as receptors for repulsive axonal guidance molecules semaphorins. The cytoplasmic domain of the semaphorin 4D (Sema4D) receptor, Plexin-B1 has two separated Ras GTPase-activating protein (GAP)-homologous domains, C1 and C2. Recently, we reported that the Rho family small GTPase Rnd1 associates with Plexin-B1, and the Plexin-B1-Rnd1 complex stimulates GTPase activity of R-Ras, inducing growth cone collapse in hippocampal neurons in response to Sema4D. However, the molecular mechanisms by which Plexin-B1 exhibits the GAP activity remain unclear. In this report, critical roles of Rnd1 and Sema4D in Plexin-B1-stimulated R-Ras GAP activity and neurite remodeling were examined. The N-terminal region of the cytoplasmic domain of Plexin-B1 containing the C1 domain interacts with the C-terminal region containing the C2 domain, and Rnd1 disrupts this interaction. On the other hand, Sema4D induces clustering of Rnd1-bound Plexin-B1, in parallel with inactivation of R-Ras in cells. Antibody clustering of the recombinant cytoplasmic domain of Plexin-B1 in the presence of Rnd1 triggers the R-Ras GAP activity. Deletion of the extracellular domain of Plexin-B1 causes ligand-independent clustering of the receptor, rendering the receptor constitutively active in the presence of Rnd1, and induces contraction of COS-7 cells and inhibition of neurite outgrowth in hippocampal neurons. These results indicate that Rnd1 opens the two R-Ras GAP domains of Plexin-B1, and Sema4D-induced receptor clustering stimulates R-Ras GAP activity and neurite remodeling in hippocampal neurons.

An electrostatic engine model for autoinhibition and activation of the epidermal growth factor receptor (EGFR/ErbB) family

We propose a new mechanism to explain autoinhibition of the epidermal growth factor receptor (EGFR/ErbB) family of receptor tyrosine kinases based on a structural model that postulates both their juxtamembrane and protein tyrosine kinase domains bind electrostatically to acidic lipids in the plasma membrane, restricting access of the kinase domain to substrate tyrosines. Ligand-induced dimerization promotes partial trans autophosphorylation of ErbB1, leading to a rapid rise in intracellular [Ca²⁺] that can activate calmodulin. We postulate the Ca²⁺/calmodulin complex binds rapidly to residues 645–660 of the juxtamembrane domain, reversing its net charge from +8 to −8 and repelling it from the negatively charged inner leaflet of the membrane. The repulsion has two consequences: it releases electrostatically sequestered phosphatidylinositol 4,5-bisphosphate (PIP₂), and it disengages the kinase domain from the membrane, allowing it to become fully active and phosphorylate an adjacent ErbB molecule or other substrate. We tested various aspects of the model by measuring ErbB juxtamembrane peptide binding to phospholipid vesicles using both a centrifugation assay and fluorescence correlation spectroscopy; analyzing the kinetics of interactions between ErbB peptides, membranes, and Ca²⁺/calmodulin using fluorescence stop flow; assessing ErbB1 activation in Cos1 cells; measuring fluorescence resonance energy transfer between ErbB peptides and PIP₂; and making theoretical electrostatic calculations on atomic models of membranes and ErbB juxtamembrane and kinase domains.

The role of disulfide bonds in the structure and function of murine epidermal growth factor (mEGF)

A systematic study using solid phase peptide synthesis has been undertaken to examine the role of the disulfide bonds in the structure and function of mEGF. A combination of one, two and three native disulfide pair analogues of an active truncated (4-48) form of mEGF have been synthesised by replacing specific cysteine residues with isosteric a-amino-n-butyric acid (Abu). Oxidation of the peptides was performed using either conventional aerobic oxidation at basic pH, in DMSO under acidic conditions or via selective disulfide formation using orthogonal protection of the cysteine pairs. The contribution of individual, or pairs of, disulfide bonds to EGF structure was evaluated by CD and (1)H-NMR spectroscopy. The mitogenic activity of each analogue was determined using Balb/c 3T3 mouse fibroblastsAs we have reported previously (Barnham et al. 1998), the disulfide bond between residues 6 and 20 can be removed with significant retention of biological activity (EC50 20-50 nM). The overall structure of this analogue was similar to that of native mEGF, indicating that the loss of the 6-20 disulfide bridge did not affect the global fold of the molecule. We now show that removal of any other disulfide bond, either singly or in pairs, results in a major disruption of the tertiary structure, and a large loss of activity (EC50>900 nM). Remarkably, the linear analogue appears to have greater activity (EC50 580 nM) than most one and two disulfide bond analogues although it does not have a definable tertiary structure.

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

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

Cholesterol dictates the freedom of EGF receptors and HER2 in the plane of the membrane

The flow of information through the epidermal growth factor receptor (EGFR) is shaped by molecular interactions in the plasma membrane. The EGFR is associated with lipid rafts, but their role in modulating receptor mobility and subsequent interactions is unclear. To investigate the role of nanoscale rafts in EGFR dynamics, we used single-molecule fluorescence imaging to track individual receptors and their dimerization partner, human epidermal growth factor receptor 2 (HER2), in the membrane of human mammary epithelial cells. We found that the motion of both receptors was interrupted by dwellings within nanodomains. EGFR was significantly less mobile than HER2. This difference was likely due to F-actin because its depolymerization led to similar diffusion patterns between the EGFR and HER2. Manipulations of membrane cholesterol content dramatically altered the diffusion pattern of both receptors. Cholesterol depletion led to almost complete confinement of the receptors, whereas cholesterol enrichment extended the boundaries of the restricted areas. Interestingly, F-actin depolymerization partially restored receptor mobility in cholesterol-depleted membranes. Our observations suggest that membrane cholesterol provides a dynamic environment that facilitates the free motion of EGFR and HER2, possibly by modulating the dynamic state of F-actin. The association of the receptors with lipid rafts could therefore promote their rapid interactions only upon ligand stimulation.

ERBB receptors and cancer: the complexity of targeted inhibitors

ERBB receptor tyrosine kinases have important roles in human cancer. In particular, the expression or activation of epidermal growth factor receptor and ERBB2 are altered in many epithelial tumours, and clinical studies indicate that they have important roles in tumour aetiology and progression. Accordingly, these receptors have been intensely studied to understand their importance in cancer biology and as therapeutic targets, and many ERBB inhibitors are now used in the clinic. We will discuss the significance of these receptors as clinical targets, in particular the molecular mechanisms underlying response.

The transmembrane domains of ErbB receptors do not dimerize strongly in micelles

The epidermal growth factor receptors (erbB) constitute an important class of single pass transmembrane receptors involved in the transduction of signals important for cell proliferation and differentiation. Receptor association is a key event in the signal transduction process, but the molecular basis of this interaction is not fully understood. Previous biochemical and genetic studies have suggested that the single transmembrane helices of these receptor proteins might play a role in stabilizing the receptor complexes. To determine if the erbB transmembrane domains could provide a driving force to stabilize the receptor dimers, we carried out a thermodynamic study of these domains expressed as C-terminal fusion proteins with staphylococcal nuclease. Similar fusion constructs have been used successfully to investigate the oligomerization and association thermodynamics of a number of transmembrane sequences, including that of glycophorin A. Using SDS-PAGE analysis and sedimentation equilibrium analytical ultracentrifugation, we do not find strong, specific homo or hetero-interactions between the transmembrane domains of the erbB receptors in micellar solutions. Our results indicate that any preferential interactions between these domains in micellar solutions are extremely modest, of the order of 1 kcal mol(-1) or less. We applied a thermodynamic formalism to assess the effect of weakly interacting TM segments on the behavior of a covalently attached soluble domain. In the case of the ligand-bound EGFR ectodomain, we find that restriction of the ectodomain to the micellar phase by a hydrophobic TM, even in the absence of strong specific interactions, is largely sufficient to account for the previously reported increase in dimerization affinity.

Retracted: Additive antitumor effects of the epidermal growth factor receptor tyrosine kinase inhibitor, gefitinib (Iressa), and the nonsteroidal antiandrogen, bicalutamide (Casodex), in prostate cancer cellsin vitro

Progression from an androgen-dependent to an androgen-independent state often occurs in patients with prostate cancer (PCa) who undergo hormonal therapy. We have investigated whether inhibition of the epidermal growth factor receptor (EGFR) signaling pathway affects the antitumor effect of a nonsteroidal antiandrogen. Gefitinib (Iressa), an EGFR tyrosine kinase inhibitor, and bicalutamide (Casodex), a nonsteroidal antiandrogen [androgen receptor (AR) antagonist], were administered alone and in combination to AR-positive human PCa cell lines. FACS analysis showed lower EGFR expression levels on AR-positive cells (LNCaP, CWR22, CWR22R 2152 and AR-transfected DU145 cell lines) compared with AR-negative cells (DU145, PC3 and TSU-Pr1). Moreover, in AR-transfected DU145 cells, chronic treatment with bicalutamide increased EGFR expression to levels similar to androgen-independent DU145 cells. All AR-positive PCa cell lines were sensitive to gefitinib (IC50 = 0.1-0.6 microM), whereas higher concentrations of bicalutamide were needed to reduce AR-positive PCa cell line proliferation (IC50 = 0.8-2.0 microM). Low doses of gefitinib increased the antitumor effects of bicalutamide by strongly reducing the IC50 of bicalutamide (approximately 10-fold). Similarly, bicalutamide increased the antiproliferative effects of gefitinib by reducing the IC50 of gefitinib (approximately 5-fold). Taken together, our data suggest that in androgen-dependent cell lines, addition of gefitinib in combination with bicalutamide results in concurrent dual inhibition of AR and EGFR/HER2 pathways. This causes a significant delay in the onset of EGFR-driven androgen independence.

Structural basis for inhibition of the epidermal growth factor receptor by cetuximab

Recent structural studies of epidermal growth factor receptor (EGFR) family extracellular regions have identified an unexpected mechanism for ligand-induced receptor dimerization that has important implications for activation and inhibition of these receptors. Here we describe the 2.8 angstroms resolution X-ray crystal structure of the antigen binding (Fab) fragment from cetuximab (Erbitux), an inhibitory anti-EGFR antibody, in complex with the soluble extracellular region of EGFR (sEGFR). The sEGFR is in the characteristic "autoinhibited" or "tethered" inactive configuration. Cetuximab interacts exclusively with domain III of sEGFR, partially occluding the ligand binding region on this domain and sterically preventing the receptor from adopting the extended conformation required for dimerization. We suggest that both these effects contribute to potent inhibition of EGFR activation.

The antitumor monoclonal antibody 806 recognizes a high-mannose form of the EGF receptor that reaches the cell surface when cells over-express the receptor

Overexpression of the EGFR is commonly caused by EGFR gene amplification and is sometimes associated with expression of a variant EGFR (de2-7 EGFR or EGFRvIII) bearing an internal deletion in its extracellular domain. mAb 806 is a novel EGFR antibody with significant antitumor activity that recognizes both the de2-7 EGFR and a subset of the wild-type (wt) EGFR when overexpressed, but does not bind the EGFR expressed in normal tissues. Recently, we demonstrated that the mAb 806 epitope is restricted to a short cysteine loop of the EGFR (amino acids 287-302) that is only available for antibody binding in a transitional form of the receptor, which occurs as the receptor changes from its inactive tethered conformation to a dimeric untethered form. The truncation associated with the de2-7 EGFR mutation renders this receptor constitutively untethered, leading to increased binding of mAb 806. We now show that mAb 806 preferentially binds the immature high-mannose wt and de2-7 EGFR precursors normally located in the endoplasmic reticulum, indicating that this form of the wt EGFR is also constitutively untethered. Using the unique specificity of mAb 806, we clearly demonstrated the presence of these high-mannose EGFR precursors on the cell surface. Given that the high-mannose forms of the wt EGFR must be untethered they may contribute to the spontaneous EGFR signaling reported in cells overexpressing the receptor. These precursor forms of the EGFR thus represent novel tumor targets and contribute to the exceptional selectivity of mAb 806 for EGFR when overexpressed in cancer cells. As our observations are likely to apply to other receptors overexpressed in cancer, they suggest a strategy for developing antitumor antibodies even when the target receptor is expressed in normal tissue.

ADAMs: key components in EGFR signalling and development

ADAM (a disintegrin and metalloprotease) proteins are membrane-anchored metalloproteases that process and shed the ectodomains of membrane-anchored growth factors, cytokines and receptors. ADAMs also have essential roles in fertilization, angiogenesis, neurogenesis, heart development and cancer. Research on ADAMs and their role in protein ectodomain shedding is emerging as a fertile ground for gathering new insights into the functional regulation of membrane proteins.

erbb3 and erbb2 Are Essential for Schwann Cell Migration and Myelination in Zebrafish

BACKGROUND

Myelin is critical for efficient axonal conduction in the vertebrate nervous system. Neuregulin (Nrg) ligands and their ErbB receptors are required for the development of Schwann cells, the glial cells that form myelin in the peripheral nervous system. Previous studies have not determined whether Nrg-ErbB signaling is essential in vivo for Schwann cell fate specification, proliferation, survival, migration, or the onset of myelination.

RESULTS

In genetic screens for mutants with disruptions in myelinated nerves, we identified mutations in erbb3 and erbb2, which together encode a heteromeric tyrosine kinase receptor for Neuregulin ligands. Phenotypic analysis shows that both genes are essential for development of Schwann cells. BrdU-incorporation studies and time-lapse analysis reveal that Schwann cell proliferation and migration, but not survival, are disrupted in erbb3 mutants. We show that Schwann cells can migrate in the absence of DNA replication. This uncoupling of proliferation and migration indicates that erbb gene function is required independently for these two processes. Pharmacological inhibition of ErbB signaling at different stages reveals a continuing requirement for ErbB function during migration and also provides evidence that ErbB signaling is required after migration for proliferation and the terminal differentiation of myelinating Schwann cells.

CONCLUSIONS

These results provide in vivo evidence that Neuregulin-ErbB signaling is essential for directed Schwann cell migration and demonstrate that this pathway is also required for the onset of myelination in postmigratory Schwann cells.

Phase I clinical study of pertuzumab, a novel HER dimerization inhibitor, in patients with advanced cancer

PURPOSE

Pertuzumab, a recombinant humanized monoclonal antibody (2C4), binds to extracellular domain II of the HER-2 receptor and blocks its ability to dimerize with other HER receptors. Pertuzumab represents a new class of targeted therapeutics known as HER dimerization inhibitors. A clinical study was conducted to investigate safety and pharmacokinetics of pertuzumab and to perform a preliminary assessment of HER dimerization inhibition as a treatment strategy.

PATIENTS AND METHODS

Patients with incurable, locally advanced, recurrent or metastatic solid tumors that had progressed during or after standard therapy were recruited to a dose-escalation study of pertuzumab (0.5 to 15 mg/kg) given intravenously every 3 weeks.

RESULTS

Twenty-one patients received pertuzumab and 19 completed at least two cycles. Pertuzumab was well tolerated. Overall, 365 adverse events were reported and 122 considered to be possibly drug related. Of these, 116 were of grade 1 to 2 intensity. The pharmacokinetics of pertuzumab were similar to other humanized immunoglobulin G antibodies, supporting a 3-week dosing regimen. Trough plasma concentrations were in excess of target concentrations at doses greater than 5 mg/kg. Two patients, one with ovarian cancer (5.0 mg/kg) and one with pancreatic islet cell carcinoma (15.0 mg/kg), achieved a partial response. Responses were documented by Response Evaluation Criteria in Solid Tumors after 1.5 and 6 months of pertuzumab therapy, and lasted for 11 and 10 months, respectively. Stable disease lasting for more than 2.5 months (range, 2.6 to 5.5 months) was observed in six patients.

CONCLUSION

These results demonstrate that pertuzumab is well tolerated, has a pharmacokinetic profile which supports 3-week dosing, and is clinically active, suggesting that inhibition of dimerization may be an effective anticancer strategy.

Quinone-induced Cdc25A inhibition causes ERK-dependent connexin phosphorylation

Gap junctional intercellular communication (GJC) varies during progression of the cell cycle. We propose here that Cdc25A, a dual specificity phosphatase crucial for cell cycle progression, is linked to connexin (Cx) phosphorylation and the modulation of GJC. Inhibition of Cdc25 phosphatases in rat liver epithelial cells employing a 1,4-naphthoquinone-based inhibitor, NSC95397, induced cell cycle arrest, tyrosine phosphorylation of the epidermal growth factor receptor (EGFR), and activation of extracellular signal-regulated kinases ERK-1 and -2. ERK activation was blocked by specific inhibitors of MAPK/ERK kinases 1/2 or of the EGFR tyrosine kinase. An EGFR-dephosphorylation assay suggested that Cdc25A interacts with the EGFR, with inhibition by NSC95397 resulting in activation of the receptor. As a consequence of ERK activation, Cx43 was phosphorylated, resulting in a downregulation of GJC. Loss of GJC was prevented by inhibition of ERK activation. In summary, cell cycle and GJC are connected via Cdc25A and the EGFR-ERK pathway.

Ligand depletion negatively controls the mitogenic activity of epidermal growth factor

EGF activates the ErbB1 receptor, but there appears only a limited correlation between its receptor binding affinity and mitogenic activity. This is indicated by our present observation that in cells with high ErbB1 expression, including SUM102 breast tumor cells, low affinity EGF/Notch chimeras have similarly high mitogenic activity as EGF, in spite of the fact that EGF is superior in inducing receptor tyrosine phosphorylation and p42/p44 MAP-kinase activity. However, as a result of receptor-mediated internalisation high-affinity ligands such as EGF are depleted much more rapidly from the extracellular medium than low-affinity EGF/Notch chimeras. As a consequence, the mitogenic activity of EGF on ErbB1 overexpressing cells is limited by substantial degradation of internalised ligand in the period before cells enter S-phase, a phenomenon that is not observed for low affinity mutant ligands. The mitogenic activity of EGF on ErbB1 overexpressing cells does therefore not only depend on the applied concentration but also on the total amount of ligand added, and is strongly underestimated when tested in a limited assay volume. No such dependence on the incubation volume was observed for EGF activity on cells with low ErbB1 expression levels and on cells for which EGF is growth inhibitory.

Bipartite inhibition of Drosophila epidermal growth factor receptor by the extracellular and transmembrane domains of Kekkon1

In Drosophila, signaling by the epidermal growth factor receptor (EGFR) is required for a diverse array of developmental decisions. Essential to these decisions is the precise regulation of the receptor's activity by both stimulatory and inhibitory molecules. To better understand the regulation of EGFR activity we investigated inhibition of EGFR by the transmembrane protein Kekkon1 (Kek1). Kek1 encodes a molecule containing leucine-rich repeats (LRR) and an immunoglobulin (Ig) domain and is the founding member of the Drosophila Kekkon family. Here we demonstrate with a series of Kek1-Kek2 chimeras that while the LRRs suffice for EGFR binding, inhibition in vivo requires the Kek1 juxta/transmembrane region. We demonstrate directly, and using a series of Kek1-EGFR chimeras, that Kek1 is not a phosphorylation substrate for the receptor in vivo. In addition, we show that EGFR inhibition is unique to Kek1 among Kek family members and that this function is not ligand or tissue specific. Finally, we have identified a unique class of EGFR alleles that specifically disrupt Kek1 binding and inhibition, but preserve receptor activation. Interestingly, these alleles map to domain V of the Drosophila EGFR, a region absent from the vertebrate receptors. Together, our results support a model in which the LRRs of Kek1 in conjunction with its juxta/transmembrane region direct association and inhibition of the Drosophila EGFR through interactions with receptor domain V.

Molecular dynamics (MD) investigations of preformed structures of the transmembrane domain of the oncogenic Neu receptor dimer in a DMPC bilayer

The critical Val/Glu mutation in the membrane spanning domain of the rat Neu receptor confers the ability for ligand-independent signaling and leads to increased dimerization and transforming ability. There is evidence that the two transmembrane interacting helices play a role in receptor activation by imposing orientation constraints to the intracellular tyrosine kinase domains. By using MD simulations we have attempted to discriminate between correct and improper helix-helix packing by examining the structural and energetic properties of preformed left-handed and right-handed structures in a fully hydrated DMPC bilayer. The best energetic balance between the residues at the helix-helix interface and the residues exposed to the lipids is obtained for helices in symmetrical left-handed interactions packed together via Glu side chain/Ala backbone interhelical hydrogen bonds. Analyses demonstrate the importance of the ATVEG motif in helix-helix packing and point to additional contacting residues necessary for association. Our findings, all consistent with experimental data, suggest that a symmetrical left-handed structure of the helices could be the transmembrane domain configuration that promotes receptor activation and transformation. The present study may provide further insight into signal transduction mechanisms of the ErbB/Neu receptors.