ERBB3-dependent AKT and ERK pathways are essential for atrioventricular cushion development in mouse embryos

ERBB family members and their ligands play an essential role in embryonic heart development and adult heart physiology. Among them, ERBB3 is a binding partner of ERBB2; the ERBB2/3 complex mediates downstream signaling for cell proliferation. ERBB3 has seven consensus binding sites to the p85 regulatory subunit of PI3K, which activates the downstream AKT pathway, leading to the proliferation of various cells. This study generated a human ERBB3 knock-in mouse expressing a mutant ERBB3 whose seven YXXM p85 binding sites were replaced with YXXA. Erbb3 knock-in embryos exhibited lethality between E12.5 to E13.5, and showed a decrease in mesenchymal cell numbers and density in AV cushions. We determined that the proliferation of mesenchymal cells in the atrioventricular (AV) cushion in Erbb3 knock-in mutant embryos was temporarily reduced due to the decrease of AKT and ERK1/2 phosphorylation. Overall, our results suggest that AKT/ERK activation by the ERBB3-dependent PI3K signaling is crucial for AV cushion morphogenesis during embryonic heart development.

Co-occurring gain-of-function mutations in HER2 and HER3 modulate HER2/HER3 activation, oncogenesis, and HER2 inhibitor sensitivity

Activating mutations in HER2 (ERBB2) drive the growth of a subset of breast and other cancers and tend to co-occur with HER3 (ERBB3) missense mutations. The HER2 tyrosine kinase inhibitor neratinib has shown clinical activity against HER2-mutant tumors. To characterize the role of HER3 mutations in HER2-mutant tumors, we integrate computational structural modeling with biochemical and cell biological analyses. Computational modeling predicts that the frequent HER3E928G kinase domain mutation enhances the affinity of HER2/HER3 and reduces binding of HER2 to its inhibitor neratinib. Co-expression of mutant HER2/HER3 enhances HER2/HER3 co-immunoprecipitation and ligand-independent activation of HER2/HER3 and PI3K/AKT, resulting in enhanced growth, invasiveness, and resistance to HER2-targeted therapies, which can be reversed by combined treatment with PI3Kα inhibitors. Our results provide a mechanistic rationale for the evolutionary selection of co-occurring HER2/HER3 mutations and the recent clinical observations that HER3 mutations are associated with a poor response to neratinib in HER2-mutant cancers.

Crystal structures of HER3 extracellular domain 4 in complex with the designed ankyrin-repeat protein D5

The members of the human epidermal growth factor receptor (HER) family are among the most intensely studied oncological targets. HER3 (ErbB3), which had long been neglected, has emerged as a key oncogene, regulating the activity of other receptors and being involved in progression and tumor escape in multiple types of cancer. Designed ankyrin-repeat proteins (DARPins) serve as antibody mimetics that have proven to be useful in the clinic, in diagnostics and in research. DARPins have previously been selected against EGFR (HER1), HER2 and HER4. In particular, their combination into bivalent binders that separate or lock receptors in their inactive conformation has proved to be a promising strategy for the design of potent anticancer therapeutics. Here, the selection of DARPins targeting extracellular domain 4 of HER3 (HER3d4) is described. One of the selected DARPins, D5, in complex with HER3d4 crystallized in two closely related crystal forms that diffracted to 2.3 and 2.0 Å resolution, respectively. The DARPin D5 epitope comprises HER3d4 residues 568-577. These residues also contribute to interactions within the tethered (inactive) and extended (active) conformations of the extracellular domain of HER3.

A human epidermal growth factor receptor 3/heregulin interaction inhibitor aptamer discovered using SELEX

The interaction of human epidermal growth factor receptor 3 (HER3) and heregulin (HRG) is involved in resistance to human epidermal growth factor receptor 2 (HER2)-targeted cancer treatment, such as therapies using anti-HER2 monoclonal antibody. Therefore, inhibition of the HER3/HRG interaction is potentially valuable therapeutic target for cancer treatment. In this study, we used in vitro selection, also known as systematic evolution of ligands by exponential enrichment (SELEX) against the extracellular domain of human HER3, and discovered a novel RNA aptamer. Pull-down and bio-layer interferometry assays showed that RNA aptamer discovered specifically bound to HER3 with a dissociation constant (K_D) of 700 nM. Pull-down assays using chemiluminescence detection also revealed that the HER3-binding RNA aptamer inhibited interactions between HER3 and human HRG. These results indicated that the novel HER3-binding RNA aptamer has potential to be used as basic tool in a range of applications involving HER3/HRG interactions, including research, therapeutic, and diagnostic applications.

A small molecule inhibitor of HER3: a proof-of-concept study

Despite being catalytically defective, pseudokinases are typically essential players of cellular signalling, acting as allosteric regulators of their active counterparts. Deregulation of a growing number of pseudokinases has been linked to human diseases, making pseudokinases therapeutic targets of interest. Pseudokinases can be dynamic, adopting specific conformations critical for their allosteric function. Interfering with their allosteric role, with small molecules that would lock pseudokinases in a conformation preventing their productive partner interactions, is an attractive therapeutic strategy to explore. As a well-known allosteric activator of epidermal growth factor receptor family members, and playing a major part in cancer progression, the pseudokinase HER3 is a relevant context in which to address the potential of pseudokinases as drug targets for the development of allosteric inhibitors. In this proof-of-concept study, we developed a multiplex, medium-throughput thermal shift assay screening strategy to assess over 100 000 compounds and identify selective small molecule inhibitors that would trap HER3 in a conformation which is unfavourable for the formation of an active HER2-HER3 heterodimer. As a proof-of-concept compound, AC3573 bound with some specificity to HER3 and abrogated HER2-HER3 complex formation and downstream signalling in cells. Our study highlights the opportunity to identify new molecular mechanisms of action interfering with the biological function of pseudokinases.

Phage Display Selection, In Vitro Characterization, and Correlative PET Imaging of a Novel HER3 Peptide

PURPOSE

HER3 (ERBB3) is a receptor tyrosine kinase that is implicated in treatment resistance across multiple cancers, including those of the breast, lung, and prostate. Overexpression of HER3 following targeted therapy can occur rapidly and heterogeneously both within a single lesion and across sites of metastasis, making protein quantification by biopsy highly challenging. A global, non-invasive methodology such as positron emission tomography (PET) imaging can permit serial quantification of HER3, providing a useful approach to monitor HER3 expression across the entire tumor burden both prior to and following treatment. PET imaging of HER3 expression may permit a more personalized approach to targeted therapy by allowing for detection of HER3-mediated resistance, in addition to informing clinical trial patient selection for novel therapies targeting HER3.

PROCEDURES

Phage display selection targeting the HER3 extracellular domain was performed in order to develop a peptide with optimal blood clearance and highly accurate HER3 quantification.

RESULTS

The selection converged to a consensus peptide sequence that was subsequently found to bind HER3 with an affinity of 270 ± 151 nM. The peptide, termed HER3P1, was bound with high selectivity to HER3 over other similar receptor tyrosine kinases such as EGFR and HER2. Furthermore, HER3P1 was able to distinguish between high and low HER3-expressing cells in vitro. The peptide was radiolabeled with Ga-68 and demonstrated to specifically bind HER3 by in vivo PET imaging. Uptake of [⁶⁸Ga]HER3P1 was highly specific for HER3-positive tumors, with tumor-to-background ratios ranging from 1.59-3.32, compared to those of HER3-negative tumors, ranging from 0.84-0.93. The uptake of [⁶⁸Ga]HER3P1 also demonstrated high (P < 0.001) correlation with protein expression as quantified by Western blot and confirmed by biodistribution.

CONCLUSIONS

HER3P1 accurately quantifies expression of HER3 by PET imaging and has potential utility as a clinical imaging agent.

Unbiased Combinatorial Screening Identifies a Bispecific IgG1 that Potently Inhibits HER3 Signaling via HER2-Guided Ligand Blockade

HER2-driven cancers require phosphatidylinositide-3 kinase (PI3K)/Akt signaling through HER3 to promote tumor growth and survival. The therapeutic benefit of HER2-targeting agents, which depend on PI3K/Akt inhibition, can be overcome by hyperactivation of the heregulin (HRG)/HER3 pathway. Here we describe an unbiased phenotypic combinatorial screening approach to identify a bispecific immunoglobulin G1 (IgG1) antibody against HER2 and HER3. In tumor models resistant to HER2-targeting agents, the bispecific IgG1 potently inhibits the HRG/HER3 pathway and downstream PI3K/Akt signaling via a "dock & block" mechanism. This bispecific IgG1 is a potentially effective therapy for breast cancer and other tumors with hyperactivated HRG/HER3 signaling.

Inhibitor-induced HER2-HER3 heterodimerisation promotes proliferation through a novel dimer interface

While targeted therapy against HER2 is an effective first-line treatment in HER2+ breast cancer, acquired resistance remains a clinical challenge. The pseudokinase HER3, heterodimerisation partner of HER2, is widely implicated in the resistance to HER2-mediated therapy. Here, we show that lapatinib, an ATP-competitive inhibitor of HER2, is able to induce proliferation cooperatively with the HER3 ligand neuregulin. This counterintuitive synergy between inhibitor and growth factor depends on their ability to promote atypical HER2-HER3 heterodimerisation. By stabilising a particular HER2 conformer, lapatinib drives HER2-HER3 kinase domain heterocomplex formation. This dimer exists in a head-to-head orientation distinct from the canonical asymmetric active dimer. The associated clustering observed for these dimers predisposes to neuregulin responses, affording a proliferative outcome. Our findings provide mechanistic insights into the liabilities involved in targeting kinases with ATP-competitive inhibitors and highlight the complex role of protein conformation in acquired resistance.

HER kinase inhibition in patients with HER2- and HER3-mutant cancers

Somatic mutations of ERBB2 and ERBB3 (which encode HER2 and HER3, respectively) are found in a wide range of cancers. Preclinical modelling suggests that a subset of these mutations lead to constitutive HER2 activation, but most remain biologically uncharacterized. Here we define the biological and therapeutic importance of known oncogenic HER2 and HER3 mutations and variants of unknown biological importance by conducting a multi-histology, genomically selected, 'basket' trial using the pan-HER kinase inhibitor neratinib (SUMMIT; clinicaltrials.gov identifier NCT01953926). Efficacy in HER2-mutant cancers varied as a function of both tumour type and mutant allele to a degree not predicted by preclinical models, with the greatest activity seen in breast, cervical and biliary cancers and with tumours that contain kinase domain missense mutations. This study demonstrates how a molecularly driven clinical trial can be used to refine our biological understanding of both characterized and new genomic alterations with potential broad applicability for advancing the paradigm of genome-driven oncology.

An in-silico approach to find a peptidomimetic targeting extracellular domain of HER3 from a HER3 Nanobody

HER3 is an important therapeutic target in cancer treatments. HER3 Nanobodies (Nbs) are a novel class of antibodies with several competitive advantages over conventional antibodies. A peptidomimetic derived from these Nbs can be considered to be a small peptide mimicking some of the molecular recognition interactions of a natural peptide or protein in a three-dimensional (3D) space, with a receptor that has improved properties. In this study, we introduce a new approach to design a peptidomimetic derived from HER3 Nb through an in silico analysis. We propose that the complementarity determining region (CDR3) of HER3 Nb is large enough to effectively interact with HER3 antigen as well as with the entire Nb. A computational analysis has been performed using Nb models retrieved from SWISS-pdb Viewer 4.1.0 (spdbv) as a target spot and HER3 extracellular domain as its antigenic target to identify the interactions between them by the protein-protein docking method. Detailed analysis of selected models with docked complex help us to identify the interacting amino acid residues between the two molecules. The results of in silico analysis show that the CDR3 of HER3 Nb might be used by itself as a peptidomimetic drug instead of the full Nb. HER3 peptidomimetic-derived HER3 Nb may reduce Nb production costs and be used as a substitute for HER3 Nb after further experimental work. The paper demonstrates the feasibility of peptidomimetics designs using bioinformatic tools.

Computing ensembles of transitions with molecular dynamics simulations

A molecular understanding of conformational change is important for connecting structure and function. Without the ability to sample on the meaningful large-scale conformational changes, the ability to infer biological function and to understand the effect of mutations and changes in environment is not possible. Our Dynamic Importance Sampling method (DIMS), part of the CHARMM simulation package, is a method that enables sampling over ensembles of transition intermediates. This chapter outlines the context for the method and the usage within the program.

Structural analysis of the EGFR/HER3 heterodimer reveals the molecular basis for activating HER3 mutations

The human epidermal growth factor receptor (HER) tyrosine kinases homo- and heterodimerize to activate downstream signaling pathways. HER3 is a catalytically impaired member of the HER family that contributes to the development of several human malignancies and is mutated in a subset of cancers. HER3 signaling depends on heterodimerization with a catalytically active partner, in particular epidermal growth factor receptor (EGFR) (the founding family member, also known as HER1) or HER2. The activity of homodimeric complexes of catalytically active HER family members depends on allosteric activation between the two kinase domains. To determine the structural basis for HER3 signaling through heterodimerization with a catalytically active HER family member, we solved the crystal structure of the heterodimeric complex formed by the isolated kinase domains of EGFR and HER3. The structure showed HER3 as an allosteric activator of EGFR and revealed a conserved role of the allosteric mechanism in activation of HER family members through heterodimerization. To understand the effects of cancer-associated HER3 mutations at the molecular level, we solved the structures of two kinase domains of HER3 mutants, each in a heterodimeric complex with the kinase domain of EGFR. These structures, combined with biochemical analysis and molecular dynamics simulations, indicated that the cancer-associated HER3 mutations enhanced the allosteric activator function of HER3 by redesigning local interactions at the dimerization interface.

Combining anti-ERBB3 antibodies specific for domain I and domain III enhances the anti-tumor activity over the individual monoclonal antibodies

Background

Inappropriate signaling through the epidermal growth factor receptor family (EGFR1/ERBB1, ERBB2/HER2, ERBB3/HER3, and ERBB4/HER4) of receptor tyrosine kinases leads to unregulated activation of multiple downstream signaling pathways that are linked to cancer formation and progression. In particular, ERBB3 plays a critical role in linking ERBB signaling to the phosphoinositide 3-kinase and Akt signaling pathway and increased levels of ERBB3-dependent signaling is also increasingly recognized as a mechanism for acquired resistance to ERBB-targeted therapies.

Methods

We had previously reported the isolation of a panel of anti-ERBB3 single-chain Fv antibodies through use of phage-display technology. In the current study scFv specific for domain I (F4) and domain III (A5) were converted into human IgG1 formats and analyzed for efficacy.

Results

Treatment of cells with an oligoclonal mixture of the A5/F4 IgGs appeared more effective at blocking both ligand-induced and ligand-independent signaling through ERBB3 than either single IgG alone. This correlated with improved ability to inhibit the cell growth both as a single agent and in combination with other ERBB-targeted therapies. Treatment of NCI-N87 tumor xenografts with the A5/F4 oligoclonal led to a statistically significant decrease in tumor growth rate that was further enhanced in combination with trastuzumab.

Conclusion

These results suggest that an oligoclonal antibody mixture may be a more effective approach to downregulate ERBB3-dependent signaling.

Expression, purification, crystallization and preliminary X-ray analysis of the HER3-9E12 Fab complex

9E12 is a fully human immunoglobulin G1/κ monoclonal antibody that is specific for the epidermal growth factor receptor 3 (HER3), the overexpression of which has been detected in many tumour types and is associated with poor survival outcomes. To date, knowledge of the molecular mechanism for targeted antibodies that directly inhibit HER3 signalling is limited. Because knowledge of such therapeutic antibodies would help basic immunological therapeutics, structural insights into the HER3-9E12 Fab complex are important. Recombinant human HER3 and Fab fragments of the 9E12 antibody were cloned, expressed and crystallized, and crystallographic data sets were collected. The crystals belonged to space group P1, with unit-cell parameters a=74.4, b=98.6, c=99.6 Å, α=106.0, β=95.0, γ=102.5° and diffracted to a resolution of 2.1 Å.

Model-based analysis of HER activation in cells co-expressing EGFR, HER2 and HER3

The HER/ErbB family of receptor tyrosine kinases drives critical responses in normal physiology and cancer, and the expression levels of the various HER receptors are critical determinants of clinical outcomes. HER activation is driven by the formation of various dimer complexes between members of this receptor family. The HER dimer types can have differential effects on downstream signaling and phenotypic outcomes. We constructed an integrated mathematical model of HER activation, and trafficking to quantitatively link receptor expression levels to dimerization and activation. We parameterized the model with a comprehensive set of HER phosphorylation and abundance data collected in a panel of human mammary epithelial cells expressing varying levels of EGFR/HER1, HER2 and HER3. Although parameter estimation yielded multiple solutions, predictions for dimer phosphorylation were in agreement with each other. We validated the model using experiments where pertuzumab was used to block HER2 dimerization. We used the model to predict HER dimerization and activation patterns in a panel of human mammary epithelial cells lines with known HER expression levels in response to stimulations with ligands EGF and HRG. Simulations over the range of expression levels seen in various cell lines indicate that: i) EGFR phosphorylation is driven by HER1-HER1 and HER1-HER2 dimers, and not HER1-HER3 dimers, ii) HER1-HER2 and HER2-HER3 dimers both contribute significantly to HER2 activation with the EGFR expression level determining the relative importance of these species, and iii) the HER2-HER3 dimer is largely responsible for HER3 activation. The model can be used to predict phosphorylated dimer levels for any given HER expression profile. This information in turn can be used to quantify the potencies of the various HER dimers, and can potentially inform personalized therapeutic approaches.

A family of cell surface molecules called the HER receptor family plays important roles in normal physiology and cancer. This family has four members, HER1-4. These receptors convert signals received from the extracellular environment into cell decisions such as growth and survival – a process termed signal transduction. In particular, HER2 and HER3 are over-expressed in a number of tumors, and their expression levels are associated with abnormal growth and poor clinical prognosis. A key step in HER-mediated signal transduction is the formation of dimer complexes between members of this family. Different dimer types have different potencies for activating normal and aberrant responses. Prediction of the dimerization pattern for a given HER expression level may pave the way for personalized therapeutic approaches targeting specific dimers. Towards this end, we constructed a mathematical model for HER dimerization and activation. We determined unknown model parameters by analyzing HER activation data collected in a panel of human mammary epithelial cells that express different levels of the HER molecules. The model enables us to quantitatively link HER expression levels to receptor dimerization and activation. Further, the model can be used to support additional quantitative investigations into the basic biology of HER-mediated signal transduction.

Inhibiting HER3-mediated tumor cell growth with affibody molecules engineered to low picomolar affinity by position-directed error-prone PCR-like diversification

The HER3 receptor is implicated in the progression of various cancers as well as in resistance to several currently used drugs, and is hence a potential target for development of new therapies. We have previously generated Affibody molecules that inhibit heregulin-induced signaling of the HER3 pathways. The aim of this study was to improve the affinity of the binders to hopefully increase receptor inhibition efficacy and enable a high receptor-mediated uptake in tumors. We explored a novel strategy for affinity maturation of Affibody molecules that is based on alanine scanning followed by design of library diversification to mimic the result from an error-prone PCR reaction, but with full control over mutated positions and thus less biases. Using bacterial surface display and flow-cytometric sorting of the maturation library, the affinity for HER3 was improved more than 30-fold down to 21 pM. The affinity is among the higher that has been reported for Affibody molecules and we believe that the maturation strategy should be generally applicable for improvement of affinity proteins. The new binders also demonstrated an improved thermal stability as well as complete refolding after denaturation. Moreover, inhibition of ligand-induced proliferation of HER3-positive breast cancer cells was improved more than two orders of magnitude compared to the previously best-performing clone. Radiolabeled Affibody molecules showed specific targeting of a number of HER3-positive cell lines in vitro as well as targeting of HER3 in in vivo mouse models and represent promising candidates for future development of targeted therapies and diagnostics.

A two-in-one antibody against HER3 and EGFR has superior inhibitory activity compared with monospecific antibodies

Extensive crosstalk among ErbB/HER receptors suggests that blocking signaling from more than one family member may be essential to effectively treat cancer and limit drug resistance. We generated a conventional IgG molecule MEHD7945A with dual HER3/EGFR specificity by phage display engineering and used structural and mutational studies to understand how a single antigen recognition surface binds two epitopes with high affinity. As a human IgG1, MEHD7945A exhibited dual action by inhibiting EGFR- and HER3-mediated signaling in vitro and in vivo and the ability to engage immune effector functions. Compared with monospecific anti-HER antibodies, MEHD7945A was more broadly efficacious in multiple tumor models, showing that combined inhibition of EGFR and HER3 with a single antibody is beneficial.

[Bacterial synthesis, purification, and solubilization of transmembrane segments of ErbB family members]

A family of epidermal growth factor receptors, ErbB, represents an important class of receptor tyrosine kinases, playing a leading role in cellular growth, development and differentiation. Transmembrane domains of these receptors transduce biochemical signals across plasma membrane via lateral homo- and heterodimerization. Relatively small size of complexes of ErbB transmembrane domains with detergents or lipids allows one to study their detailed spatial structure using three-dimensional heteronuclear high-resolution NMR spectroscopy. Here, we describe the effective expression system and purification procedure for preparative-scale production of transmembrane peptides from four representatives of ErbB family, ErbB1, ErbB2, ErbB3, ErbB4, for structural studies. The recombinant peptides were produced in Escherichia coli BL21(DE3)pLysS as C-terminal extensions of thioredoxin A. The fusion protein cleavage was accomplished with the light subunit of human enterokinase. Several (10-30) milligrams of purified isotope-labeled transmembrane peptides were isolated with the use of a simple and convenient procedure, which consists of consecutive steps of immobilized metal affinity chromatography and cation-exchange chromatography. The purified peptides were reconstituted in lipid/detergent environment (micelles or bicelles) and characterized using dynamic light scattering, CD and NMR spectroscopy. The data obtained indicate that the purified ErbB transmembrane peptides are suitable for structural and dynamic studies of their homo- and heterodimer complexes using high resolution NMR spectroscopy.

Novel anticancer targets: revisiting ERBB2 and discovering ERBB3

Aberrant receptor expression or functioning of the epidermal growth factor receptor (Erbb) family plays a crucial part in the development and evolution of cancer. Inhibiting the signalling activity of individual receptors in this family has advanced the treatment of a range of human cancers. In this Review we re-evaluate the role of two important family members, ERBB2 (also known as HER2) and ERBB3 (also known as HER3), and explore the mechanisms of action and preclinical and clinical data for new therapies that target signalling through these pivotal receptors. These new therapies include tyrosine kinase inhibitors, antibody-chemotherapy conjugates, heat-shock protein inhibitors and antibodies that interfere with the formation of ERBB2-ERBB3 dimers.

Calorimetric investigation of phosphorylated and non-phosphorylated peptide ligand binding to the human Grb7-SH2 domain

Grb7 is a member of the Grb7 family of proteins, which also includes Grb10 and Grb14. All three proteins have been found to be overexpressed in certain cancers and cancer cell lines. In particular, Grb7 (along with the receptor tyrosine kinase erbB2) is overexpressed in 20-30% of breast cancers. In general, growth factor receptor bound (Grb) proteins bind to activated membrane-bound receptor tyrosine kinases (RTKs; e.g., the epidermal growth factor receptor, EGFR) through their Src homology 2 (SH2) domains. In particular, Grb7 binds to erbB2 (a.k.a. EGFR2) and may be involved in cell signaling pathways that promote the formation of metastases and inflammatory responses. In previous studies, we reported the solution structure and the backbone relaxation behavior of the Grb7-SH2/erbB2 peptide complex. In this study, isothermal titration calorimetry studies have been completed by measuring the thermodynamic binding parameters of several phosphorylated and non-phosphorylated peptides representative of natural Grb7 receptor ligands as well as ligands developed through combinatorial peptide screening methods. The entirety of these calorimetric studies is interpreted in an effort to describe the specific ligand binding characteristics of the Grb7 protein.

Ligand-Induced Structural Transitions in ErbB Receptor Extracellular Domains

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

A secreted isoform of ErbB3 promotes osteonectin expression in bone and enhances the invasiveness of prostate cancer cells

The propensity for prostate cancer to metastasize to bone led us and others to propose that bidirectional interactions between prostate cancer cells and bone are critical for the preferential metastasis of prostate cancer to bone. We identified previously a secreted isoform of ErbB3 (p45-sErbB3) in bone marrow supernatant samples from men with prostate cancer and bone metastasis and showed by immunohistochemical analysis of human tissue specimens that p45-sErbB3 was highly expressed in metastatic prostate cancer cells in bone. Here, we show that p45-sErbB3 stimulated mouse calvaria to secrete factors that increased the invasiveness of prostate cancer cells in a Boyden chamber invasion assay. Using gene array analysis to identify p45-sErbB3-responsive genes, we found that p45-sErbB3 up-regulated the expression of osteonectin/SPARC, biglycan, and type I collagen in calvaria. We further show that recombinant osteonectin increased the invasiveness of PC-3 cells, whereas osteonectin-neutralizing antibodies blocked this p45-sErbB3-induced invasiveness. These results indicate that p45-sErbB3 enhances the invasiveness of PC-3 cells in part by stimulating the secretion of osteonectin by bone. Thus, p45-sErbB3 may mediate the bidirectional interactions between prostate cancer cells and bone via osteonectin.

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

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

A dimerization hierarchy in the transmembrane domains of the HER receptor family

Bitopic membrane proteins offer an opportunity for studying transmembrane domain interactions without the structural complexity inherent to multitopic integral membrane proteins. To date, only homomeric associations have been extensively studied quantitatively. Here we propose to assess the thermodynamics of heteromeric associations, which opens the way to investigating specificity and selectivity. A very interesting system of biological relevance with single transmembrane domains possibly involved in interactions with different partners is the EGFR receptor family. The four members, all tyrosine kinase receptors, are involved in an interaction network that potentially leads to a complete set of homo- and heterodimers, ideally suited to such a study. Furthermore, the transmembrane domains of these receptors have been previously implicated in their function in the past by mutations in the transmembrane domain leading to constitutive activation. We demonstrate, using a fluorescence-based measurement of interaction energies, a hierarchy of transmembrane domain interactions ranging from a noninteractive pair to strong dimerization. We propose a structural model based on the crystal structure of the EGFR dimer, to show how the dimeric structure favors these interactions. The correlation we observe between transmembrane domain and whole receptor interaction hierarchies opens a new perspective, suggesting a role for transmembrane receptor domains in the modulation of receptor signaling.

Negative Constraints Underlie the ErbB Specificity of Epidermal Growth Factor-like Ligands

Epidermal growth factor (EGF)-like growth factors bind their ErbB receptors in a highly selective manner, but the molecular basis for this specificity is poorly understood. We have previously shown that certain residues in human EGF (Ser(2)-Asp(3)) and TGFalpha (Glu(26)) are not essential for their binding to ErbB1 but prevent binding to ErbB3 and ErbB4. In the present study, we have used a phage display approach to affinity-optimize the C-terminal linear region of EGF-like growth factors for binding to each ErbB receptor and thereby shown that Arg(45) in EGF impairs binding to both ErbB3 and ErbB4. By omitting all these so-called negative constraints from EGF, we designed a ligand designated panerbin that binds ErbB1, ErbB3, and ErbB4 with similarly high affinity as their wild-type ligands. Homology models, based on the known crystal structure of TGFalpha-bound ErbB1, showed that panerbin is able to bind ErbB1, ErbB3, and ErbB4 in a highly similar manner with respect to position and number of interaction sites. Upon in silico introduction of the experimentally known negative constraints into panerbin, we found that Arg(45) induced local charge repulsion and Glu(26) induced steric hindrance in a receptor-specific manner, whereas Ser(2)-Asp(3) impaired binding due to a disordered conformation. Furthermore, radiolabeled panerbin was used to quantify the level of all three receptors on human breast cancer cells in a single radioreceptor assay. It is concluded that the ErbB specificity of EGF-like growth factors primarily results from the presence of a limited number of residues that impair the unintended interaction with other ErbB receptors.

Insights into the evolution of the ErbB receptor family and their ligands from sequence analysis

Background

In the time since we presented the first molecular evolutionary study of the ErbB family of receptors and the EGF family of ligands, there has been a dramatic increase in genomic sequences available. We have utilized this greatly expanded data set in this study of the ErbB family of receptors and their ligands.

Results

In our previous analysis we postulated that EGF family ligands could be characterized by the presence of a splice site in the coding region between the fourth and fifth cysteines of the EGF module and the placement of that module near the transmembrane domain. The recent identification of several new ligands for the ErbB receptors supports this characterization of an ErbB ligand; further, applying this characterization to available sequences suggests additional potential ligands for these receptors, the EGF modules from previously identified proteins: interphotoreceptor matrix proteoglycan-2, the alpha and beta subunit of meprin A, and mucins 3, 4, 12, and 17. The newly available sequences have caused some reorganizations of relationships among the ErbB ligand family, but they add support to the previous conclusion that three gene duplication events gave rise to the present family of four ErbB receptors among the tetrapods.

Conclusion

This study provides strong support for the hypothesis that the presence of an easily identifiable sequence motif can distinguish EGF family ligands from other EGF-like modules and reveals several potential new EGF family ligands. It also raises interesting questions about the evolution of ErbB2 and ErbB3: Does ErbB2 in teleosts function differently from ErbB2 in tetrapods in terms of ligand binding and intramolecular tethering? When did ErbB3 lose kinase activity, and what is the functional significance of the divergence of its kinase domain among teleosts?

Mutation of the PI3' kinase gene in a human colon carcinoma cell line, HCC2998

HCC2998 is a highly differentiated human colon carcinoma cell line, which has been shown to be converted to a poorly differentiated one after expression of a constitutively active phosphatidylinositol 3-kinase (PI3' kinase). These cells express aberrant sizes of a regulatory subunit of PI3' kinase, p85alpha, with molecular weights of 50 and 76 kDa at a very low level. To elucidate how these cells express these proteins, we analyzed mutations within the p85alpha gene. DNA sequencing analysis revealed that these mutant proteins were generated by independent point mutations in the two alleles of the p85alpha gene: one in the coding sequence, and the other in the acceptor sequence for splicing. Introduction of wild-type p85alpha into HCC2998 cells induced slight rounding of the cells and enhancement of mucin secretion. At the same time, a membrane receptor, ErbB3, was phosphorylated on tyrosine, which in turn, binds to PI3' kinase. Since ErbB3 is upstream of PI3' kinase, it is likely that there is an autocrine loop in which PI3' kinase is activated by ErbB3, which may contribute to dedifferentiation of the cells.

Regulation of early Xenopus development by ErbB signaling

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

Computational analysis of molecular basis of 1:1 interactions of NRG-1beta wild-type and variants with ErbB3 and ErbB4

The neuregulin/ErbB system is a growth factor/receptor cascade that has been proven to be essential in the development of the heart and the sympathetic nervous system. However, the basis of the specificity of ligand-receptor recognition remains to be elucidated. In this study, the structures of NRG-1beta/ErbB3 and NRG-1beta/ErbB4 complexes were modeled based on the available structures of the homologous proteins. The binding free energies of NRG-1beta to ErbB3 and ErbB4 were calculated using the molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) computational method. In addition, computational alanine-scanning mutagenesis was performed in the binding site of NRG-1beta and the difference in the binding free energies between NRG-1beta mutants and the receptors was calculated. The results specify the contribution of each residue at the interaction interfaces to the binding affinity of NRG-1beta with ErbB3 and ErbB4, identifying several important interaction residue pairs that are in agreement with previously acquired experimental data. This indicates that the presented structural models of NRG-1beta/ErbB3 and NRG-1beta/ErbB4 complexes are reliable and could be used to guide future studies, such as performing desirable mutations on NRG-1beta to increase the binding affinity and selectivity to the receptor and discovering new therapeutic agents for the treatment of heart failure.

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.

Phosphotyrosine interactome of the ErbB-receptor kinase family

Interactions between short modified peptide motifs and modular protein domains are central events in cell signal-transduction. We determined interaction partners to all cytosolic tyrosine residues of the four members of the ErbB-receptor family in an unbiased fashion by quantitative proteomics using pull-down experiments with pairs of phosphorylated and nonphosphorylated synthetic peptides. Each receptor had characteristic preferences for interacting proteins and most interaction partners had multiple binding sites on each receptor. EGFR and ErbB4 had several docking sites for Grb2, while ErbB3 was characterized by six binding sites for PI3K. We identified STAT5 as a direct binding partner to EGFR and ErbB4 and discovered new recognition motifs for Shc and STAT5. The overall pattern of interaction partners of EGFR and ErbB4 suggests similar roles during signaling through their respective ligands. Phosphorylation kinetics of several tyrosine resides was measured by mass spectrometry and correlated with interaction partner preference. Our results demonstrate that system-wide mapping of peptide-protein interactions sites is possible, and suggest shared and unique roles of ErbB-receptor family members in downstream signaling.

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

ErbB receptors associate in a ligand-dependent or -independent manner, and overexpression of epidermal growth factor receptor (ErbB1) or ErbB2 results in ligand-independent activation. Ligand-independent activation is poorly understood, and dimerization alone is not sufficient for activation. ErbB receptors also form higher order oligomers, but the mechanism of oligomer formation and their contribution to signaling are not known. The kinase-deficient ErbB3 as well as its extracellular domains are particularly prone to ligand-independent oligomerization, and oligomers are destabilized by binding of the ligand heregulin. In contrast, ligand binding facilitates heterodimerization with ErbB2 and is expected to stabilize an extended conformation of the ErbB3 extracellular domain (ECD) in which the dimerization interface is exposed. In the absence of ligand, ErbB3 can adopt a closed conformation that is held together by an intramolecular tether. We used a constitutively extended form of the ErbB3-ECD to analyze the conformation of the ECD in oligomers and the mechanism of oligomer disruption by heregulin. The extended conformation of the ECD forms oligomers more readily, suggesting the crystallographically defined dimer interface is one of the interfaces involved in oligomerization. Heregulin destabilizes oligomeric complexes but not dimers, which are neither stabilized nor disrupted by ligand binding, indicating a distinct second interface in oligomers of ErbB3. Cross-linking and activation studies on membrane-embedded ErbB3/ErbB2 chimeras confirm this dual effect of heregulin. Most of the ErbB3-ECD on the cell surface is apparently kept in an open conformation through oligomerization, and the resulting oligomers adopt a conformation representing a state of reduced activity.

ErbB4 Expression in Neural Progenitor Cells (ST14A) Is Necessary to Mediate Neuregulin-1β1-induced Migration

Activation of the receptor tyrosine kinase ErbB4 leads to various cellular responses such as proliferation, survival, differentiation, and chemotaxis. Two pairs of naturally occurring ErbB4 isoforms differing in their juxtamembrane (JMa/JMb) and C termini (cyt1/cyt2) have been described. To examine the role of ErbB4 in neuron migration, we cloned and stably transfected each of the four ErbB4 isoforms in ST14A cells (a neural progenitor cell line derived from the striatum of embryonic day 14 rats) endogenously expressing the other members of the ErbB family: ErbB1, ErbB2, and ErbB3. Using immunoprecipitation assays, we showed that the neuregulin-1beta1 (NRG1beta1) stimulus induced ErbB4 tyrosine phosphorylation and phosphatidylinositol 3-kinase (PI3K) recruitment and activation (as demonstrated by Akt phosphorylation) either directly (ErbB4 cyt1 isoform) or indirectly (ErbB4 cyt2 isoform). We examined the ability of the four ErbB4 isoforms to induce chemotaxis and cell proliferation in response to NRG1beta1 stimulation. Using migration assays, we observed that only ErbB4-expressing cells stimulated with NRG1beta1 showed a significant increase in migration, whereas the growth rate remained unchanged. Additional assays showed that inhibition of PI3K (but not of phospholipase Cgamma) dramatically reduced migratory activity. Our data show that ErbB4 signaling via PI3K activation plays a fundamental role in controlling NRG1beta1-induced migration.

The leucine-rich repeat protein LRIG1 is a negative regulator of ErbB family receptor tyrosine kinases

The molecular mechanisms by which mammalian receptor tyrosine kinases are negatively regulated remain largely unexplored. Previous genetic and biochemical studies indicate that Kekkon-1, a transmembrane protein containing leucine-rich repeats and an immunoglobulin-like domain in its extracellular region, acts as a feedback negative regulator of epidermal growth factor (EGF) receptor signaling in Drosophila melanogaster development. Here we tested whether the related human LRIG1 (also called Lig-1) protein can act as a negative regulator of EGF receptor and its relatives, ErbB2, ErbB3, and ErbB4. We observed that in co-transfected 293T cells, LRIG1 forms a complex with each of the ErbB receptors independent of growth factor binding. We further observed that co-expression of LRIG1 with EGF receptor suppresses cellular receptor levels, shortens receptor half-life, and enhances ligand-stimulated receptor ubiquitination. Finally, we observed that co-expression of LRIG1 suppresses EGF-stimulated transformation of NIH3T3 fibroblasts and that the inducible expression of LRIG1 in PC3 prostate tumor cells suppresses EGF- and neuregulin-1-stimulated cell cycle progression. Our observations indicate that LRIG1 is a negative regulator of the ErbB family of receptor tyrosine kinases and suggest that LRIG1-mediated receptor ubiquitination and degradation may contribute to the suppression of ErbB receptor function.

ErbB3/HER3 does not homodimerize upon neuregulin binding at the cell surface

To understand signaling by the neuregulin (NRG) receptor ErbB3/HER3, it is important to know whether ErbB3 forms homodimers upon ligand binding. Previous biophysical studies suggest that the ErbB3 extracellular region remains monomeric when bound to NRG. We used a chimeric receptor approach to address this question in living cells, fusing the extracellular region of ErbB3 to the kinase-active intracellular domain of ErbB1. The ErbB3/ErbB1 chimera responded to NRG only if ErbB2 was co-expressed in the same cells, whereas an ErbB4/ErbB1 chimera responded without ErbB2. We, therefore, suggest that ErbB3 is an obligate heterodimerization partner because of its inability to homodimerize.

Truncated ErbB2 receptor (p95ErbB2) is regulated by heregulin through heterodimer formation with ErbB3 yet remains sensitive to the dual EGFR/ErbB2 kinase inhibitor GW572016

The expression of the NH2 terminally truncated ErbB2 receptor (p95ErbB2) in breast cancer correlates with metastatic disease progression compared with the expression of full-length p185ErbB2. We now show that heregulin (HRG), but not EGF, stimulates p95ErbB2 phosphorylation in BT474 breast cancer cells. Furthermore, phospho-p95ErbB2 forms heterodimers with ErbB3, but not EGFR, while p185ErbB2 heterodimerizes with both EGFR and ErbB3. The predilection of p95ErbB2 to heterodimerize with ErbB3 provides an explanation for its regulation by HRG, an ErbB3 ligand. GW572016, a reversible small molecule inhibitor of EGFR and ErbB2 tyrosine kinases, inhibits baseline p95ErbB2 phosphorylation in BT474 cells and tumor xenografts. Inhibition of p95ErbB2, p185ErbB2, and EGFR phosphorylation by GW572016 resulted in the inhibition of downstream phospho-Erk1/2, phospho-AKT, and cyclin D steady-state protein levels. Increased phosphorylation of p95ErbB2 and AKT in response to HRG was abrogated to varying degrees by GW572016. In contrast, trastuzumab did not inhibit p95ErbB2 phosphorylation or the expression of downstream phospho-Erk1/2, phospho-AKT, or cyclin D. It is tempting to speculate that trastuzumab resistance may be mediated in part by the selection of p95ErbB2-expressing breast cancer cells capable of exerting potent growth and prosurvival signals through p95ErbB2-ErbB3 heterodimers. Thus, p95ErbB2 represents a target for therapeutic intervention, and one that is sensitive to GW572016 therapy.

Update on HER-kinase-directed therapy in prostate cancer

The human epidermal growth factor receptor (HER) family of receptor tyrosine kinases is part of a network of pathways that are involved in the development and progression of prostate cancer. HER-kinase receptors include epidermal growth factor receptor (EGFR), HER2, HER3, and HER4, which must combine as dimers to affect signaling. Different combinations of receptors produce different qualities and levels of pathway activation. Among HER-family receptors, HER2 activation is particularly important in breast cancer, as HER2 gene amplification is associated with a distinct clinical course and response to treatment with a HER2-directed therapy (trastuzumab). Although HER2 can be over-expressed in prostate cancer, there is no clinical data to support the use of trastuzumab for prostate cancer patients. Preclinical and clinical data show that the activation of the HER-kinase axis is important for the progression of prostate cancer to androgen-independent disease. Data points towards the importance of inhibiting multiple members of the HER-kinase family to achieve more complete blockade of this axis for cancers other than HER2-overexpressing breast cancer. Multiple pharmaceutical agents that block the HER-kinase axis are currently being tested for patients with prostate cancer. These include antibodies, tyrosine kinase inhibitors, and novel strategies which seek to decrease HER2 expression.

Apigenin induces apoptosis through proteasomal degradation of HER2/neu in HER2/neu-overexpressing breast cancer cells via the phosphatidylinositol 3-kinase/Akt-dependent pathway

Apigenin is a low toxicity and non-mutagenic phytopolyphenol and protein kinase inhibitor. It exhibits anti-proliferating effects on human breast cancer cells. Here we examined several human breast cancer cell lines having different levels of HER2/neu expression and found that apigenin exhibited potent growth-inhibitory activity in HER2/neu-overexpressing breast cancer cells but was much less effective for those cells expressing basal levels of HER2/neu. Induction of apoptosis was also observed in HER2/neu-overexpressing breast cancer cells in a dose- and time-dependent manner. However, the one or more molecular mechanisms of apigenin-induced apoptosis in HER2/neu-overexpressing breast cancer cells remained to be elucidated. A cell survival pathway involving phosphatidylinositol 3-kinase (PI3K), and Akt is known to play an important role in inhibiting apoptosis in response to HER2/neu-overexpressing breast cancer cells, which prompted us to investigate whether this pathway plays a role in apigenin-induced apoptosis in HER2/neu-overexpressing breast cancer cells. Our results showed that apigenin inhibits Akt function in tumor cells in a complex manner. First, apigenin directly inhibited the PI3K activity while indirectly inhibiting the Akt kinase activity. Second, inhibition of HER2/neu autophosphorylation and transphosphorylation resulting from depleting HER2/neu protein in vivo was also observed. In addition, apigenin inhibited Akt kinase activity by preventing the docking of PI3K to HER2/HER3 heterodimers. Therefore, we proposed that apigenin-induced cellular effects result from loss of HER2/neu and HER3 expression with subsequent inactivation of PI3K and AKT in cells that are dependent on this pathway for cell proliferation and inhibition of apoptosis. This implies that the inhibition of the HER2/HER3 heterodimer function provided an especially effective strategy for blocking the HER2/neu-mediated transformation of breast cancer cells. Our results also demonstrated that apigenin dissociated the complex of HER2/neu and GRP94 that preceded the depletion of HER2/neu. Apigenin-induced degradation of mature HER2/neu involves polyubiquitination of HER2/neu and subsequent hydrolysis by the proteasome.

Cyclin-dependent kinase-5 is involved in neuregulin-dependent activation of phosphatidylinositol 3-kinase and Akt activity mediating neuronal survival

The phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway plays an important role in mediating survival signals in wide variety of neurons and cells. Recent studies show that Akt also regulates metabolic pathways to regulate cell survival. In this study, we reported that cyclin-dependent kinase-5 (Cdk5) regulates Akt activity and cell survival through the neuregulin-mediated PI 3-kinase signaling pathway. We found that brain extracts of Cdk5-/-mice display a lower PI 3-kinase activity and phosphorylation of Akt compared with that in wild type mice. Moreover, we demonstrated that Cdk5 phosphorylated Ser-1176 in the neuregulin receptor ErbB2 and phosphorylated Thr-871 and Ser-1120 in the ErbB3 receptor. We identified the Ser-1120 sequence RSRSPR in ErbB3 as a novel phosphorylation consensus sequence of Cdk5. Finally, we found that Cdk5 activity is involved in neuregulin-induced Akt activity and neuregulin-mediated neuronal survival. These findings suggest that Cdk5 may exert a key role in promoting neuronal survival by regulating Akt activity through the neuregulin/PI 3-kinase signaling pathway.

Inhibition of heregulin signaling by an aptamer that preferentially binds to the oligomeric form of human epidermal growth factor receptor-3

Human epidermal growth factor receptor-3 (HER3) is a member of the type I receptor tyrosine kinase family. Several members of this family are overexpressed in various carcinomas. Specifically, HER2 is found to be overexpressed in 20-30% of breast cancers. In contrast to epidermal growth factor receptor or HER2, the kinasedeficient HER3 self-associates readily at low nanomolar concentrations and in the absence of its ligands, various isoforms of heregulin (hrg). Binding of hrg disrupts HER3 oligomerization and leads to the formation of signaling-competent heterodimers, preferentially with HER2. Elevated levels of HER3 contribute to increased drug resistance observed in HER2-overexpressing cells. We have used the SELEX (systematic evolution of ligands by exponential enrichment) methodology to select RNA aptamers against the oligomeric state of the extracellular domains of HER3 (HER3ECD, monomeric molecular mass 82,000 Da). One of the aptamers, A30, binds with high affinity to a limited number of binding sites in the oligomeric state of HER3ECD. Binding of A30 and hrg are not competitive. Instead, the disruption of HER3 oligomers by hrg results in an approximately 10-fold increase in total binding sites, but the newly created binding sites are of lower affinity. High-affinity binding of A30 inhibits hrg-dependent tyrosine phosphorylation of HER2 and the hrg-induced growth response of MCF7 cells. As an example of an aptamer against a large macromolecular protein complex, A30 can serve as a tool for the analysis of receptor interactions and may serve as a lead compound for the development of inhibitors against overexpressed receptor tyrosine kinases in carcinomas.

The ErbB2/ErbB3 heterodimer functions as an oncogenic unit: ErbB2 requires ErbB3 to drive breast tumor cell proliferation

ErbB2 is a receptor tyrosine kinase whose activity in normal cells depends on dimerization with another ligand-binding ErbB receptor. In contrast, amplification of c-erbB2 in tumors results in dramatic overexpression and constitutive activation of the receptor. Breast cancer cells overexpressing ErbB2 depend on its activity for proliferation, because treatment of these cells with ErbB2-specific antagonistic antibodies or kinase inhibitors blocks tumor cells in the G1 phase of the cell cycle. Intriguingly, loss of ErbB2 signaling is accompanied by a decrease in the phosphotyrosine content of ErbB3. On the basis of these results, it has been proposed that ErbB3 might be a partner for ErbB2 in promoting cellular transformation. To test this hypothesis and directly examine the role of the "kinase dead" ErbB3, we specifically ablated its expression with a designer transcription factor (E3). By infection of ErbB2-overexpressing breast cancer cells with a retrovirus expressing E3, we show that ErbB3 is an essential partner in the transformation process. Loss of functional ErbB2 or ErbB3 has similar effects on cell proliferation and cell cycle regulators. Furthermore, expression of constitutively active protein kinase B rescues the proliferative block induced as a consequence of loss of ErbB2 or ErbB3 signaling. These results demonstrate that ErbB2 overexpression and activity alone are insufficient to promote breast tumor cell division. Furthermore, we identify ErbB3's role, which is to couple active ErbB2 to the phosphatidylinositol 3-kinase/protein kinase B pathway. Thus, the ErbB2/ErbB3 dimer functions as an oncogenic unit to drive breast tumor cell proliferation.

Identification of novel ErbB3-interacting factors using the split-ubiquitin membrane yeast two-hybrid system

Analysis of membrane protein interactions is difficult because of the hydrophobic nature of these proteins, which often renders conventional biochemical and genetic assays fruitless. This is a substantial problem because proteins that are integral or associated with membranes represent approximately one-third of all proteins in a typical eukaryotic cell. We have shown previously that the modified split-ubiquitin system can be used as a genetic assay for the in vivo detection of interactions between the two characterized yeast transmembrane proteins, Ost1p and Wbp1p. This so-called split-ubiquitin membrane yeast two-hybrid (YTH) system uses the split-ubiquitin approach in which reconstitution of two ubiquitin halves is mediated by a protein-protein interaction. Here we converted the split-ubiquitin membrane YTH system into a generally applicable in vivo screening approach to identify interacting partners of a particular mammalian transmembrane protein. We have demonstrated the effectiveness of this approach by using the mammalian ErbB3 receptor as bait and have identified three previously unknown ErbB3-interacting proteins. In addition, we have confirmed one of the newly found interactions between ErbB3 and the membrane-associated RGS4 protein by coimmunoprecipitating the two proteins from human cells. We expect the split-ubiquitin membrane YTH technology to be valuable for the identification of potential interacting partners of integral membrane proteins from many model organisms.

Selective formation of ErbB-2/ErbB-3 heterodimers depends on the ErbB-3 affinity of epidermal growth factor-like ligands

EGF-like growth factors activate their ErbB receptors by promoting receptor-mediated homodimerization or, alternatively, by the formation of heterodimers with the orphan ErbB-2 through an as yet unknown mechanism. To investigate the selectivity in dimer formation by ligands, we have applied the phage display approach to obtain ligands with modified C-terminal residues that discriminate between ErbB-2 and ErbB-3 as dimerization partners. We used the epidermal growth factor/transforming growth factor alpha chimera T1E as the template molecule because it binds to ErbB-3 homodimers with low affinity and to ErbB-2/ErbB-3 heterodimers with high affinity. Many phage variants were selected with enhanced binding affinity for ErbB-3 homodimers, indicating that C-terminal residues contribute to the interaction with ErbB-3. These variants were also potent ligands for ErbB-2/ErbB-3 heterodimers despite negative selection for such heterodimers. In contrast, phage variants positively selected for binding to ErbB-2/ErbB-3 heterodimers but negatively selected for binding to ErbB-3 homodimers can be considered as "second best" ErbB-3 binders, which require ErbB-2 heterodimerization for stable complex formation. Our findings imply that epidermal growth factor-like ligands bind ErbB-3 through a multi-domain interaction involving at least both linear endings of the ligand. Apparently the ErbB-3 affinity of a ligand determines whether it can form only ErbB-2/ErbB-3 complexes or also ErbB-3 homodimers. Because no separate binding domain for ErbB-2 could be identified, our data support a model in which ErbB heterodimerization occurs through a receptor-mediated mechanism and not through bivalent ligands.

The crystal structure of a truncated ErbB2 ectodomain reveals an active conformation, poised to interact with other ErbB receptors

ErbB2 does not bind ligand, yet appears to be the major signaling partner for other ErbB receptors by forming heteromeric complexes with ErbB1, ErbB3, or ErbB4. The crystal structure of residues 1-509 of ErbB2 at 2.5 A resolution reveals an activated conformation similar to that of the EGFR when complexed with ligand and very different from that seen in the unactivated forms of ErbB3 or EGFR. The structure explains the inability of ErbB2 to bind known ligands and suggests why ErbB2 fails to form homodimers. Together, the data suggest a model in which ErbB2 is already in the activated conformation and ready to interact with other ligand-activated ErbB receptors.

Differential regulation of tumor angiogenesis by distinct ErbB homo- and heterodimers

Interactions between cancer cells and their microenvironment are critical for the development and progression of solid tumors. This study is the first to examine the role of all members of the ErbB tyrosine kinase receptors (epidermal growth factor receptor [EGFR], ErbB-2, ErbB-3, or ErbB-4), expressed singly or as paired receptor combinations, in the regulation of angiogenesis both in vitro and in vivo. Comparison of all receptor combinations reveals that EGFR/ErbB-2 and ErbB-2/ErbB-3 heterodimers are the most potent inducers of vascular endothelial growth factor (VEGF) mRNA expression compared with EGFR/ErbB-3, EGFR/ErbB-4, ErbB-2/ErbB-4, and ErbB-3/ErbB-4. Immunohistochemistry of tumor xenografts overexpressing these heterodimers shows increased VEGF expression and remarkably enhanced vascularity. Enhanced VEGF expression is associated with increased VEGF transcription. Deletional analysis reveals that ErbB-mediated transcriptional up-regulation of VEGF involves a hypoxia-inducible factor 1-independent responsive region located between nucleotides -88 to -66 of the VEGF promoter. Mutational analysis reveals that the Sp-1 and AP-2 transcription factor binding elements within this region are required for up-regulation of VEGF by heregulin beta1 and that this up-regulation is dependent on the activity of extracellular signal-related protein kinases. These results emphasize the biological implications of cell signaling diversity among members of the ErbB receptor family in regulation of the tumor microenvironment.

ErbB receptor tyrosine kinase inhibitors as therapeutic agents

The ErbB family of receptor tyrosine kinases comprise four members: EGFR, ErbB2, ErbB3 and ErbB4. All are essential for normal development and participate in the functioning of normal cells. ErbB receptors, particularly EGFR and ErbB2 are commonly deregulated in certain prevalent forms of human cancer. Recently a number of small molecule inhibitors of the tyrosine kinase activity of these receptors have been developed. Some of these agents, known as TKIs, are progressing through clinical trials in patients with aberrant ErbB receptor expression in their tumors. This article provides a brief overview on the structure and biology of ErbB receptors and their ligands before discussing in detail the development and current status of ErbB receptor TKIs. These agents are shown to inhibit multiple features of cancer cells including proliferation, survival, invasion and angiogenesis. It is clear from recent studies that not all cancer cells that overexpress ErbB receptors will be sensitive to TKIs. Potential explanations for resistance to these molecules are reviewed. Finally the prospect of using TKIs in combination with existing chemotherapeutic agents is discussed.

Structure of the extracellular region of HER3 reveals an interdomain tether

We have determined the 2.6 angstrom crystal structure of the entire extracellular region of human HER3 (ErbB3), a member of the epidermal growth factor receptor (EGFR) family. The structure consists of four domains with structural homology to domains found in the type I insulin-like growth factor receptor. The HER3 structure reveals a contact between domains II and IV that constrains the relative orientations of ligand-binding domains and provides a structural basis for understanding both multiple-affinity forms of EGFRs and conformational changes induced in the receptor by ligand binding during signaling. These results also suggest new therapeutic approaches to modulating the behavior of members of the EGFR family.

Disabling receptor ensembles with rationally designed interface peptidomimetics

Members of the erbB family receptor tyrosine kinases (erbB1, erbB2, erbB3, and erbB4) are overexpressed in a variety of human cancers and represent important targets for the structure-based drug design. Homo- and heterodimerization (oligomerization) of the erbB receptors are known to be critical events for receptor signaling. To block receptor self-associations, we have designed a series of peptides derived from potential dimerization surfaces in the extracellular subdomain IV of the erbB receptors (erbB peptides). In surface plasmon resonance (BIAcore) studies, the designed peptides have been shown to selectively bind to the erbB receptor ectodomains and isolated subdomain IV of erbB2 with submicromolar affinities and to inhibit heregulin-induced interactions of erbB3 with different erbB receptors. A dose-dependent inhibition of native erbB receptor dimerization by the erbB peptides has been observed in 32D cell lines transfected with different combinations of erbB receptors. The peptides effectively inhibited growth of two types of transformed cells overexpressing different erbB receptors, T6-17 and 32D, in standard MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) and cell viability assays. The study identifies distinct loops within the membrane-proximal part of the subdomain IV as potential receptor-receptor interaction sites for the erbB receptors and demonstrates the possibility of disabling receptor activity by structure-based targeting of the dimerization interfaces. Molecular models for possible arrangement of the erbB1.EGF complex, consistent with the involvement of subdomain IV in inter-receptor interactions, are proposed. Small dimerization inhibitors described herein can be useful as probes to elucidate different erbB signaling pathways and may be developed as therapeutic agents.

Role of the N-terminus of epidermal growth factor in ErbB-2/ErbB-3 binding studied by phage display

Epidermal growth factor (EGF) binds with high affinity to the EGF receptor, also known as ErbB-1, but upon replacement of the N-terminal linear region by neuregulin (NRG) 1 or transforming growth factor (TGF) alpha sequences it gains in addition high affinity for ErbB-2/ErbB-3 heterodimers. However, these chimeras weakly bind to ErbB-3 alone. To further dissect the ligand binding selectivity of the ErbB network, we have applied the phage display technique to examine the role of the linear N-terminal region in EGF for interaction with ErbB-2/ErbB-3 heterodimers. A library of EGF variants was constructed in which residues 2, 3, and 4 were randomly mutated, followed by selection for binding to intact MDA-MB-453 cells that overexpress ErbB-2 and ErbB-3 but lack ErbB-1. Analysis of the selected phage EGF variants revealed clones with high binding affinity to ErbB-2/ErbB-3 while maintaining high affinity to ErbB-1. In these variants, Trp (or alternatively His) was almost exclusively present at position 2, while specific combinations of hydrophobic, basic, and small residues were found at positions 3 and 4. The mitogenic activity of the phage EGF variants corresponded with their relative binding affinity. Two of the selected EGF variants, EGF/WVS and EGF/WRS, were further characterized as recombinant proteins. In contrast to previously characterized chimeras of EGF with NRG-1 or TGF-alpha, these variants did not only show high binding affinity for ErbB-2/ErbB-3 heterodimers but also for ErbB-3 alone. These data show that the linear N-terminal region of EGF-like growth factors is directly involved in binding to ErbB-3.

c-erbB-3: a nuclear protein in mammary epithelial cells

c-erbB receptors are usually located in cell membranes and are activated by extracellular binding of EGF-like growth factors. Unexpectedly, using immunofluorescence we found high levels of c-erbB-3 within the nuclei of MTSV1-7 immortalized nonmalignant human mammary epithelial cells. Nuclear localization was mediated by the COOH terminus of c-erbB-3, and a nuclear localization signal was identified by site-directed mutagenesis and by transfer of the signal to chicken pyruvate kinase. A nuclear export inhibitor caused accumulation of c-erbB-3 in the nuclei of other mammary epithelial cell lines as demonstrated by immunofluorescence and biochemical cell fractionation, suggesting that c-erbB-3 shuttles between nuclear and nonnuclear compartments in these cells. Growth of MTSV1-7 on permeable filters induced epithelial polarity and concentration of c-erbB-3 within the nucleoli. However, the c-erbB-3 ligand heregulin beta1 shifted c-erbB-3 from the nucleolus into the nucleoplasm and then into the cytoplasm. The subcellular localization of c-erbB-3 obviously depends on exogenous stimuli and on the stage of epithelial polarity and challenges the specific function of c-erbB-3 as a transmembrane receptor protein arguing for additional, as yet unidentified, roles of c-erbB-3 within the nucle(ol)us of mammary epithelial cells.