Mechanism of activation and inhibition of the HER4/ErbB4 kinase

The Nudix hydrolase CDP-chase, a CDP-choline pyrophosphatase, is an asymmetric dimer with two distinct enzymatic activities

A Nudix enzyme from Bacillus cereus (NCBI RefSeq accession no. NP_831800) catalyzes the hydrolysis of CDP-choline to produce CMP and phosphocholine. Here, we show that in addition, the enzyme has a 3'→5' RNA exonuclease activity. The structure of the free enzyme, determined to a 1.8-Å resolution, shows that the enzyme is an asymmetric dimer. Each monomer consists of two domains, an N-terminal helical domain and a C-terminal Nudix domain. The N-terminal domain is placed relative to the C-terminal domain such as to result in an overall asymmetric arrangement with two distinct catalytic sites: one with an "enclosed" Nudix pyrophosphatase site and the other with a more open, less-defined cavity. Residues that may be important for determining the asymmetry are conserved among a group of uncharacterized Nudix enzymes from Gram-positive bacteria. Our data support a model where CDP-choline hydrolysis is catalyzed by the enclosed Nudix site and RNA exonuclease activity is catalyzed by the open site. CDP-Chase is the first identified member of a novel Nudix family in which structural asymmetry has a profound effect on the recognition of substrates.

A multiscale modeling approach to investigate molecular mechanisms of pseudokinase activation and drug resistance in the HER3/ErbB3 receptor tyrosine kinase signaling network

Multiscale modeling provides a powerful and quantitative platform for investigating the complexity inherent in intracellular signaling pathways and rationalizing the effects of molecular perturbations on downstream signaling events and ultimately, on the cell phenotype. Here we describe the application of a multiscale modeling scheme to the HER3/ErbB3 receptor tyrosine kinase (RTK) signaling network, which regulates critical cellular processes including proliferation, migration and differentiation. The HER3 kinase is a topic of current interest and investigation, as it has been implicated in mechanisms of resistance to tyrosine kinase inhibition (TKI) of EGFR and HER2 in the treatment of many human malignancies. Moreover, the commonly regarded status of HER3 as a catalytically inactive 'pseudokinase' has recently been challenged by our previous study, which demonstrated robust residual kinase activity for HER3. Through our multiscale model, we investigate the most significant molecular interactions that contribute to potential mechanisms of HER3 activity and the physiological relevance of this activity to mechanisms of drug resistance in an ErbB-driven tumor cell in silico. The results of our molecular-scale simulations support the characterization of HER3 as a weakly active kinase that, in contrast to its fully-active ErbB family members, depends upon a unique hydrophobic interface to coordinate the alignment of specific catalytic residues required for its activity. Translating our molecular simulation results of the uniquely active behavior of the HER3 kinase into a physiologically relevant environment, our HER3 signaling model demonstrates that even a weak level of HER3 activity may be sufficient to induce AKT signaling and TKI resistance in the context of an ErbB signaling-dependent tumor cell, and therefore therapeutic targeting of HER3 may represent a superior treatment strategy for specific ErbB-driven cancers.

Molecular dynamics analysis of conserved hydrophobic and hydrophilic bond-interaction networks in ErbB family kinases

The EGFR (epidermal growth factor receptor)/ErbB/HER (human EGFR) family of kinases contains four homologous receptor tyrosine kinases that are important regulatory elements in key signalling pathways. To elucidate the atomistic mechanisms of dimerization-dependent activation in the ErbB family, we have performed molecular dynamics simulations of the intracellular kinase domains of three members of the ErbB family (those with known kinase activity), namely EGFR, ErbB2 (HER2) and ErbB4 (HER4), in different molecular contexts: monomer against dimer and wild-type against mutant. Using bioinformatics and fluctuation analyses of the molecular dynamics trajectories, we relate sequence similarities to correspondence of specific bond-interaction networks and collective dynamical modes. We find that in the active conformation of the ErbB kinases, key subdomain motions are co-ordinated through conserved hydrophilic interactions: activating bond-networks consisting of hydrogen bonds and salt bridges. The inactive conformations also demonstrate conserved bonding patterns (albeit less extensive) that sequester key residues and disrupt the activating bond network. Both conformational states have distinct hydrophobic advantages through context-specific hydrophobic interactions. We show that the functional (activating) asymmetric kinase dimer interface forces a corresponding change in the hydrophobic and hydrophilic interactions that characterize the inactivating bond network, resulting in motion of the αC-helix through allostery. Several of the clinically identified activating kinase mutations of EGFR act in a similar fashion to disrupt the inactivating bond network. The present molecular dynamics study reveals a fundamental difference in the sequence of events in EGFR activation compared with that described for the Src kinase Hck.

Structural analysis of the mechanism of inhibition and allosteric activation of the kinase domain of HER2 protein

Aberrant signaling of ErbB family members human epidermal growth factor 2 (HER2) and epidermal growth factor receptor (EGFR) is implicated in many human cancers, and HER2 expression is predictive of human disease recurrence and prognosis. Small molecule kinase inhibitors of EGFR and of both HER2 and EGFR have received approval for the treatment of cancer. We present the first high resolution crystal structure of the kinase domain of HER2 in complex with a selective inhibitor to understand protein activation, inhibition, and function at the molecular level. HER2 kinase domain crystallizes as a dimer and suggests evidence for an allosteric mechanism of activation comparable with previously reported activation mechanisms for EGFR and HER4. A unique Gly-rich region in HER2 following the α-helix C is responsible for increased conformational flexibility within the active site and could explain the low intrinsic catalytic activity previously reported for HER2. In addition, we solved the crystal structure of the kinase domain of EGFR in complex with a HER2/EGFR dual inhibitor (TAK-285). Comparison with previously reported inactive and active EGFR kinase domain structures gave insight into the mechanism of HER2 and EGFR inhibition and may help guide the design and development of new cancer drugs with improved potency and selectivity.

Catalytic control in the EGF receptor and its connection to general kinase regulatory mechanisms

In contrast to the active conformations of protein kinases, which are essentially the same for all kinases, inactive kinase conformations are structurally diverse. Some inactive conformations are, however, observed repeatedly in different kinases, perhaps reflecting an important role in catalysis. In this review, we analyze one of these recurring conformations, first identified in CDK and Src kinases, which turned out to be central to understanding of how kinase domain of the EGF receptor is activated. This mechanism, which involves the stabilization of the active conformation of an α helix, has features in common with mechanisms operative in several other kinases.

Carboxyl-group footprinting maps the dimerization interface and phosphorylation-induced conformational changes of a membrane-associated tyrosine kinase

Her4 is a transmembrane receptor tyrosine kinase belonging to the ErbB-EGFR family. It plays a vital role in the cardiovascular and nervous systems, and mutations in Her4 have been found in melanoma and lung cancer. The kinase domain of Her4 forms a dimer complex, called the asymmetric dimer, which results in kinase activation. Although a crystal structure of the Her4 asymmetric dimer is known, the dimer affinity and the effect of the subsequent phosphorylation steps on kinase domain conformation are unknown. We report here the use of carboxyl-group footprinting MS on a recombinant expressed, Her4 kinase-domain construct to address these questions. Carboxyl-group footprinting uses a water-soluble carbodiimide, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, in the presence of glycine ethyl ester, to modify accessible carboxyl groups on glutamate and aspartate residues. Comparisons of Her4 kinase-domain monomers versus dimers and of unphosphorylated versus phosphorylated dimers were made to map the dimerization interface and to determine phosphorylation induced-conformational changes. We detected 37 glutamate and aspartate residues that were modified, and we quantified their extents of modification by liquid chromatography MS. Five residues showed changes in carboxyl-group modification. Three of these residues are at the predicted dimer interface, as shown by the crystal structure, and the remaining two residues are on loops that likely have altered conformation in the kinase dimer. Incubating the Her4 kinase dimers with ATP resulted in dramatic increase in Tyr-850 phosphorylation, located on the activation loop, and this resulted in a conformational change in this loop, as evidenced by reduction in carboxyl-group modification. The kinase monomer-dimer equilibrium was measured using a titration format in which the extent of carboxyl-group footprinting was mathematically modeled to give the dimer association constant (1.5-6.8 × 10(12) dm(2)/mol). This suggests that the kinase-domain makes a significant contribution to the overall dimerization affinity of the full-length Her4 protein.

Analysis of Somatic Mutations in Cancer: Molecular Mechanisms of Activation in the ErbB Family of Receptor Tyrosine Kinases

The ErbB/EGFR/HER family of kinases consists of four homologous receptor tyrosine kinases which are important regulatory elements in many cellular processes, including cell proliferation, differentiation, and migration. Somatic mutations in, or over-expression of, the ErbB family is found in many cancers and is correlated with a poor prognosis; particularly, clinically identified mutations found in non-small-cell lung cancer (NSCLC) of ErbB1 have been shown to increase its basal kinase activity and patients carrying these mutations respond remarkably to the small tyrosine kinase inhibitor gefitinib. Here, we analyze the potential effects of the currently catalogued clinically identified mutations in the ErbB family kinase domains on the molecular mechanisms of kinase activation. Recently, we identified conserved networks of hydrophilic and hydrophobic interactions characteristic to the active and inactive conformation, respectively. Here, we show that the clinically identified mutants influence the kinase activity in distinctive fashion by affecting the characteristic interaction networks.

Investigating Molecular Mechanisms of Activation and Mutation of the HER2 Receptor Tyrosine Kinase through Computational Modeling and Simulation

Human epidermal growth factor receptor 2 (HER2)/ErbB2 is a receptor tyrosine kinase belonging to the EGFR/ErbB family and is overexpressed in 20-30% of human breast cancers. Since there is a growing effort to develop pharmacological inhibitors of the HER2 kinase for the treatment of breast cancer, it is clinically valuable to rationalize how specific mutations impact the molecular mechanism of receptor activation. Although several crystal structures of the ErbB kinases have been solved, the precise mechanism of HER2 activation remains unknown, and it has been suggested that HER2 is unique in its requirement for phosphorylation of Y877, a key tyrosine residue located in the activation loop (A-loop). In our studies, discussed here, we have investigated the mechanisms that are important in HER2 kinase domain regulation and compared them with the other ErbB family members, namely EGFR and ErbB4, to determine the molecular basis for HER2's unique mode of activation. We apply computational simulation techniques at the atomic level and at the electronic structure (quantum mechanical) level to elucidate details of the mechanisms governing the kinase domains of these ErbB members. Through analysis of our simulation results, we have discovered potential regulatory mechanisms common to EGFR, HER2, and ErbB4, including a tight coupling between the A-loop and catalytic loop that may contribute to alignment of residues required for catalysis in the active kinase. We further postulate an autoinhibitory mechanism whereby the inactive kinase is stabilized through sequestration of catalytic residues. In HER2, we also predict a role for phosphorylated Y877 in bridging a network of hydrogen bonds that fasten the A-loop in its active conformation, suggesting that HER2 may be unique among the ErbB members in requiring A-loop tyrosine phosphorylation for functionality. In EGFR, HER2, and ErbB4, we discuss the possible effects of activating mutations. Delineation of the activation mechanism of HER2 in the context of the other ErbB members is crucial for understanding how the activated kinase might interact with downstream molecules and couple to signaling cascades that promote cancer. Our comparative analysis furthers insight into the mechanics of activation of the HER2 kinase and enables us to predict the effect of an identified insertion mutation on HER2 activation. Further understanding of the mechanism of HER2 kinase activation at the atomic scale and how it couples to downstream signaling at the cellular scale will elucidate predictive molecular phenotypes that may indicate likelihood of response to specific therapies for HER2-mediated cancers.

Co-conserved features associated with cis regulation of ErbB tyrosine kinases

Background

The epidermal growth factor receptor kinases, or ErbB kinases, belong to a large sub-group of receptor tyrosine kinases (RTKs), which share a conserved catalytic core. The catalytic core of ErbB kinases have functionally diverged from other RTKs in that they are activated by a unique allosteric mechanism that involves specific interactions between the kinase core and the flanking Juxtamembrane (JM) and COOH-terminal tail (C-terminal tail). Although extensive studies on ErbB and related tyrosine kinases have provided important insights into the structural basis for ErbB kinase functional divergence, the sequence features that contribute to the unique regulation of ErbB kinases have not been systematically explored.

Methodology/Principal Findings

In this study, we use a Bayesian approach to identify the selective sequence constraints that most distinguish ErbB kinases from other receptor tyrosine kinases. We find that strong ErbB kinase-specific constraints are imposed on residues that tether the JM and C-terminal tail to key functional regions of the kinase core. A conserved RIxKExE motif in the JM-kinase linker region and a glutamine in the inter-lobe linker are identified as two of the most distinguishing features of the ErbB family. While the RIxKExE motif tethers the C-terminal tail to the N-lobe of the kinase domain, the glutamine tethers the C-terminal tail to hinge regions critical for inter-lobe movement. Comparison of the active and inactive crystal structures of ErbB kinases indicates that the identified residues are conformationally malleable and can potentially contribute to the cis regulation of the kinase core by the JM and C-terminal tail. ErbB3, and EGFR orthologs in sponges and parasitic worms, diverge from some of the canonical ErbB features, providing insights into sub-family and lineage-specific functional specialization.

Conclusion/Significance

Our analysis pinpoints key residues for mutational analysis, and provides new clues to cancer mutations that alter the canonical modes of ErbB kinase regulation.

Cytohesins are cytoplasmic ErbB receptor activators

Signaling by ErbB receptors requires the activation of their cytoplasmic kinase domains, which is initiated by ligand binding to the receptor ectodomains. Cytoplasmic factors contributing to the activation are unknown. Here we identify members of the cytohesin protein family as such factors. Cytohesin inhibition decreased ErbB receptor autophosphorylation and signaling, whereas cytohesin overexpression stimulated receptor activation. Monitoring epidermal growth factor receptor (EGFR) conformation by anisotropy microscopy together with cell-free reconstitution of cytohesin-dependent receptor autophosphorylation indicate that cytohesins facilitate conformational rearrangements in the intracellular domains of dimerized receptors. Consistent with cytohesins playing a prominent role in ErbB receptor signaling, we found that cytohesin overexpression correlated with EGF signaling pathway activation in human lung adenocarcinomas. Chemical inhibition of cytohesins resulted in reduced proliferation of EGFR-dependent lung cancer cells in vitro and in vivo. Our results establish cytohesins as cytoplasmic conformational activators of ErbB receptors that are of pathophysiological relevance.

EGFR-mutated lung cancer: a paradigm of molecular oncology

The development of EGFR tyrosine kinase inhibitors for clinical use in non-small cell lung cancer and the subsequent discovery of activating EGFR mutations have led to an explosion of knowledge in the fields of EGFR biology, targeted therapeutics and lung cancer research. EGFR-mutated adenocarcinoma of the lung has clearly emerged as a unique clinical entity necessitating the routine introduction of molecular diagnostics into our current diagnostic algorithms and leading to the evidence-based preferential usage of EGFR-targeted agents for patients with EGFR-mutant lung cancers. This review will summarize our current understanding of the functional role of activating mutations, key downstream signaling pathways and regulatory mechanisms, pivotal primary and acquired resistance mechanisms, structure-function relationships and ultimately the incorporation of molecular diagnostics and small molecule EGFR tyrosine kinase inhibitors into our current treatment paradigms.

Geldanamycin selectively targets the nascent form of ERBB3 for degradation

Heat shock protein 90 (HSP90) targets a broad spectrum of client proteins with divergent modes of interaction and consequences. The homologous epidermal growth factor receptor (EGFR) and ERBB2 receptors as well as kinase-deficient mutants thereof differ in their requirement for HSP90 in the nascent versus mature state of the receptor. Specific features of the kinase domain have been implicated for the selective association of HSP90 with mature ERBB2. We evaluated the role of HSP90 for the homologous ERBB3 receptor. ERBB3 is naturally kinase deficient, a central mediator in cell survival and stress response and the primary dimerization partner for ERBB2 in signaling. Cellular studies indicate that, similar to EGFR, the geldanamycin (GA) sensitivity of ERBB3 and HSP90 binding resides in the nascent state and is dependent on the presence of the kinase domain of ERBB3. Furthermore, despite its intrinsic lack of kinase activity and in contrast to the reported GA sensitivity of mature and kinase-deficient EGFR, the GA sensitivity of the nascent state of ERBB3 appears to be exclusive. Geldanamycin disrupts the interaction of ERBB3 and HSP90 and inhibits ERBB3 maturation at an early stage of synthesis, prior to export from the ER. Studies with a photo-convertible fusion protein of ERBB3 suggest geldanamycin sensitivity at a later stage in maturation, possibly through the putative role of HSP90 in structural proofreading.

Structural basis for negative cooperativity in growth factor binding to an EGF receptor

Transmembrane signaling by the epidermal growth factor receptor (EGFR) involves ligand-induced dimerization and allosteric regulation of the intracellular tyrosine kinase domain. Crystallographic studies have shown how ligand binding induces dimerization of the EGFR extracellular region but cannot explain the "high-affinity" and "low-affinity" classes of cell-surface EGF-binding sites inferred from curved Scatchard plots. From a series of crystal structures of the Drosophila EGFR extracellular region, we show here how Scatchard plot curvature arises from negatively cooperative ligand binding. The first ligand-binding event induces formation of an asymmetric dimer with only one bound ligand. The unoccupied site in this dimer is structurally restrained, leading to reduced affinity for binding of the second ligand, and thus negative cooperativity. Our results explain the cell-surface binding characteristics of EGF receptors and suggest how individual EGFR ligands might stabilize distinct dimeric species with different signaling properties.

Cell signaling by receptor tyrosine kinases

Recent structural studies of receptor tyrosine kinases (RTKs) have revealed unexpected diversity in the mechanisms of their activation by growth factor ligands. Strategies for inducing dimerization by ligand binding are surprisingly diverse, as are mechanisms that couple this event to activation of the intracellular tyrosine kinase domains. As our understanding of these details becomes increasingly sophisticated, it provides an important context for therapeutically countering the effects of pathogenic RTK mutations in cancer and other diseases. Much remains to be learned, however, about the complex signaling networks downstream from RTKs and how alterations in these networks are translated into cellular responses.

ErbB3/HER3 intracellular domain is competent to bind ATP and catalyze autophosphorylation

ErbB3/HER3 is one of four members of the human epidermal growth factor receptor (EGFR/HER) or ErbB receptor tyrosine kinase family. ErbB3 binds neuregulins via its extracellular region and signals primarily by heterodimerizing with ErbB2/HER2/Neu. A recently appreciated role for ErbB3 in resistance of tumor cells to EGFR/ErbB2-targeted therapeutics has made it a focus of attention. However, efforts to inactivate ErbB3 therapeutically in parallel with other ErbB receptors are challenging because its intracellular kinase domain is thought to be an inactive pseudokinase that lacks several key conserved (and catalytically important) residues-including the catalytic base aspartate. We report here that, despite these sequence alterations, ErbB3 retains sufficient kinase activity to robustly trans-autophosphorylate its intracellular region--although it is substantially less active than EGFR and does not phosphorylate exogenous peptides. The ErbB3 kinase domain binds ATP with a K(d) of approximately 1.1 microM. We describe a crystal structure of ErbB3 kinase bound to an ATP analogue, which resembles the inactive EGFR and ErbB4 kinase domains (but with a shortened alphaC-helix). Whereas mutations that destabilize this configuration activate EGFR and ErbB4 (and promote EGFR-dependent lung cancers), a similar mutation conversely inactivates ErbB3. Using quantum mechanics/molecular mechanics simulations, we delineate a reaction pathway for ErbB3-catalyzed phosphoryl transfer that does not require the conserved catalytic base and can be catalyzed by the "inactive-like" configuration observed crystallographically. These findings suggest that ErbB3 kinase activity within receptor dimers may be crucial for signaling and could represent an important therapeutic target.

Resiliency and vulnerability in the HER2-HER3 tumorigenic driver

About 25% of breast cancers harbor the amplified oncogene human epidermal growth factor receptor 2 (HER2) and are dependent on HER2 kinase function, identifying HER2 as a vulnerable target for therapy. However, HER2-HER3 signaling is buffered so that it is protected against a nearly two-log inhibition of HER2 catalytic activity; this buffering is driven by the negative regulation of HER3 by Akt. We have now further characterized HER2-HER3 signaling activity and have shown that the compensatory buffering prevents apoptotic tumor cell death from occurring as a result of the combined loss of mitogen-activated protein kinase (MAPK) and Akt signaling. To overcome the cancer cells' compensatory mechanisms, we coadministered a phosphoinositide 3-kinase-mammalian target of rapamycin inhibitor and a HER2 tyrosine kinase inhibitor (TKI). This treatment strategy proved equivocal because it induced both TKI-sensitizing and TKI-desensitizing effects and robust cross-compensation of MAPK and Akt signaling pathways. Noting that HER2-HER3 activity was completely inhibited by higher, fully inactivating doses of TKI, we then attempted to overcome the cells' compensatory buffering with this higher dose. This treatment crippled all downstream signaling and induced tumor apoptosis. Although such high doses of TKI are toxic in vivo when given continuously, we found that intermittent doses of TKI administered to mice produced sequential cycles of tumor apoptosis and ultimately complete tumor regression in mouse models, with little toxicity. This strategy for inactivation of HER2-HER3 tumorigenic activity is proposed for clinical testing.

Her4 and Her2/neu tyrosine kinase domains dimerize and activate in a reconstituted in vitro system

Her4 (ErbB-4) and Her2/neu (ErbB-2) are receptor-tyrosine kinases belonging to the epidermal growth factor receptor (EGFR) family. Crystal structures of EGFR and Her4 kinase domains demonstrate kinase dimerization and activation through an allosteric mechanism. The kinase domains form an asymmetric dimer, where the C-lobe surface of one monomer contacts the N-lobe of the other monomer. EGFR kinase dimerization and activation in vitro was previously reported using a nickel-chelating lipid-liposome system, and we now apply this system to all other members of the EGFR family. Polyhistidine-tagged Her4, Her2/neu, and Her3 kinase domains are bound to these nickel-liposomes and are brought to high local concentration, mimicking what happens to full-length receptors in vivo following ligand binding. Addition of nickel-liposomes to Her4 kinase domain results in 40-fold activation in kinase activity and marked enhancement of C-terminal tail autophosphorylation. Activation of Her4 shows a sigmoidal dependence on kinase concentration, consistent with a cooperative process requiring kinase dimerization. Her2/neu kinase activity is also activated by nickel-liposomes, and is increased further by heterodimerization with Her3 or Her4. The ability of Her3 and Her4 to heterodimerize and activate other family members is studied in vitro. Her3 kinase domain readily activates Her2/neu but is a poor activator of Her4, which differs from the prediction made by the asymmetric dimer model. Mutation of Her3 residues (952)ENI(954) to the corresponding sequence in Her4 enhanced the ability of Her3 to activate Her4, demonstrating that sequence differences on the C-lobe surface influence the heterodimerization and activation of ErbB kinase domains.

Structural analysis of the catalytically inactive kinase domain of the human EGF receptor 3

The kinase domain of human epidermal growth factor receptor (HER) 3/ErbB3, a member of the EGF receptor (EGFR) family, lacks several residues that are critical for catalysis. Because catalytic activity in EGFR family members is switched on by an allosteric interaction between kinase domains in an asymmetric kinase domain dimer, HER3 might be specialized to serve as an activator of other EGFR family members. We have determined the crystal structure of the HER3 kinase domain and show that it appears to be locked into an inactive conformation that resembles that of EGFR and HER4. Although the crystal structure shows that the HER3 kinase domain binds ATP, we confirm that it is catalytically inactive but can serve as an activator of the EGFR kinase domain. The HER3 kinase domain forms a dimer in the crystal, mediated by hydrophobic contacts between the N-terminal lobes of the kinase domains. This N-lobe dimer closely resembles a dimer formed by inactive HER4 kinase domains in crystal structures determined previously, and molecular dynamics simulations suggest that the HER3 and HER4 N-lobe dimers are stable. The kinase domains of HER3 and HER4 form similar chains in their respective crystal lattices, in which N-lobe dimers are linked together by reciprocal exchange of C-terminal tails. The conservation of this tiling pattern in HER3 and HER4, which is the closest evolutionary homolog of HER3, might represent a general mechanism by which this branch of the HER receptors restricts ligand-independent formation of active heterodimers with other members of the EGFR family.

Analysis of the tyrosine kinome in melanoma reveals recurrent mutations in ERBB4

Tyrosine phosphorylation is important in signaling pathways underlying tumorigenesis. We performed a mutational analysis of the protein tyrosine kinase (PTK) gene family in cutaneous metastatic melanoma. We identified 30 somatic mutations affecting the kinase domains of 19 PTKs and subsequently evaluated the entire coding regions of the genes encoding these 19 PTKs for somatic mutations in 79 melanoma samples. We found ERBB4 mutations in 19% of individuals with melanoma and found mutations in two other kinases (FLT1 and PTK2B) in 10% of individuals with melanomas. We examined seven missense mutations in the most commonly altered PTK gene, ERBB4, and found that they resulted in increased kinase activity and transformation ability. Melanoma cells expressing mutant ERBB4 had reduced cell growth after shRNA-mediated knockdown of ERBB4 or treatment with the ERBB inhibitor lapatinib. These studies could lead to personalized therapeutics specifically targeting the kinases that are mutationally altered in individual melanomas.

EGFR juxtamembrane domain, membranes, and calmodulin: kinetics of their interaction

Calcium/calmodulin (Ca/CaM) binds to the intracellular juxtamembrane domain (JMD) of the epidermal growth factor receptor (EGFR). The basic JMD also binds to acidic lipids in the inner leaflet of the plasma membrane, and this interaction may contribute an extra level of autoinhibition to the receptor. Binding of a ligand to the EGFR produces a rapid increase in intracellular calcium, [Ca2+]i, and thus Ca/CaM. How does Ca/CaM compete with the plasma membrane for the JMD? Does Ca/CaM directly pull the JMD off the membrane or does Ca/CaM only bind to the JMD after it has dissociated spontaneously from the bilayer? To answer this question, we studied the effect of Ca/CaM on the rate of dissociation of fluorescent JMD peptides from phospholipid vesicles by making kinetic stop-flow measurements. Ca/CaM increases the rate of dissociation: an analysis of the differential equations that describe the dissociation shows that Ca/CaM must directly pull the basic JMD peptide off the membrane surface. These measurements lead to a detailed atomic-level mechanism for EGFR activation that reconciles the existence of preformed EGFR dimers/oligomers with the Kuriyan allosteric model for activation of the EGFR kinase domains.

Positive and negative symptoms in schizophrenia: the NMDA receptor hypofunction hypothesis, neuregulin/ErbB4 and synapse regression

Carlsson has put forward the hypothesis that the positive and negative symptoms of schizophrenia are due to failure of mesolimbic and mesocortical projections consequent on hypofunction of the glutamate N-methyl-d-aspartate (NMDA) receptor. The hypothesis has been recently emphasized in this Journal that the loss of synaptic spines with NMDA receptors, which can be precipitated by stress, can explain the emergence of positive symptoms such as hallucinations and that this synapse regression involves molecules such as neuregulin and its receptor ErbB4 that have been implicated in schizophrenia. In this essay these two hypotheses are brought together in a single scheme in which emphasis is placed on the molecular pathways from neuregulin/ErbB4, to modulation of the NMDA receptors, subsequent changes in the synaptic spine's cytoskeletal apparatus and so regression of the spines. It is suggested that identification of the molecular constituents of this pathway will allow synthesis of suitable substances for removing the hypofunction of NMDA receptors and so the phenotypic consequences that flow from this hypofunction.

Ectodomain orientation, conformational plasticity and oligomerization of ErbB1 receptors investigated by molecular dynamics

Epidermal growth factor receptor (EGFR; ErbB1, HER1 in humans) is a receptor tyrosine kinase triggering signals across the plasma membranes of cells to determine cell fate. We have used molecular dynamics simulations to investigate structural models of ErbB1 ectodomains. We show that, with minor rearrangements, the ErbB1 back-to-back dimer can align almost flat on the cell membrane. This is in contrast to the traditional picture of ErbB1 dimers standing proud of the membrane, but in line with recent FRET and EM experiments. Interaction with the membrane leads to conformational changes in the dimer, which further stabilize the back-to-back interface. On the membrane, two dimers can associate forming a tetramer. This is enabled by a head-to-head interface, involving the ligand binding side of the ectodomain, and which significantly enhances ligand binding. A weak head-to-head interface has been seen in crystal structures, but is found to stabilise appreciably in our simulation. We also find that the domains IV, connecting the receptor to the membrane, weakly interact with each other. These simulations illustrate some of the flexibility of the ErbB1 ectodomains, and may help to explain recent experimental results.

In vitro enzymatic characterization of near full length EGFR in activated and inhibited states

The epidermal growth factor receptor (EGFR) is a single-pass transmembrane protein with an extracellular ligand-binding region and a cytoplasmic tyrosine kinase. Ligand binding activates the tyrosine kinase, which in turn initiates signaling cascades that influence cell proliferation and differentiation. EGFR activity is essential for normal development of many multicellular organisms, and inappropriate activation of EGFR is associated with multiple human cancers. Several drugs targeting EGFR activity are approved cancer therapies, and new EGFR-targeted therapies are being actively pursued. Much of what is known about EGFR structure and function is derived from studies of soluble receptor fragments. We report here an approach to producing an active, membrane-spanning form of EGFR of suitable purity, homogeneity, and quantity for structural and functional studies. We show that EGFR is capable of direct autophosphorylation of tyrosine 845, which is located on its kinase activation loop, and that the kinase activity of EGFR is approximately 500-fold higher in the presence of EGF vs the inhibitory anti-EGFR antibody cetuximab. The potencies of the small molecule EGFR kinase inhibitors erlotinib and lapatinib for various forms of EGFR were measured, and the therapeutic and mechanistic implications of these results considered.

Mechanism for activation of the EGF receptor catalytic domain by the juxtamembrane segment

Signaling by the epidermal growth factor receptor requires an allosteric interaction between the kinase domains of two receptors, whereby one activates the other. We show that the intracellular juxtamembrane segment of the receptor, known to potentiate kinase activity, is able to dimerize the kinase domains. The C-terminal half of the juxtamembrane segment latches the activated kinase domain to the activator, and the N-terminal half of this segment further potentiates dimerization, most likely by forming an antiparallel helical dimer that engages the transmembrane helices of the activated receptor. Our data are consistent with a mechanism in which the extracellular domains block the intrinsic ability of the transmembrane and cytoplasmic domains to dimerize and activate, with ligand binding releasing this block. The formation of the activating juxtamembrane latch is prevented by the C-terminal tails in a structure of an inactive kinase domain dimer, suggesting how alternative dimers can prevent ligand-independent activation.

The juxtamembrane region of the EGF receptor functions as an activation domain

In several growth factor receptors, the intracellular juxtamembrane (JM) region participates in autoinhibitory interactions that must be disrupted for tyrosine kinase activation. Using alanine scanning mutagenesis and crystallographic approaches, we define a domain within the JM region of the epidermal growth factor receptor (EGFR) that instead plays an activating--rather than autoinhibitory--role. Mutations in the C-terminal 19 residues of the EGFR JM region abolish EGFR activation. In a crystal structure of an asymmetric dimer of the tyrosine kinase domain, the JM region of an acceptor monomer makes extensive contacts with the C lobe of a donor monomer, thus stabilizing the dimer. We describe how an uncharacterized lung cancer mutation in this JM activation domain (V665M) constitutively activates EGFR by augmenting its capacity to act as an acceptor in the asymmetric dimer. This JM mutant promotes cellular transformation by EGFR in vitro and is tumorigenic in a xenograft assay.

Atomistic insights into regulatory mechanisms of the HER2 tyrosine kinase domain: a molecular dynamics study

HER2 (ErbB2/Neu) is a receptor tyrosine kinase belonging to the epidermal growth factor receptor (EGFR)/ErbB family and is overexpressed in 20-30% of human breast cancers. Although several crystal structures of ErbB kinases have been solved, the precise mechanism of HER2 activation remains unknown, and it has been suggested that HER2 is unique in its requirement for phosphorylation of Y877, a key tyrosine residue located in the activation loop. To elucidate mechanistic details of kinase domain regulation, we performed molecular dynamics simulations of a homology-modeled HER2 kinase structure in active and inactive conformations. Principal component analysis of the atomistic fluctuations reveals a tight coupling between the activation loop and catalytic loop that may contribute to alignment of residues required for catalysis in the active kinase. The free energy perturbation method is also employed to predict a role for phosphorylated Y877 in stabilizing the kinase conformations. Finally, simulation results are presented for a HER2/EGFR heterodimer and reveal that the dimeric interface induces a rearrangement of the alphaC helix toward the active conformation. Elucidation of the molecular regulatory mechanisms in HER2 will help establish structure-function relationships in the wild-type kinase, as well as predict mutations with a propensity for constitutive activation in HER2-mediated cancers.

Comparative analysis of mutant tyrosine kinase chemical rescue

Protein tyrosine kinases are critical cell signaling enzymes. These enzymes have a highly conserved Arg residue in their catalytic loop which is present two residues or four residues downstream from an absolutely conserved Asp catalytic base. Prior studies on protein tyrosine kinases Csk and Src revealed the potential for chemical rescue of catalytically deficient mutant kinases (Arg to Ala mutations) by small diamino compounds, particularly imidazole; however, the potency and efficiency of rescue was greater for Src. This current study further examines the structural and kinetic basis of rescue for mutant Src as compared to mutant Abl tyrosine kinase. An X-ray crystal structure of R388A Src revealed the surprising finding that a histidine residue of the N-terminus of a symmetry-related kinase inserts into the active site of the adjacent Src and mimics the hydrogen-bonding pattern seen in wild-type protein tyrosine kinases. Abl R367A shows potent and efficient rescue more comparable to Src, even though its catalytic loop is more like that of Csk. Various enzyme redesigns of the active sites indicate that the degree and specificity of rescue are somewhat flexible, but the overall properties of the enzymes and rescue agents play an overarching role. The newly discovered rescue agent 2-aminoimidazole is about as efficient as imidazole in rescuing R/A Src and Abl. Rate vs pH studies with these imidazole analogues suggest that the protonated imidazolium is the preferred form for chemical rescue, consistent with structural models. The efficient rescue seen with mutant Abl points to the potential of this approach to be used effectively to analyze Abl phosphorylation pathways in cells.

ErbB receptors in the biology and pathology of the aerodigestive tract

The most common sites of malignancies in the aerodigestive tract include the lung, head and neck and the esophagus. Esophageal adenocarcinomas (EA), esophageal squamous cell carcinomas (ESCC), and squamous cell carcinomas of the head and neck (SCCHN) are the primary focus of this review. Traditional treatment for aerodigestive tract cancers includes primary chemoradiotherapy (CRT) or surgical resection followed by radiation (or CRT). Recent developments in treatment have focused increasingly on molecular targeting strategies including cetuximab (a monoclonal antibody against epidermal growth factor receptor (EGFR)). Cetuximab was FDA approved in 2006 for treatment of SCCHN, underscoring the importance of understanding the biology of these malignancies. EGFR is a member of the ErbB family of growth factor receptor tyrosine kinases. The major pathways activated by ErbB receptors include Ras/Raf/MAPK; PI3K/AKT; PLCgamma and STATs, all of which lead to the transcription of target genes that may contribute to aerodigestive tumor progression. This review explores the expression of ErbB receptors in EA, ESCC and SCCHN and the signaling pathways of EGFR in SCCHN.

Ligand-induced ErbB receptor dimerization

Structural studies have provided important new insights into how ligand binding promotes homodimerization and activation of the EGF receptor and the other members of the ErbB family of receptor tyrosine kinases. These structures have also suggested possible explanations for the unique properties of ErbB2, which has no known ligand and can cause cell transformation (and tumorigenesis) by simple overexpression. In parallel with these advances, studies of the EGF receptor at the cell surface increasingly argue that the structural studies are missing key mechanistic components. This is particularly evident in the structural prediction that EGF binding linked to receptor dimerization should be positively cooperative, whereas cell-surface EGF-binding studies suggest negative cooperativity. In this review, I summarize studies of ErbB receptor extracellular regions in solution and of intact receptors at the cell surface, and attempt to reconcile the differences suggested by the two approaches. By combining results obtained with receptor 'parts', it is qualitatively possible to explain some models for the properties of the whole receptor. These considerations underline the need to consider the intact ErbB receptors as intact allosterically regulated enzymes, and to combine cellular and structural studies into a complete picture.

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

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

Molecular systems biology of ErbB1 signaling: bridging the gap through multiscale modeling and high-performance computing

The complexity in intracellular signaling mechanisms relevant for the conquest of many diseases resides at different levels of organization with scales ranging from the subatomic realm relevant to catalytic functions of enzymes to the mesoscopic realm relevant to the cooperative association of molecular assemblies and membrane processes. Consequently, the challenge of representing and quantifying functional or dysfunctional modules within the networks remains due to the current limitations in our understanding of mesoscopic biology, i.e., how the components assemble into functional molecular ensembles. A multiscale approach is necessary to treat a hierarchy of interactions ranging from molecular (nm, ns) to signaling (microm, ms) length and time scales, which necessitates the development and application of specialized modeling tools. Complementary to multiscale experimentation (encompassing structural biology, mechanistic enzymology, cell biology, and single molecule studies) multiscale modeling offers a powerful and quantitative alternative for the study of functional intracellular signaling modules. Here, we describe the application of a multiscale approach to signaling mediated by the ErbB1 receptor which constitutes a network hub for the cell's proliferative, migratory, and survival programs. Through our multiscale model, we mechanistically describe how point-mutations in the ErbB1 receptor can profoundly alter signaling characteristics leading to the onset of oncogenic transformations. Specifically, we describe how the point mutations induce cascading fragility mechanisms at the molecular scale as well as at the scale of the signaling network to preferentially activate the survival factor Akt. We provide a quantitative explanation for how the hallmark of preferential Akt activation in cell-lines harboring the constitutively active mutant ErbB1 receptors causes these cell-lines to be addicted to ErbB1-mediated generation of survival signals. Consequently, inhibition of ErbB1 activity leads to a remarkable therapeutic response in the addicted cell lines.

Interactome of ErbB4 unveiled

MacBeath and colleagues (Kaushansky et al., 2008) use a protein array technology to find binding partners of ErbB4 in a genome-wide and quantitative fashion, shedding new light on how ErbB4 initiates cellular signaling events and why ErbB4 is not a potent oncogene.

ERBB3/HER3 and ERBB2/HER2 duet in mammary development and breast cancer

ERBB3/HER3 is one of the four members of the epidermal growth factor receptor (ERBB) family. It is activated by binding to ligands Neuregulin-1 and Neuregulin-2. Since ERBB3 lacks intrinsic kinase activity, signal transduction occurs through formation of heterodimers with EGFR, ERBB2, and ERBB4. ERBB3 is a signaling specialist since it has six binding sites for the p85 SH2 adapter subunit of phosphoinositide 3' kinases. These lipid kinases coordinate regulation of metabolism, cell size, proliferation, survival, and angiogenesis. Not surprisingly, ERBB3 signaling has been linked to cancer etiology and progression. In breast cancer, the partnership of ERBB2 and ERBB3 may be crucial for the aggressive properties of cancers with ERBB2 amplification, and may contribute to pre-existing and acquired resistance to therapy. This partnership creates opportunities for improving efficacy of ERBB-targeted pharmaceuticals, by interfering with coupling of ERBB2 to ERBB3 through dimerization inhibitors, and by use of therapeutic compounds that target AKT-dependent pathways activated through ERBB3. Additional therapeutic opportunities may be identified through better understanding of how ERBBs are regulated and deployed in normal mammary gland processes. Work using mouse models has identified the main processes regulated by each of the four ERBBs, which has practical implications in understanding breast cancer etiology, and eventual development of better prognostic, predictive, and therapeutic tools.