Oligomers of ERBB3 have two distinct interfaces that differ in their sensitivity to disruption by heregulin

Construction of a Mass-Tagged Oligo Probe Set for Revealing Protein Ratiometric Relationship Associated with EGFR-HER2 Heterodimerization in Living Cells

Protein dimerization, as the most common form of protein-protein interaction, can manifest more significant roles in cellular signaling than individual monomers. For example, excessive formation of EGFR-HER2 dimer has been implicated in cancer development and therapeutic resistance in addition to the overexpression of EGFR and HER2 proteins. Thus, quantitative evaluation of these heterodimers in living cells and revelation of their ratiometric relationship with protein monomers in dimerization may provide insights into clinical cancer management. To achieve this goal, the prerequisite is protein heterodimer quantification. Given the current lack of quantitative methods, we constructed a mass-tagged oligo nanoprobe set for quantification of EGFR-HER2 dimer in living cells. The mass-tagged oligo nanoprobe set contained two targeting probes (nucleic acid aptamers), a connector probe, a hairpin probe, and a photocleavable mass-tagged probe. Two distinct aptamers can recognize target protein monomers and initiate the subsequent hybridization cascade involving binding to the connector probe, formation of an initiator strand, opening of a hairpin probe, and ensuing hybridization with a photocleavable mass-tagged probe. Ultimately, the mass tag was released under ultraviolet light and then subjected to mass spectrometric analysis. In this way, the information regarding the interaction between two protein monomers was successfully converted to the quantitative signal of the mass tag. Using the assay, the expression level of EGFR-HER2 dimer and its relationship with individual protein monomers were determined in four breast cancer cell lines. We are among the first to obtain the absolute level of protein heterodimer, and this quantitative information may be vital in understanding the molecular basis of cancer.

It Takes More than Two to Tango: Complex, Hierarchal, and Membrane-Modulated Interactions in the Regulation of Receptor Tyrosine Kinases

The search for an understanding of how cell fate and motility are regulated is not a purely scientific undertaking, but it can also lead to rationally designed therapies against cancer. The discovery of tyrosine kinases about half a century ago, the subsequent characterization of certain transmembrane receptors harboring tyrosine kinase activity, and their connection to the development of human cancer ushered in a new age with the hope of finding a treatment for malignant diseases in the foreseeable future. However, painstaking efforts were required to uncover the principles of how these receptors with intrinsic tyrosine kinase activity are regulated. Developments in molecular and structural biology and biophysical approaches paved the way towards better understanding of these pathways. Discoveries in the past twenty years first resulted in the formulation of textbook dogmas, such as dimerization-driven receptor association, which were followed by fine-tuning the model. In this review, the role of molecular interactions taking place during the activation of receptor tyrosine kinases, with special attention to the epidermal growth factor receptor family, will be discussed. The fact that these receptors are anchored in the membrane provides ample opportunities for modulatory lipid-protein interactions that will be considered in detail in the second part of the manuscript. Although qualitative and quantitative alterations in lipids in cancer are not sufficient in their own right to drive the malignant transformation, they both contribute to tumor formation and also provide ways to treat cancer. The review will be concluded with a summary of these medical aspects of lipid-protein interactions.

Mechanisms of Action of EGFR Tyrosine Kinase Receptor Incorporated in Extracellular Vesicles

EGFR and some of the cognate ligands extensively traffic in extracellular vesicles (EVs) from different biogenesis pathways. EGFR belongs to a family of four homologous tyrosine kinase receptors (TKRs). This family are one of the major drivers of cancer and is involved in several of the most frequent malignancies such as non-small cell lung cancer, breast cancer, colorectal cancer and ovarian cancer. The carrier EVs exert crucial biological effects on recipient cells, impacting immunity, pre-metastatic niche preparation, angiogenesis, cancer cell stemness and horizontal oncogene transfer. While EV-mediated EGFR signalling is important to EGFR-driven cancers, little is known about the precise mechanisms by which TKRs incorporated in EVs play their biological role, their stoichiometry and associations to other proteins relevant to cancer pathology and EV biogenesis, and their means of incorporation in the target cell. In addition, it remains unclear whether different subtypes of EVs incorporate different complexes of TKRs with specific functions. A raft of high spatial and temporal resolution methods is emerging that could solve these and other questions regarding the activity of EGFR and its ligands in EVs. More importantly, methods are emerging to block or mitigate EV activity to suppress cancer progression and drug resistance. By highlighting key findings and areas that remain obscure at the intersection of EGFR signalling and EV action, we hope to cross-fertilise the two fields and speed up the application of novel techniques and paradigms to both.

Mammalian Expression and In Situ Biotinylation of Extracellular Protein Targets for Directed Evolution

Directed evolution is a powerful tool for the selection of functional ligands from molecular libraries. Extracellular domains (ECDs) of cell surface receptors are common selection targets for therapeutic and imaging agent development. Unfortunately, these proteins are often post-translationally modified and are therefore unsuitable for expression in bacterial systems. Directional immobilization of these targets is further hampered by the absence of biorthogonal groups for site-specific chemical conjugation. We have developed a nonadherent mammalian expression system for rapid, high-yield expression of biotinylated ECDs. ECDs from EGFR, HER2, and HER3 were site-specifically biotinylated in situ and recovered from the cell culture supernatant with yields of up to 10 mg/L at >90% purity. Biotinylated ECDs also contained a protease cleavage site for rapid and selective release of the ECD after immobilization on avidin/streptavidin resins and library binding. A model mRNA display selection round was carried out against the HER2 ECD with the HER2 affibody expressed as an mRNA-protein fusion. HER2 affibody-mRNA fusions were selectively released by thrombin and quantitative PCR revealed substantial improvements in the enrichment of functional affibody-mRNA fusions relative to direct PCR amplification of the resin-bound target. This methodology allows rapid purification of high-quality targets for directed evolution and selective elution of functional sequences at the conclusion of each selection round.

Homo- and Heteroassociations Drive Activation of ErbB3

Dimerization or the formation of higher-order oligomers is required for the activation of ErbB receptor tyrosine kinases. The heregulin (HRG) receptor, ErbB3, must heterodimerize with other members of the family, preferentially ErbB2, to form a functional signal transducing complex. Here, we applied single molecule imaging capable of detecting long-lived and mobile associations to measure their stoichiometry and mobility and analyzed data from experiments globally, taking the different lateral mobility of monomeric and dimeric molecular species into account. Although ErbB3 was largely monomeric in the absence of stimulation and ErbB2 co-expression, a small fraction was present as constitutive homodimers exhibiting a ∼40% lower mobility than monomers. HRG stimulation increased the homodimeric fraction of ErbB3 significantly and reduced the mobility of homodimers fourfold compared to constitutive homodimers. Expression of ErbB2 elevated the homodimeric fraction of ErbB3 even in unstimulated cells and induced a ∼2-fold reduction in the lateral mobility of ErbB3 homodimers. The mobility of ErbB2 was significantly lower than that of ErbB3, and HRG induced a less pronounced decrease in the diffusion coefficient of all ErbB2 molecules and ErbB3/ErbB2 heterodimers than in the mobility of ErbB3. The slower diffusion of ErbB2 compared to ErbB3 was abolished by depolymerizing actin filaments, whereas ErbB2 expression induced a substantial rearrangement of microfilaments, implying a bidirectional interaction between ErbB2 and actin. HRG stimulation of cells co-expressing ErbB3 and ErbB2 led to the formation of ErbB3 homodimers and ErbB3/ErbB2 heterodimers in a competitive fashion. Although pertuzumab, an antibody binding to the dimerization arm of ErbB2, completely abolished the formation of constitutive and HRG-induced ErbB3/ErbB2 heterodimers, it only slightly blocked ErbB3 homodimerization. The results imply that a dynamic equilibrium exists between constitutive and ligand-induced homo- and heterodimers capable of shaping transmembrane signaling.

Prevalence and influence on outcome of HER2/neu, HER3 and NRG1 expression in patients with metastatic colorectal cancer

Our aim was to explore the impact of the HER2/neu, HER3 receptor as well as their ligands' neuregulin (NRG1) expression on the outcome of patients with metastatic colorectal cancer (mCRC). NRG1, HER2/neu and HER3 expression was evaluated in 208 patients with mCRC receiving 5-FU/LV plus irinotecan or irinotecan plus oxaliplatin as the first-line treatment. Biomarker expression was correlated with the outcome of patients. NRG1 (low: 192 vs. high: 16), HER2/neu (low: 201 vs. high: 7) and HER3 (low: 69 vs. high: 139) expressions were assessed in 208 patients. High versus low NRG1 expression significantly affected progression-free survival (PFS) [4.7 vs. 8.2 months, hazard ratio (HR): 2.45; 95% confidence interval (CI): 1.45-4.13; P=0.001], but not overall survival (OS) (15.5 vs. 20.7 months, HR: 1.33; 95% CI: 0.76-2.35; P=0.32). High versus low HER3 expression (PFS: 7.1 vs. 8.8 months, HR: 1.11; 95% CI: 0.82-1.50; P=0.50; OS: 19.8 vs. 21.1 months, HR: 0.95; 95% CI: 0.70-1.30; P=0.75) and high compared with low HER2/neu expression (PFS: 7.7 vs. 8.0 months, HR: 1.07; 95% CI: 0.71-1.60; P=0.75; OS: 16.6 vs. 21.1 months, HR: 1.13; 95% CI: 0.75-1.71; P=0.57) did not influence outcome. High NRG1 expression was associated with inferior PFS in the FIRE-1 trial. We did not detect a prognostic impact of HER2/neu and HER3 overexpression in mCRC. The frequency of overexpression was comparable with other studies.

APIP, an ERBB3-binding partner, stimulates erbB2-3 heterodimer formation to promote tumorigenesis

Despite the fact that the epidermal growth factor (EGF) family member ERBB3 (HER3) is deregulated in many cancers, the list of ERBB3-interacting partners remains limited. Here, we report that the Apaf-1-interacting protein (APIP) stimulates heregulin-β1 (HRG-β1)/ERBB3-driven cell proliferation and tumorigenesis. APIP levels are frequently increased in human gastric cancers and gastric cancer-derived cells. Cell proliferation and tumor formation are repressed by APIP downregulation and stimulated by its overexpression. APIP's role in the ERBB3 pathway is not associated with its functions within the methionine salvage pathway. In response to HRG-β1, APIP binds to the ERBB3 receptor, leading to an enhanced binding of ERBB3 and ERBB2 that results in sustained activations of ERK1/2 and AKT protein kinases. Furthermore, HRG-β1/ERBB3-dependent signaling is gained in APIP transgenic mouse embryonic fibroblasts (MEFs), but not lost in Apip^−/− MEFs. Our findings offer compelling evidence that APIP plays an essential role in ERBB3 signaling as a positive regulator for tumorigenesis, warranting future development of therapeutic strategies for ERBB3-driven gastric cancer.

EGF and NRG induce phosphorylation of HER3/ERBB3 by EGFR using distinct oligomeric mechanisms

Heteromeric interactions between the catalytically impaired human epidermal growth factor receptor (HER3/ERBB3) and its catalytically active homologs EGFR and HER2 are essential for their signaling. Different ligands can activate these receptor pairs but lead to divergent signaling outcomes through mechanisms that remain largely unknown. We used stochastic optical reconstruction microscopy (STORM) with pair-correlation analysis to show that EGF and neuregulin (NRG) can induce different extents of HER3 clustering that are dependent on the nature of the coexpressed HER receptor. We found that the presence of these clusters correlated with distinct patterns and mechanisms of receptor phosphorylation. NRG induction of HER3 phosphorylation depended on the formation of the asymmetric kinase dimer with EGFR in the absence of detectable higher-order oligomers. Upon EGF stimulation, HER3 paralleled previously observed EGFR behavior and formed large clusters within which HER3 was phosphorylated via a noncanonical mechanism. HER3 phosphorylation by HER2 in the presence of NRG proceeded through still another mechanism and involved the formation of clusters within which receptor phosphorylation depended on asymmetric kinase dimerization. Our results demonstrate that the higher-order organization of HER receptors is an essential feature of their ligand-induced behavior and plays an essential role in lateral cross-activation of the receptors. We also show that HER receptor ligands exert unique effects on signaling by modulating this behavior.

High Density or Urban Sprawl: What Works Best in Biology?

With new approaches in imaging-from new tools or reagents to processing algorithms-come unique opportunities and challenges to our understanding of biological processes, structures, and dynamics. Although innovations in super-resolution imaging are affording novel perspectives into how molecules structurally associate and localize in response to, or in order to initiate, specific signaling events in the cell, questions arise as to how to interpret these observations in the context of biological function. Just as each neighborhood in a city has its own unique vibe, culture, and indeed density, recent work has shown that membrane receptor behavior and action is governed by their localization and association state. There is tremendous potential in developing strategies for tracking how the populations of these molecular neighborhoods change dynamically.

Piecing it together: Unraveling the elusive structure-function relationship in single-pass membrane receptors

The challenge of crystallizing single-pass plasma membrane receptors has remained an obstacle to understanding the structural mechanisms that connect extracellular ligand binding to cytosolic activation. For example, the complex interplay between receptor oligomerization and conformational dynamics has been, historically, only inferred from static structures of isolated receptor domains. A fundamental challenge in the field of membrane receptor biology, then, has been to integrate experimentally observable dynamics of full-length receptors (e.g. diffusion and conformational flexibility) into static structural models of the disparate domains. In certain receptor families, e.g. the ErbB receptors, structures have led somewhat linearly to a putative model of activation. In other families, e.g. the tumor necrosis factor (TNF) receptors, structures have produced divergent hypothetical mechanisms of activation and transduction. Here, we discuss in detail these and other related receptors, with the goal of illuminating the current challenges and opportunities in building comprehensive models of single-pass receptor activation. The deepening understanding of these receptors has recently been accelerated by new experimental and computational tools that offer orthogonal perspectives on both structure and dynamics. As such, this review aims to contextualize those technological developments as we highlight the elegant and complex conformational communication between receptor domains. This article is part of a Special Issue entitled: Interactions between membrane receptors in cellular membranes edited by Kalina Hristova.

EGFR oligomerization organizes kinase-active dimers into competent signalling platforms

Epidermal growth factor receptor (EGFR) signalling is activated by ligand-induced receptor dimerization. Notably, ligand binding also induces EGFR oligomerization, but the structures and functions of the oligomers are poorly understood. Here, we use fluorophore localization imaging with photobleaching to probe the structure of EGFR oligomers. We find that at physiological epidermal growth factor (EGF) concentrations, EGFR assembles into oligomers, as indicated by pairwise distances of receptor-bound fluorophore-conjugated EGF ligands. The pairwise ligand distances correspond well with the predictions of our structural model of the oligomers constructed from molecular dynamics simulations. The model suggests that oligomerization is mediated extracellularly by unoccupied ligand-binding sites and that oligomerization organizes kinase-active dimers in ways optimal for auto-phosphorylation in trans between neighbouring dimers. We argue that ligand-induced oligomerization is essential to the regulation of EGFR signalling.

Epidermal growth factor receptors have been shown to oligomerise upon binding to their cognate ligands. Here, the authors use biochemical, biophysical and cell biology techniques to analyse the structures of these oligomers, and argue that these formations are required for signalling.

Orchestration of ErbB3 signaling through heterointeractions and homointeractions

Members of the ErbB family of receptor tyrosine kinases are capable of both homointeractions and heterointeractions. Because each receptor has a unique set of binding sites for downstream signaling partners and differential catalytic activity, subtle shifts in their combinatorial interplay may have a large effect on signaling outcomes. The overexpression and mutation of ErbB family members are common in numerous human cancers and shift the balance of activation within the signaling network. Here we report the development of a spatial stochastic model that addresses the dynamics of ErbB3 homodimerization and heterodimerization with ErbB2. The model is based on experimental measures for diffusion, dimer off-rates, kinase activity, and dephosphorylation. We also report computational analysis of ErbB3 mutations, generating the prediction that activating mutations in the intracellular and extracellular domains may be subdivided into classes with distinct underlying mechanisms. We show experimental evidence for an ErbB3 gain-of-function point mutation located in the C-lobe asymmetric dimerization interface, which shows enhanced phosphorylation at low ligand dose associated with increased kinase activity.

Recruitment of the adaptor protein Grb2 to EGFR tetramers

Adaptor protein Grb2 binds phosphotyrosines in the epidermal growth factor (EGF) receptor (EGFR) and thereby links receptor activation to intracellular signaling cascades. Here, we investigated how recruitment of Grb2 to EGFR is affected by the spatial organization and quaternary state of activated EGFR. We used the techniques of image correlation spectroscopy (ICS) and lifetime-detected Förster resonance energy transfer (also known as FLIM-based FRET or FLIM–FRET) to measure ligand-induced receptor clustering and Grb2 binding to activated EGFR in BaF/3 cells. BaF/3 cells were stably transfected with fluorescently labeled forms of Grb2 (Grb2–mRFP) and EGFR (EGFR–eGFP). Following stimulation of the cells with EGF, we detected nanometer-scale association of Grb2–mRFP with EGFR–eGFP clusters, which contained, on average, 4 ± 1 copies of EGFR–eGFP per cluster. In contrast, the pool of EGFR–eGFP without Grb2–mRFP had an average cluster size of 1 ± 0.3 EGFR molecules per punctum. In the absence of EGF, there was no association between EGFR–eGFP and Grb2–mRFP. To interpret these data, we extended our recently developed model for EGFR activation, which considers EGFR oligomerization up to tetramers, to include recruitment of Grb2 to phosphorylated EGFR. The extended model, with adjustment of one new parameter (the ratio of the Grb2 and EGFR copy numbers), is consistent with a cluster size distribution where 2% of EGFR monomers, 5% of EGFR dimers, <1% of EGFR trimers, and 94% of EGFR tetramers are associated with Grb2. Together, our experimental and modeling results further implicate tetrameric EGFR as the key signaling unit and call into question the widely held view that dimeric EGFR is the predominant signaling unit.

RETRACTED: An ATP-competitive inhibitor modulates the allosteric function of the HER3 pseudokinase

Human epidermal growth factor receptor 3 (HER3) is a receptor tyrosine kinase that lacks catalytic activity but is essential for cellular homeostasis due to its ability to allosterically activate EGFR and HER2. Although catalytically inactive, HER3 binds ATP tightly, hinting at a possible role of the nucleotide-binding pocket in modulating HER3 function. We report a structure of the HER3 pseudokinase bound to the ATP-competitive inhibitor bosutinib. Previously solved structures show that bosutinib can potently interact with multiple kinase domain conformations. In complex with HER3, bosutinib binds to yet another conformation, which is nearly identical to that observed in the HER3-ATP complex. Interestingly, occupation of the ATP-binding site by bosutinib improves the ability of HER3 to act as an allosteric activator of EGFR in vitro by increasing the affinity of the HER3-EGFR heterodimer in a membrane-dependent manner.

erbB3 is an active tyrosine kinase capable of homo- and heterointeractions

Often considered to be a "dead" kinase, erbB3 is implicated in escape from erbB-targeted cancer therapies. Here, heregulin stimulation is shown to markedly upregulate kinase activity in erbB3 immunoprecipitates. Intact, activated erbB3 phosphorylates tyrosine sites in an exogenous peptide substrate, and this activity is abolished by mutagenesis of lysine 723 in the catalytic domain. Enhanced erbB3 kinase activity is linked to heterointeractions with catalytically active erbB2, since it is largely blocked in cells pretreated with lapatinib or pertuzumab. erbB2 activation of erbB3 is not dependent on equal surface levels of these receptors, since it occurs even in erbB3-transfected CHO cells with disproportionally small amounts of erbB2. We tested a model in which transient erbB3/erbB2 heterointeractions set the stage for erbB3 homodimers to be signaling competent. erbB3 homo- and heterodimerization events were captured in real time on live cells using single-particle tracking of quantum dot probes bound to ligand or hemagglutinin tags on recombinant receptors.

Dynamic analysis of the epidermal growth factor (EGF) receptor-ErbB2-ErbB3 protein network by luciferase fragment complementation imaging

ErbB3 is a member of the ErbB family of receptor tyrosine kinases. It is unique because it is the only member of the family whose kinase domain is defective. As a result, it is obliged to form heterodimers with other ErbB receptors to signal. In this study, we characterized the interaction of ErbB3 with the EGF receptor and ErbB2 and assessed the effects of Food and Drug Administration-approved therapeutic agents on these interactions. Our findings support the concept that ErbB3 exists in preformed clusters that can be dissociated by NRG-1β and that it interacts with other ErbB receptors in a distinctly hierarchical fashion. Our study also shows that all pairings of the EGF receptor, ErbB2, and ErbB3 form ligand-independent dimers/oligomers. The small-molecule tyrosine kinase inhibitors erlotinib and lapatinib differentially enhance the dimerization of the various ErbB receptor pairings, with the EGFR/ErbB3 heterodimer being particularly sensitive to the effects of erlotinib. The data suggest that the physiological effects of these drugs may involve not only inhibition of tyrosine kinase activity but also a dynamic restructuring of the entire network of receptors.

Asymmetric kinase dimer formation is crucial for the activation of oncogenic EGFRvIII but not for ERBB3 phosphorylation

Background

Formation of asymmetric kinase dimers is required for wt-EGFR activation upon ligand stimulation. The role of receptor dimerization in oncogenic EGFRvIII mutant activation is not completely understood and the molecular details of EGFRvIII interactions within homo-dimers and hetero-dimers are not elucidated yet.

Findings

By employing mutations that disrupt the asymmetric kinase dimer interface in EGFRvIII, we demonstrate that the mechanism of oncogenic EGFRvIII mutant activation is similar to that of the full-length wild-type EGFR. Surprisingly, the monomeric EGFRvIII lacks autophosphorylation and the formation of asymmetric kinase dimers is indispensable for oncogenic kinase activation. In addition, we show that ERBB3 can act as an activator of EGFRvIII by forming asymmetric kinase dimer in a ligand-independent manner. Interestingly, we found that the formation of asymmetric kinase dimer is dispensable for ERBB3 phosphorylation by the activated EGFR kinase as well as the ERBB2 kinase thus revealing a novel model for receptor function.

Conclusions

Lateral signaling is a novel mechanism of signal propagation via ERBB3 upon activation by EGFR/ERBB2 kinase even in the absence of their ability to form asymmetric kinase dimers.

ErbB receptors and PKC regulate PC12 neuronal-like differentiation and sodium current elicitation

Excitability, neurite outgrowth and their specification are very important features in the establishment of neuronal differentiation. We have studied a conditioned medium (CM) from sciatic nerve which is able to induce a neuronal-like differentiation of PC12 cells. Previously, we have demonstrated that supplementing this CM with a generic inhibitor (k252a), which mainly inhibits tropomyosin-related kinase receptors (Trk receptors) and protein kinase C (PKC), caused neurite elongation, sodium current induction and axon development. In the present work, we are showing that the enhancement of neurite length and induction of sodium currents induced by CM+k252a were prevented by ErbB receptor inhibition. Additionally, we demonstrated that specific inhibition of PKC produced a similar effect to that exerted by k252a in CM-treated cells, specifically by increasing the percentage of differentiated cells with long neurites and inducing sodium currents. Moreover, CM changed the mRNA levels for ErbB2 and ErbB3 increasing them 6- and 36-folds respectively compared to their control. The inclusion of k252a with CM changed the ErbB1, ErbB2 and ErbB3 mRNA proportions increasing those eight-, seven- and fivefolds respectively. From this point, it is clear that appropriate ErbB receptor levels and PKC inhibition are necessary to enhance the effect of the CM in inducing the neuronal-like differentiation of PC12 cells. In summary, we demonstrated the involvement of ErbB receptors in the regulation of neurite elongation and sodium current induction in PC12 cells and propose that these processes could be initiated by ErbB receptors followed by a fine regulation of PKC signaling. These findings might implicate a novel interplay between ErbB receptors and PKC in the regulation of these molecular mechanisms.

Novel anti-ErbB3 monoclonal antibodies show therapeutic efficacy in xenografted and spontaneous mouse tumors

The role of the ErbB3 receptor in signal transduction is to augment the signaling repertoire of active heterodimeric ErbB receptor complexes through activating the PI3K/AKT pathway, which in turn promotes survival and proliferation. ErbB3 has recently been proposed to be involved in acquired resistance to tyrosine kinase inhibitors (TKIs), and is therefore a promising new drug cancer target. Since ErbB3 is a kinase defective receptor, it cannot be targeted by small molecule inhibitors, whereas monoclonal antibodies may offer a viable strategy for pharmacological intervention. In this study, we have utilized DNA electroporation (DNA-EP) to generate a set of novel hybridomas directed against human ErbB3, which have been characterized for their biochemical and functional properties and selected for their ability to negatively regulate the ErbB3-mediated signaling pathway. In vitro, the anti-ErbB3 antibodies modulate the growth rate of cancer cells of different origins. In vivo they show antitumoral properties in a xenograft model of human pancreatic tumor and in the ErbB2-driven carcinogenesis genetically engineered mouse model (GEMM) for mammary tumor, the BALB/neuT. Our data confirm that downregulating the ErbB3-mediated signals with the use of anti-ErbB3 monoclonal antibodies is both feasible and relevant for therapeutic purposes and provides new opportunities for novel anti-ErbB3 combinatory strategies for cancer treatment.

Dynamically varying interactions between heregulin and ErbB proteins detected by single-molecule analysis in living cells

Heregulin (HRG) belongs to the family of EGFs and activates the receptor proteins ErbB3 and ErbB4 in a variety of cell types to regulate cell fate. The interactions between HRG and ErbB3/B4 are important to the pathological mechanisms underlying schizophrenia and some cancers. Here, we observed the reaction kinetics between fluorescently labeled single HRG molecules and ErbB3/B4 on the surfaces of MCF-7 human breast cancer cells. The equilibrium association and the dissociation from equilibrium were also measured using single-molecule imaging techniques. The unitary association processes mirrored the EGF and ErbB1 interactions in HeLa cells [Teramura Y, et al. (2006) EMBO J 25:4215-4222], suggesting that the predimerization of the receptors, followed by intermediate formation (between the first and second ligand-binding events to a receptor dimer), accelerated the formation of doubly liganded signaling dimers of the receptor molecules. However, the dissociation analysis suggested that the first HRG dissociation from the doubly liganded dimer was rapid, but the second dissociation from the singly liganded dimer was slow. The dissociation rate constant from the liganded monomer was intermediate. The dynamic changes in the association and dissociation kinetics in relation to the dimerization of ErbB displayed negative cooperativity, which resulted in apparent low- and high-affinity sites of HRG association on the cell surface.

ErbB3 inhibitory surrobodies inhibit tumor cell proliferation in vitro and in vivo

ErbB3 is an important regulator of tumorigenesis and is implicated in development of resistance to several currently used oncology drugs. We have identified ErbB3 inhibitors based on a novel biologic scaffold termed a surrobody. Two of these inhibitors appear to work by a previously unrecognized mechanism of action. As a consequence, they not only inhibited cell proliferation and intracellular signaling driven by stimulation with the ErbB3 ligand neuregulin (NRG), but also inhibited signaling and proliferation that was driven by overexpression of ErbB2 in the absence of ligand stimulation. In addition, the surrobodies inhibited tumor growth in vivo in both ErbB2-overexpressing and nonoverexpressing cells. In ErbB2-overexpressing cells, both of the anti-ErbB3 surrobodies significantly augmented the activities of trastuzumab, lapatinib, and GDC-0941, agents that inhibit cell proliferation by different mechanisms. Moreover, although NRG diminished the efficacy of these agents, when they were combined with anti-ErbB3 surrobodies the affect of NRG was abrogated. In this capacity, the anti-ErbB3 surrobodies were more effective than the ErbB2/ErbB3 dimerization inhibitory antibody pertuzumab. Despite the fact that these surrobodies appear to engage ErbB3 differently than previously described anti-ErbB3 antibodies, they retain all of the beneficial characteristics of this class of agents, including the ability to augment drugs that inhibit EGF receptor. These anti-ErbB3 agents, therefore, show substantial promise for development as single agents or in combination with other ErbB-directed antibodies or small molecules and may provide for a broader range of therapeutic indications than previously described anti-ErbB3 antibodies.

Targeting ErbB3: the New RTK(id) on the Prostate Cancer Block

Most prostate cancers (PCa) are critically reliant on functional androgen receptor (AR) signaling. At its onset, PCa is androgen-dependent and although temporarily halted by surgically or pharmacologically blocking the AR (androgen ablation), the disease ultimately recurs as an aggressive, fatal castration resistant prostate cancer (CRPC). FDA-approved treatments like docetaxel, a chemotherapeutic agent, and Provenge, a cancer vaccine, extend survival by a scant 3 and 4 months, respectively. It is clear that more effective drugs targeting CRPC are urgently needed. The ErbB family (EGFR/ErbB1, ErbB2/HER2/neu, ErbB3/HER3 and ErbB4/HER4) of receptor tyrosine kinases (RTKs) have long been implicated in PCa initiation and progression, but inhibitors of ErbB1 and ErbB2 (prototypic family members) fared poorly in PCa clinical trials. Recent research suggests that another family member ErbB3 abets emergence of the castration-resistant phenotype. Considerable efforts are being directed towards understanding ErbB3-mediated molecular mechanisms of castration resistance and searching for novel ways of inhibiting ErbB3 activity via rational drug design. Antibody-based therapy that prevents ligand binding to ErbB3 appears promising and fully-humanized antibodies that inhibit ligand-induced phosphorylation of ErbB3 are currently in early development. Small molecule tyrosine kinase inhibitors are also being vigorously pursued, as are siRNA-based approaches and combination treatment strategies- the simultaneous suppression of ErbB3 and its signaling partners or downstream effectors - with the primary purpose of undermining the resiliency of ErbB3-mediated signal transduction. This review summarizes the existing literature and reinforces the importance of ErbB3 as a therapeutic target in the clinical management of prostate cancer.

Engineered bivalent ligands to bias ErbB receptor-mediated signaling and phenotypes

The ErbB receptor family is dysregulated in many cancers, and its therapeutic manipulation by targeted antibodies and kinase inhibitors has resulted in effective chemotherapies. However, many malignancies remain refractory to current interventions. We describe a new approach that directs ErbB receptor interactions, resulting in biased signaling and phenotypes. Due to known receptor-ligand affinities and the necessity of ErbB receptors to dimerize to signal, bivalent ligands, formed by the synthetic linkage of two neuregulin-1β (NRG) moieties, two epidermal growth factor (EGF) moieties, or an EGF and a NRG moiety, can potentially drive homotypic receptor interactions and diminish formation of HER2-containing heterodimers, which are implicated in many malignancies and are a prevalent outcome of stimulation by native, monovalent EGF, or NRG. We demonstrate the therapeutic potential of this approach by showing that bivalent NRG (NN) can bias signaling in HER3-expressing cancer cells, resulting in some cases in decreased migration, inhibited proliferation, and increased apoptosis, whereas native NRG stimulation increased the malignant potential of the same cells. Hence, this new approach may have therapeutic relevance in ovarian, breast, lung, and other cancers in which HER3 has been implicated.

Evaluation of the anti-HER2 C6.5 diabody as a PET radiotracer to monitor HER2 status and predict response to trastuzumab treatment

PURPOSE

The rapid tumor targeting and pharmacokinetic properties of engineered antibodies make them potentially suitable for use in imaging strategies to predict and monitor response to targeted therapies. This study aims to evaluate C6.5 diabody (C6.5 db), a noncovalent anti-HER2 single-chain Fv dimer, as a radiotracer for predicting response to HER2-targeted therapies such as trastuzumab.

EXPERIMENTAL DESIGN

Immunodeficient mice bearing established HER2-positive tumor xenografts were injected with radioiodinated C6.5 db and imaged by PET/CT. Radiotracer biodistribution was quantified by biopsied tumor and normal tissues. Potential competition between trastuzumab and C6.5 db was examined in vitro by flow cytometry and coimmunoprecipitations.

RESULTS

Biodistribution analysis of mice bearing xenografts with varying HER2 density revealed that the tumor uptake of (125)I-C6.5 db correlates with HER2 tumor density. In vitro competition experiments suggest that the C6.5 db targets an epitope on HER2 that is distinct from that bound by trastuzumab. Treatment of mice affected with SK-OV-3 tumor with trastuzumab for 3 days caused a 42% (P = 0.002) decrease in tumor uptake of (125)I-C6.5 db. This is consistent with a dramatic decrease in the tumor PET signal of (124)I-C6.5 db after trastuzumab treatment. Furthermore, mice affected with BT-474 tumor showed an approximately 60% decrease (P = 0.0026) in C6.5 db uptake after 6 days of trastuzumab treatment. Immunohistochemistry of excised xenograft sections and in vitro flow cytometry revealed that the decreased C6.5 db uptake on trastuzumab treatment is not associated with HER2 downregulation.

CONCLUSIONS

These studies suggest that (124)I-C6.5 db-based imaging can be used to evaluate HER2 levels as a predictor of response to HER2-directed therapies.

The role of HER3, the unpretentious member of the HER family, in cancer biology and cancer therapeutics

Many types of human cancer are characterized by deregulation of the human epidermal growth factor receptor (HER) family of tyrosine kinase receptors. In some cancers, genomic events causing overactivity of individual HER family members are etiologically linked with the pathogenesis of these cancers, and constitute the driving signaling function underlying their tumorigenic behavior. HER3 stands out among this family as the only member lacking catalytic kinase function. Cancers with driving HER3 amplifications or mutations have not been found, and studies of its expression in tumors have been only weakly provocative. However, substantial evidence, predominantly from experimental models, now suggest that its non-catalytic functions are critically important in many cancers driven by its' HER family partners. Furthermore, new insights into the mechanism of activation in the HER family has provided clear evidence of functionality in the HER3 kinase domain. The convergence of structural, mechanistic, and experimental evidence highlighting HER3 functions that may be critical in tumorigenesis have now led to renewed efforts towards identification of cancers or subtypes of cancers wherein HER3 function may be important in tumor progression or drug resistance. It appears now that its failure to earn the traditional definition of an oncogene has allowed the tumor promoting functions of HER3 to elude the effects of cancer therapeutics. But experimental science has now unmasked the unpretentious role of HER3 in cancer biology, and the next generation of cancer therapies will undoubtedly perform much better because of it.

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.

Transmembrane helix-helix interactions involved in ErbB receptor signaling

Among the many transmembrane receptor classes, the receptor tyrosine kinases represent an important superfamily, involved in many cellular processes like embryogenesis, development and cell division. Deregulation and dysfunctions of these receptors can lead to various forms of cancer and other diseases. Mostly, only fragmented knowledge exists about functioning of the entire receptors, and many studies have been performed on isolated receptor domains. In this review we focus on the function of the ErbB family of receptor tyrosine kinases with a special emphasis on the role of the transmembrane domain and on the mechanisms underlying regulated and deregulated signaling. Many general aspects of ErbB receptor structure and function have been analyzed and described. All human ErbBs appear to form homo- and heterodimers within cellular membranes and the single transmembrane domain of the receptors is involved in dimerization. Additionally, only defined structures of the transmembrane helix dimer allows signaling of ErbB receptors.

HER3 comes of age: new insights into its functions and role in signaling, tumor biology, and cancer therapy

The human epidermal growth family (HER) of tyrosine kinase receptors underlies the pathogenesis of many types of human cancer. The oncogenic functions of three of the HER proteins can be unleashed through amplification, overexpression, or mutational activation. This has formed the basis for the development of clinically active targeted therapies. However, the third member HER3 is catalytically inactive, not found to be mutated or amplified in cancers, and its role and functions have remained shrouded in mystery. Recent evidence derived primarily from experimental models now seems to implicate HER3 in the pathogenesis of several types of cancer. Furthermore, the failure to recognize the central role of HER3 seems to underlie resistance to epidermal growth factor receptor (EGFR)- or HER2-targeted therapies in some cancers. Structural and biochemical studies have now greatly enhanced our understanding of signaling in the HER family and revealed the previously unrecognized activating functions embodied in the catalytically impaired kinase domain of HER3. This renewed interest and mechanistic basis has fueled the development of new classes of HER3-targeting agents for cancer therapy. However, identifying HER3-dependent tumors presents a formidable challenge and the success of HER3-targeting approaches depends entirely on the development and power of predictive tools.

Ligand-independent HER2/HER3/PI3K complex is disrupted by trastuzumab and is effectively inhibited by the PI3K inhibitor GDC-0941

Herceptin (trastuzumab) is the backbone of HER2-directed breast cancer therapy and benefits patients in both the adjuvant and metastatic settings. Here, we describe a mechanism of action for trastuzumab whereby antibody treatment disrupts ligand-independent HER2/HER3 interactions in HER2-amplified cells. The kinetics of dissociation parallels HER3 dephosphorylation and uncoupling from PI3K activity, leading to downregulation of proximal and distal AKT signaling, and correlates with the antiproliferative effects of trastuzumab. A selective and potent PI3K inhibitor, GDC-0941, is highly efficacious both in combination with trastuzumab and in the treatment of trastuzumab-resistant cells and tumors.

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

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

Higher-order association states of cellular ERBB3 probed with photo-cross-linkable aptamers

Nucleic acid aptamers are rapidly gaining prominence as diagnostic tools, targeting reagents, and potential therapeutics. To extend the use of aptamers into the biochemical analysis of protein interactions on the surface of live cells, we converted an enzymatically generated RNA aptamer into a photo-cross-linkable affinity tag through the replacement of all uracils with 4-thiouracil. Specifically, we converted a previously selected, inhibitory aptamer that binds the soluble extracellular domains of the ERBB3 receptor into a targeted and highly specific cross-linking reagent in a live cell setting. Since the photo-cross-linkable aptamer has two functionalities, targeted and highly selective as well as unspecific cross-linking capability, the attachment of this inhibitory aptamer converts ERBB3 into a passive and signaling incompetent probe of its immediate receptor environment. This approach detects receptor clustering of endogenous ERBB3 in the breast cancer cell line MCF7 at levels as low as 25000 receptors per cell and at aptamer concentrations as low as 20 nM. Our analysis also indicates that ERBB3 receptors are apparently segregated from ERBB2 receptors in their resting state, and both ligand-activated ERBB3 and ERBB2 do not share the same microenvironment as inactive ERBB3.

Quantitative characterization of the large-scale association of ErbB1 and ErbB2 by flow cytometric homo-FRET measurements

The association of receptor tyrosine kinases is a key step in the initiation of growth factor-mediated signaling. Although the ligand-induced dimerization of inactive, monomeric receptors was the central dogma of receptor tyrosine kinase activation for decades, the existence of larger oligomers is now accepted. Both homoassociations and heteroassociations are of extreme importance in the epidermal growth factor (EGF) receptor family, leading to diverse and robust signaling. We present a statistically reliable, flow-cytometric homo-fluorescence resonance energy transfer method for the quantitative characterization of large-scale receptor clusters. We assumed that a fraction of a certain protein species is monomeric, whereas the rest are present in homoclusters of N-mers. We measured fluorescence anisotropy as a function of the saturation of fluorescent antibody binding, and fitted the model to the anisotropy data yielding the fraction of monomers and the cluster size. We found that ErbB2 formed larger homoclusters than ErbB1. Stimulation with EGF and heregulin led to a decrease in ErbB2 homocluster size, whereas ErbB1 homoclusters became larger after EGF stimulation. The activation level of ErbB2 was inversely proportional to its homocluster size. We conclude that homoclusters of ErbB1 and ErbB2 behave in a fundamentally different way. Whereas huge ErbB2 clusters serve as a reservoir of inactive coreceptors and dissociate upon stimulation, small ErbB1 homoclusters form higher-order oligomers after ligand binding.

The ERBB3 receptor in cancer and cancer gene therapy

ERBB3, a member of the epidermal growth factor receptor (EGFR) family, is unique in that its tyrosine kinase domain is functionally defective. It is activated by neuregulins, by other ERBB and nonERBB receptors as well as by other kinases, and by novel mechanisms. Downstream it interacts prominently with the phosphoinositol 3-kinase/AKT survival/mitogenic pathway, but also with GRB, SHC, SRC, ABL, rasGAP, SYK and the transcription regulator EBP1. There are likely important but poorly understood roles for nuclear localization and for secreted isoforms. Studies of ERBB3 expression in primary cancers and of its mechanistic contributions in cultured cells have implicated it, with varying degrees of certainty, with causation or sustenance of cancers of the breast, ovary, prostate, certain brain cells, retina, melanocytes, colon, pancreas, stomach, oral cavity and lung. Recent results link high ERBB3 activity with escape from therapy targeting other ERBBs in lung and breast cancers. Thus a wide and centrally important role for ERBB3 in cancer is becoming increasingly apparent. Several approaches for targeting ERBB3 in cancers have been tested or proposed. Small inhibitory RNA (siRNA) to ERBB3 or AKT is showing promise as a therapeutic approach to treatment of lung adenocarcinoma.

Mapping ErbB receptors on breast cancer cell membranes during signal transduction

Distributions of ErbB receptors on membranes of SKBR3 breast cancer cells were mapped by immunoelectron microscopy. The most abundant receptor, ErbB2, is phosphorylated, clustered and active. Kinase inhibitors ablate ErbB2 phosphorylation without dispersing clusters. Modest co-clustering of ErbB2 and EGFR, even after EGF treatment, suggests that both are predominantly involved in homointeractions. Heregulin leads to dramatic clusters of ErbB3 that contain some ErbB2 and EGFR and abundant PI 3-kinase. Other docking proteins, such as Shc and STAT5, respond differently to receptor activation. Levels of Shc at the membrane increase two- to five-fold with EGF, whereas pre-associated STAT5 becomes strongly phosphorylated. These data suggest that the distinct topography of receptors and their docking partners modulates signaling activities.

Similar enzymes, different structures: phthalate dioxygenase is an alpha3alpha3 stacked hexamer, not an alpha3beta3 trimer like "normal" Rieske oxygenases

Phthalate dioxygenase (PDO) is a member of a class of bacterial oxygenases that contain both Rieske [2Fe-2S] and Fe(II) mononuclear centers. Recent crystal structures of several Rieske dioxygenases showed that they exist as alpha(3)beta(3) multimers with subunits arranged head-to-tail in alpha and beta stacked planar rings. The structure of PDO, which consists of only alpha-subunits, remains to be solved. Although similar to other Rieske dioxygenases in many aspects, PDO was shown to differ in the mechanism of catalysis. Gel filtration and analytical centrifugation experiments, supplemented with mass spectrometric analysis (both ESI-MS and ESI-GEMMA), in this work showed a hexameric arrangement of subunits in the PDO multimer. Our proposed model for the subunit arrangement in PDO postulates two alpha(3) planar rings one on top the other, similar to the alpha(3)beta(3) arrangement in other Rieske dioxygenases. Unlike other Rieske dioxygenases, this arrangement brings two Rieske and two mononuclear centers, all on separate subunits, into proximity, allowing their cooperation for catalysis. Potential reasons necessitating this unusual structural arrangement are discussed.

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

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

Unligated Epidermal Growth Factor Receptor Forms Higher Order Oligomers within Microclusters on A431 Cells That Are Sensitive to Tyrosine Kinase Inhibitor Binding

Characterization of the association states of the unligated epidermal growth factor receptor (EGFR) is important in understanding the mechanism of EGFR tyrosine kinase activation in a tumor cell environment. We analyzed, in detail, the association states of unligated, immunotagged EGFR on the surface of intact epidermoid carcinoma A431 cells, using AlexaFluor488 and AlexaFluor546 anti-EGFR antibody, mAb528, as probes. Image correlation microscopy revealed the presence of unligated EGFR in submicron scale clusters containing an average of 10-30 receptors (mean cluster density = 32 +/- 9 clusters per square micron). Lifetime-based Förster resonance energy transfer (FRET) techniques as a function of acceptor:donor labeling ratio disclosed a clustering of the unligated EGFR in clusters containing an average of four receptors on the nanometer (<10 nm) scale. The relationship between the nanoscale and submicron scale associations was determined using a new analysis that combines nanoscale information from lifetime-detected FRET imaging with submicron scale information obtained with image correlation microscopy. This analysis revealed the presence of monomers (or small oligomers) and larger clusters containing 15-30 receptors that were partially associated on the sub-10 nm scale. Pretreatment of the cells with the tyrosine kinase inhibitor AG1478 caused a partial dispersal of the submicron clusters (mean cluster density = 85 +/- 15 clusters per square micron; mean degree of association = 4-10 receptors per cluster) and reduced the level of FRET down to our limit of detection. These results are consistent with a higher order nanoscale receptor organization of the unligated receptor population that is partially controlled by the kinase domains. The ramifications of the results to mechanisms of EGFR activation in a tumor cell environment are discussed.

Lectin-like oxidized low-density lipoprotein receptor (LOX-1) functions as an oligomer and oligomerization is dependent on receptor density

Lectin-like oxidized low-density lipoprotein (LDL) receptor (LOX-1) exists as a homodimer formed by an intermolecular disulfide bond. Although the dimer is the minimum structural unit of LOX-1 on cell membranes, LOX-1 can form larger noncovalent oligomeric complexes. But, the functional unit of LOX-1 is not known. We quantitatively analyzed the correlation between cyan fluorescent protein-tagged LOX-1 expression and the fluorescence-labeled ligand (DiD-AcLDL) binding ability on each cell. The results clearly indicate that there is a threshold level of expression that enables LOX-1 to bind ligand. Above this threshold level, the ability of LOX-1 to bind ligand was proportional to its level of expression. Using the membrane impermeable crosslinker BS(3), we detected oligomers (primarily hexamers) only on the cell lines that stably expressed LOX-1 above the threshold level. In contrast, little oligomer or ligand binding was detected in cell lines expressing LOX-1 below the threshold level. Moreover, oligomerization was independent of ligand binding. These results indicate that the functional unit of LOX-1 is an oligomer and that oligomerization of LOX-1 is dependent on the receptor density on the plasma membrane.

Modulation of porcine wound repair with a transfected ErbB3 gene and relevant EGF-like ligands

Our in vivo study used an ErbB3 receptor transfection strategy to determine if topical application of EGF-like ligands would enhance repair. Partial-thickness porcine wounds transfected with adenoviral particles containing an ErbB3 receptor gene or a vehicle beta-galactosidase gene were introduced and wounds were concomitantly supplied with a variety of EGF-like ligands--EGF, epiregulin (EPR), heparin binding EGF (HB-EGF), and heregulin/neuregulin (HRG). Comparisons of cutaneous repair (resurfacing, dermal depth, proliferation, macrophage infiltration, microvascular density, apoptosis) were assessed after a 5-day healing interval. Differential effects were noted. In wounds transfected with additional ErbB3, either EPR or HB-EGF promoted resurfacing greater than EGF, HRG, or controls. Dermal responses differed significantly after EPR or HB-EGF treatments compared to EGF, HRG, ErbB3 only, or empty vehicle. Hallmarks of enhanced wound maturity were noted in EPR- and HB-EGF-treated wounds transfected with ErbB3. Our data confirmed that an ErbB3-driven pathway mediates a net positive influence in an in vivo model closely resembling human repair. The sensitivity in this system was sufficient to reveal differential outcomes following stimulation with various EGF ligands. We conclude that selective stimulation through an ErbB3-driven pathway shows promise as a therapeutic strategy to hasten wound maturity.

The N-terminal domains of neuregulin 1 confer signal attenuation

Degradation of activated ERBB receptors is an important mechanism for signal attenuation. However, compared with epidermal growth factor (EGF) receptor, the ERBB2/ERBB3 signaling pair is considered to be attenuation-deficient. The ERBB2/ERBB3 ligands of the neuregulin family rely on an EGF-like domain for signaling and are generated from larger membrane-bound precursors. In contrast to EGF, which is processed to yield a 6-kDa peptide ligand, mature neuregulins retain a variety of segments N-terminal to the EGF-like domain. Here we evaluate the role of the N-terminal domain of neuregulin 1 in signaling and turnover of ERBB2/ERBB3. Our data suggest that whereas the EGF-like domain of neuregulin 1 is required and sufficient for the formation of active receptor heterodimers, the presence of the N-terminal Ig-like domain is required for efficient signal attenuation. This manifests itself for both ERBB2 and ERBB3 but is more pronounced and coupled directly to degradation for ERBB3. When stimulated with only the EGF-like domain, ERBB3 shows degradation rates comparable with constitutive turnover, but stimulation with full-length neuregulin 1 resulted in receptor degradation at rates that are comparable with activated EGF receptor. Most of the enhancement in down-regulation was maintained after replacing the Ig-like domain with a thioredoxin protein of comparable size but different amino acid composition, suggesting that the physical presence but not specific properties of the Ig-like domain are needed. This sequence-independent effect of the N-terminal domain correlates with an enhanced ability of full-size neuregulin 1 to disrupt higher order oligomers of the ERBB3 extracellular domains in vitro.

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.

The extracellular domains of ErbB3 retain high ligand binding affinity at endosome pH and in the locked conformation

The extracellular, ligand binding regions of ErbB receptors consist of four domains that can assume at least two alternative conformations, extended and locked. The locked conformation, observed in several crystal structures, is held together by a noncovalent intramolecular tether and is incompatible with current models for receptor dimerization and ligand activation. Based on structures of ligand-receptor complexes in the extended conformation, the high affinity ligand binding pocket between domains I and III is disrupted in the locked conformation. Therefore the biological role of the locked conformation is not clear. To address the impact of the locked conformation on ligand binding, we compared extracellular domains of wild-type ErbB3, mutant domains in a constitutively locked or extended conformation and partial extracellular domain constructs. We found that the constitutively locked receptor domains and truncated constructs carrying only domains I-II or III-IV strongly bind ligand, albeit with reduced affinity compared to wild-type receptor. This suggests that the locked conformation cannot be discounted for ligand binding. The significant binding by both partial interfaces in domains I and III also suggests that "partial bivalency" may be the reason for the low nanomolar and high picomolar binding observed for ErbB3 in the respective "low" and high affinity states. In contrast to EGFR (ErbB1), ErbB3 retains high ligand binding affinity at an endosome-comparable pH in both the extended and locked conformations. Ligand affinity for the locked conformation even improves at low pH. For ErbB3, the contribution of domain I to ligand binding is strong and increases at low pH while its contribution is thought to be minimal for EGFR, regardless of pH. This shift in domain contribution and pH dependency provides a mechanistic explanation for some of the divergent properties of EGFR and ErbB3.