Disabling receptor ensembles with rationally designed interface peptidomimetics

Peptides Targeting HER2-Positive Breast Cancer Cells and Applications in Tumor Imaging and Delivery of Chemotherapeutics

Breast cancer represents the most common cancer type and one of the major leading causes of death in the female worldwide population. Overexpression of HER2, a transmembrane glycoprotein related to the epidermal growth factor receptor, results in a biologically and clinically aggressive breast cancer subtype. It is also the primary driver for tumor detection and progression and, in addition to being an important prognostic factor in women diagnosed with breast cancer, HER2 is a widely known therapeutic target for drug development. The aim of this review is to provide an updated overview of the main approaches for the diagnosis and treatment of HER2-positive breast cancer proposed in the literature over the past decade. We focused on the different targeting strategies involving antibodies and peptides that have been explored with their relative outcomes and current limitations that need to be improved. The review also encompasses a discussion on targeted peptides acting as probes for molecular imaging. By using different types of HER2-targeting strategies, nanotechnology promises to overcome some of the current clinical challenges by developing novel HER2-guided nanosystems suitable as powerful tools in breast cancer imaging, targeting, and therapy.

Development of a Novel Anti-HER2 Monoclonal Antibody H2Mab-181 for Gastric Cancer

Human epidermal growth factor receptor 2 (HER2) is a type I transmembrane 185 kDa protein. HER2 is expressed in a variety of normal tissue types and cancer cells. HER2 is associated with cell proliferation, differentiation, and migration. The overexpression of HER2 has been observed in a number of cancers, including breast and gastric cancers. Gastric cancer is one of the most common cancers worldwide, with an annual case rate of ∼1 million people diagnosed with the disease. Trastuzumab is a humanized anti-HER2 monoclonal antibody (mAb) that has been utilized in gastric cancer therapy. In this study, we have developed a novel anti-HER2 mAb, H₂Mab-181 (IgG₁, kappa), through the immunization of mice with a purified recombinant extracellular domain of HER2. H₂Mab-181 can specifically and sensitively detect HER2 in both flow cytometry and Western blot applications in gastric cancer cell lines and can also be utilized in immunohistochemical analyses of gastric cancer tissues. Together, H₂Mab-181 could be useful for the diagnosis and therapy in gastric cancers.

Rational Design of Constrained Peptides as Protein Interface Inhibitors

The lack of progress in developing targeted therapeutics directed at protein–protein complexes has been due to the absence of well-defined ligand-binding pockets and the extensive intermolecular contacts at the protein–protein interface. Our laboratory has developed approaches to dissect protein–protein complexes focusing on the superfamilies of erbB and tumor necrosis factor (TNF) receptors by the combined use of structural biology and computational biology to facilitate small molecule development. We present a perspective on the development and application of peptide inhibitors as well as immunoadhesins to cell surface receptors performed in our laboratory.

Molecular Targeting of Epidermal Growth Factor Receptor (EGFR) and Vascular Endothelial Growth Factor Receptor (VEGFR)

Epidermal growth factor receptor (EGFR) and vascular endothelial growth factor receptor (VEGFR) are two extensively studied membrane-bound receptor tyrosine kinase proteins that are frequently overexpressed in many cancers. As a result, these receptor families constitute attractive targets for imaging and therapeutic applications in the detection and treatment of cancer. This review explores the dynamic structure and structure-function relationships of these two growth factor receptors and their significance as it relates to theranostics of cancer, followed by some of the common inhibition modalities frequently employed to target EGFR and VEGFR, such as tyrosine kinase inhibitors (TKIs), antibodies, nanobodies, and peptides. A summary of the recent advances in molecular imaging techniques, including positron emission tomography (PET), single-photon emission computerized tomography (SPECT), computed tomography (CT), magnetic resonance imaging (MRI), and optical imaging (OI), and in particular, near-IR fluorescence imaging using tetrapyrrolic-based fluorophores, concludes this review.

Neuroprotection: Pro-survival and Anti-neurotoxic Mechanisms as Therapeutic Strategies in Neurodegeneration

Neurotrophins (NTs) are a subset of the neurotrophic factor family. These growth factors were originally named based on the nerve growth functional assays used to identify them. NTs act as paracrine or autocrine factors for cells expressing NT receptors. The receptors and their function have been studied primarily in cells of the nervous system, but are also present in the cardiovascular, endocrine, and immune systems, as well as in many neoplastic cells. The signals activated by NTs can be varied, depending on cellular stage and context, healthy or disease states, and depending on whether the specific NTs and their receptors are expressed in the relevant cells. In the healthy central and peripheral adult nervous systems, NTs drive neuronal survival, phenotype, synaptic maintenance, and function. Deficiencies of the NT/NT receptor axis are causally associated with disease onset or disease progression. Paradoxically, NTs can also drive synaptic loss and neuronal death. In the embryonic stage this activity is essential for proper developmental pruning of the nervous system, but in the adult it can be associated with neurodegenerative disease. Given their key role in neuronal survival and death, NTs and NT receptors have long been considered therapeutic targets to achieve neuroprotection. The first neuroprotective approaches consisted of enhancing neuronal survival signals using NTs. Later strategies selectively targeted receptors to induce survival signals specifically, while avoiding activation of death signals. Recently, the concept of selectively targeting receptors to reduce neuronal death signals has emerged. Here, we review the rationale of each neuroprotective strategy with respect to the complex cell biology and pharmacology of each target receptor.

Activation of the EGF Receptor by Ligand Binding and Oncogenic Mutations: The "Rotation Model"

The epidermal growth factor receptor (EGFR) plays vital roles in cellular processes including cell proliferation, survival, motility, and differentiation. The dysregulated activation of the receptor is often implicated in human cancers. EGFR is synthesized as a single-pass transmembrane protein, which consists of an extracellular ligand-binding domain and an intracellular kinase domain separated by a single transmembrane domain. The receptor is activated by a variety of polypeptide ligands such as epidermal growth factor and transforming growth factor α. It has long been thought that EGFR is activated by ligand-induced dimerization of the receptor monomer, which brings intracellular kinase domains into close proximity for trans-autophosphorylation. An increasing number of diverse studies, however, demonstrate that EGFR is present as a pre-formed, yet inactive, dimer prior to ligand binding. Furthermore, recent progress in structural studies has provided insight into conformational changes during the activation of a pre-formed EGFR dimer. Upon ligand binding to the extracellular domain of EGFR, its transmembrane domains rotate or twist parallel to the plane of the cell membrane, resulting in the reorientation of the intracellular kinase domain dimer from a symmetric inactive configuration to an asymmetric active form (the “rotation model”). This model is also able to explain how oncogenic mutations activate the receptor in the absence of the ligand, without assuming that the mutations induce receptor dimerization. In this review, we discuss the mechanisms underlying the ligand-induced activation of the preformed EGFR dimer, as well as how oncogenic mutations constitutively activate the receptor dimer, based on the rotation model.

Preparation and Identification of HER2 Radioactive Ligands and Imaging Study of Breast Cancer-Bearing Nude Mice

OBJECTIVE: A micro-molecule peptide TP1623 of ^99mTc-human epithelial growth factor receptor 2 (HER2) was prepared and the feasibility of using it as a HER2-positive molecular imaging agent for breast cancer was evaluated. METHODS: TP1623 was chemically synthesized and labeled with ^99mTc. The labeling ratio and stability were detected. HER2 expression levels of breast cancer cells (SKBR3 and MDA-MB-231) and cell binding activity were measured. Biodistribution of ^99mTC-TP1623 in normal mice was detected. SKBR3/MDA-MB-231-bearing nude mice models with high/low expressions of HER2 were established. Tumor tissues were stained with hematoxylin–eosin (HE) and measured by immunohistochemistry to confirm the formation of tumors and HER2 expression. SPECT imaging was conducted for HER2-overexpressing SKBR3-bearing nude mice. The T/NT ratio was calculated and compared with that of MDA-MB-231-bearing nude mice with low HER2 expression. The competitive inhibition image was used to discuss the specific binding of ^99mTc- TP1623 and the tumor. RESULTS: The labeling ratio of ^99mTc-TP1623, specific activity, and radiochemical purity (RCP) after 6 h at room temperature were (97.39 ± 0.23)%, (24.61 ± 0.06) TBq/mmol, and (93.25 ± 0.06)%, respectively. HER2 of SKBR3 and MDA-MB-231 cells showed high and low expression levels by immunohistochemistry, respectively. The in vitro receptor assays indicated that specific binding of TP1623 and HER2 was retained. Radioactivity in the brain was always at the lowest level, while the clearance rate of blood and the excretion rate of the kidneys were fast. HE staining showed that tumor cells were observed in SKBR3- and MDA-MB-231-bearing nude mice, with significant heteromorphism and increased mitotic count. The imaging of mice showed that targeted images could be made of ^99mTc-TP1623 in high HER2-expressing tumors, while no obvious development was shown in tumors in low HER2-expressing nude mice. No development was visible in tumors in competitive inhibition of imaging, which indicates the combination of ^99mTc-TP1623 and tumor was mediated by HER2. CONCLUSION: High labeling ratio and specific activity of ^99mTc-TP1623 is successfully prepared; it is a molecular imaging agent for HER2-positive tumors that has potential applicative value.

Improved drug delivery and therapeutic efficacy of PEgylated liposomal doxorubicin by targeting anti-HER2 peptide in murine breast tumor model

Targeted cancer therapy is a powerful therapeutic strategy to management of cancer. HER2 as an anticancer target has long been studied. Its overexpression plays an important role in the pathogenesis and progressiveness of breast and other cancers. To establish efficient and reliable drug delivery to HER2-overexpressing cells, the authors of this study have developed anti-HER2 (ErbB2) peptide-liposomal formulations of doxorubicin (DOX) by an engineered breast tumor-targeting peptide ligand, AHNP, Anti-HER2/neu peptide, (FCDGFYACYADV) with three glycine amino acids as spacer before its original sequencing. Towards this goal, PEGylated liposome doxorubicin (PLD) bearing different ligand densities of AHNP was prepared and characterized for their size, zeta potential and peptide conjugation. The AHNP functionalization and density effects on breast tumor cell uptake, selective cytotoxicity, prevention of tumor growth and the tissue biodistribution of encapsulated DOX were studied in mice bearing TUBO breast cancer tumor model. The findings demonstrated that increasing the ligand density of AHNP increases cytotoxicity and cell-uptake in SKBR3 and TUBO cells which overexpress HER2 but not in MDA-MB-231with low HER2 expression profile. The anticancer activity was also superior for targeted liposomal DOX with more AHNP densities. Overall, the results showed that optimum AHNP density functionalization of PLD can significantly improve selectivity and the therapeutic index of liposomal DOX in the treatment of HER2 positive breast cancer and merits further investigation.

Blockade of a key region in the extracellular domain inhibits HER2 dimerization and signaling

BACKGROUND

Several treatment strategies target the human epidermal growth factor receptor 2 (HER2) in breast carcinomas, including monoclonal antibodies directed against HER2's extracellular domain (ECD) and small molecule inhibitors of its tyrosine kinase activity. Yet, novel therapies are needed that prevent HER2 dimerization with other HER family members, because current treatments are only partially effective.

METHODS

To test the hypothesis that HER2 activation requires a protein sequence in the HER2-ECD that mediates HER2 homo- and heterodimerization, we introduced a series of deletion mutations in the third subdomain of HER2-ECD. These deletion mutants were retrovirally expressed in breast cancer (BC) cells that naturally overexpress HER2 and in noncancerous, HER2-negative breast epithelial cells. One-factor analysis of variance or Student's t test were used to analyze differences. All statistical tests were two-sided.

RESULTS

The smallest deletion in the ECD domain of HER2, which removed only 16 amino acids (HER2-ECDΔ451-466), completely disrupted the oncogenic potential of HER2. In contrast to wild-type HER2, the mutant-inhibited anchorage-independent growth (mean number of colonies: mutant, 70, 95% confidence interval [CI] = 55 to 85; wild-type, 400, 95% CI = 320 to 480, P < .001) increased sensitivity to paclitaxel treatment in both transformed and nontransformed cells. Overexpression of HER2Δ451-466 efficiently inhibited activation of HER1, HER2, and HER3 in all cell lines tested.

CONCLUSIONS

These findings reveal that an essential "activating" sequence exists in the extracellular domain of HER2. Disruption of this sequence disables the HER2 dimerization loop, blocks subsequent activation of HER2-driven oncogenic signaling, and generates a dominant-negative form of HER2. Reagents specifically against this molecular activation switch may represent a novel targeted approach for the management of HER2-overexpressing carcinomas.

Novel Peptidomimetics for Inhibition of HER2:HER3 Heterodimerization in HER2-Positive Breast Cancer

The current approach to treating HER2-overexpressed breast cancer is the use of monoclonal antibodies or a combination of antibodies with traditional chemotherapeutic agents or kinase inhibitors. Our approach is to target clinically validated HER2 domain IV with peptidomimetics and inhibit the protein-protein interactions (PPI) of HERs. Unlike antibodies, peptidomimetics have advantages in terms of stability, modification, and molecular size. We have designed peptidomimetics (compounds 5 and 9) that bind to HER2 domain IV, inhibit protein-protein interactions, and decrease cell viability in breast cancer cells with HER2 overexpression. We have shown, using enzyme fragment complementation and proximity ligation assays, that peptidomimetics inhibit the PPI of HER2:HER3. Compounds 5 and 9 suppressed the tumor growth in a xenograft mouse model. Furthermore, we have shown that these compounds inhibit PPI of HER2:HER3 and phosphorylation of HER2 as compared to control in tissue samples derived from in vivo studies. The stability of the compounds was also investigated in mouse serum, and the compounds exhibited stability with a half-life of up to 3 h. These results suggest that the novel peptidomimetics we have developed target the extracellular domain of HER2 protein and inhibit HER2:HER3 interaction, providing a novel method to treat HER2-positive cancer.

Comparative analysis of evolutionarily conserved motifs of epidermal growth factor receptor 2 (HER2) predicts novel potential therapeutic epitopes

Overexpression of human epidermal growth factor receptor 2 (HER2) is associated with tumor aggressiveness and poor prognosis in breast cancer. With the availability of therapeutic antibodies against HER2, great strides have been made in the clinical management of HER2 overexpressing breast cancer. However, de novo and acquired resistance to these antibodies presents a serious limitation to successful HER2 targeting treatment. The identification of novel epitopes of HER2 that can be used for functional/region-specific blockade could represent a central step in the development of new clinically relevant anti-HER2 antibodies. In the present study, we present a novel computational approach as an auxiliary tool for identification of novel HER2 epitopes. We hypothesized that the structurally and linearly evolutionarily conserved motifs of the extracellular domain of HER2 (ECD HER2) contain potential druggable epitopes/targets. We employed the PROSITE Scan to detect structurally conserved motifs and PRINTS to search for linearly conserved motifs of ECD HER2. We found that the epitopes recognized by trastuzumab and pertuzumab are located in the predicted conserved motifs of ECD HER2, supporting our initial hypothesis. Considering that structurally and linearly conserved motifs can provide functional specific configurations, we propose that by comparing the two types of conserved motifs, additional druggable epitopes/targets in the ECD HER2 protein can be identified, which can be further modified for potential therapeutic application. Thus, this novel computational process for predicting or searching for potential epitopes or key target sites may contribute to epitope-based vaccine and function-selected drug design, especially when x-ray crystal structure protein data is not available.

Mechanisms of activation of receptor tyrosine kinases: monomers or dimers

Receptor tyrosine kinases (RTKs) play essential roles in cellular processes, including metabolism, cell-cycle control, survival, proliferation, motility and differentiation. RTKs are all synthesized as single-pass transmembrane proteins and bind polypeptide ligands, mainly growth factors. It has long been thought that all RTKs, except for the insulin receptor (IR) family, are activated by ligand-induced dimerization of the receptors. An increasing number of diverse studies, however, indicate that RTKs, previously thought to exist as monomers, are present as pre-formed, yet inactive, dimers prior to ligand binding. The non-covalently associated dimeric structures are reminiscent of those of the IR family, which has a disulfide-linked dimeric structure. Furthermore, recent progress in structural studies has provided insight into the underpinnings of conformational changes during the activation of RTKs. In this review, I discuss two mutually exclusive models for the mechanisms of activation of the epidermal growth factor receptor, the neurotrophin receptor and IR families, based on these new insights.

A novel small-molecule tumor necrosis factor α inhibitor attenuates inflammation in a hepatitis mouse model

Overexpression of tumor necrosis factor α (TNFα) is a hallmark of many inflammatory diseases, including rheumatoid arthritis, inflammatory bowel disease, and septic shock and hepatitis, making it a potential therapeutic target for clinical interventions. To explore chemical inhibitors against TNFα activity, we applied computer-aided drug design combined with in vitro and cell-based assays and identified a lead chemical compound, (E)-4-(2-(4-chloro-3-nitrophenyl) (named as C87 thereafter), which directly binds to TNFα, potently inhibits TNFα-induced cytotoxicity (IC50 = 8.73 μM) and effectively blocks TNFα-triggered signaling activities. Furthermore, by using a murine acute hepatitis model, we showed that C87 attenuates TNFα-induced inflammation, thereby markedly reducing injuries to the liver and improving animal survival. Thus, our results lead to a novel and highly specific small-molecule TNFα inhibitor, which can be potentially used to treat TNFα-mediated inflammatory diseases.

Design, synthesis and characterization of peptidomimetic conjugate of BODIPY targeting HER2 protein extracellular domain

Among the EGFRs, HER2 is a major heterodimer partner and also has important implications in the formation of particular tumors. Interaction of HER2 protein with other EGFR proteins can be modulated by small molecule ligands and, hence, these protein-protein interactions play a key role in biochemical reactions related to control of cell growth. A peptidomimetic (compound 5-1) that binds to HER2 protein extracellular domain and inhibits protein-protein interactions of EGFRs was conjugated with BODIPY (4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene). Conjugation of BODIPY to the peptidomimetic was investigated by different approaches. The conjugate was characterized for its ability to bind to HER2 overexpressing SKBR-3 and BT-474 cells. Furthermore, cellular uptake of conjugate of BODIPY was studied in the presence of membrane tracker and Lyso tracker using confocal microscopy. Our results suggested that fluorescently labeled compound 5-7 binds to the extracellular domain and stays in the membrane for nearly 24 h. After 24 h there is an indication of internalization of the conjugate. Inhibition of protein-protein interaction and downstream signaling effect of compound 5-1 was also studied by proximity ligation assay and Western blot analysis. Results suggested that compound 5-1 inhibit protein-protein interactions of HER2-HER3 and phosphorylation of HER2 in a time-dependent manner.

Identification of a novel antagonist of the ErbB1 receptor capable of inhibiting migration of human glioblastoma cells

BACKGROUND

Receptors of the ErbB family are involved in the development of various cancers, and the inhibition of these receptors represents an attractive therapeutic concept. Upon ligand binding, ErbB receptors become activated as homo- or heterodimers, leading to the activation of downstream signaling cascades that result in the facilitation of cell proliferation and migration. A region of the extracellular part of the receptor, termed the 'dimerization arm', is important for the formation of receptor dimers and represents an attractive target for the design of ErbB inhibitors.

METHODS

An ErbB1 targeting peptide, termed Herfin-1, was designed based on a model of the tertiary structure of the EGF-EGFR ternary complex. The binding kinetics of this peptide were determined employing surface plasmon resonance analyses. ErbB1-4 expression and phosphorylation in human glioblastoma cell lines U87 and U118 were determined by Western blotting using specific antibodies. Cell proliferation was determined by MTS staining. Cell migration was examined using a Chemotaxis Migration Kit. Neurite outgrowth from primary cerebellar granule neurons was evaluated by fluorescence microscopy and image processing.

RESULTS

The present study shows that Herfin-1 functions as an ErbB1 antagonist. It binds to the extracellular domain of ErbB1 with a KD value of 361 nM. In U87 and U118 cells, both expressing high levels of ErbB1, Herfin-1 inhibits EGF-induced ErbB1 phosphorylation and cell migration. Additionally, Herfin-1 was found to increase neurite outgrowth in cerebellar granule neurons, likely through the inhibition of a sustained weak ErbB1 activation.

CONCLUSIONS

Targeting the ErbB1 receptor dimerization interface is a promising strategy to inhibit receptor activation in ErbB1-expressing glioma cells.

Inhibition of protein-protein interaction of HER2-EGFR and HER2-HER3 by a rationally designed peptidomimetic

Protein-protein interactions (PPI) play a crucial role in many biological processes and modulation of PPI using small molecules to target hot spots has therapeutic value. As a model system we will use PPI of human epidermal growth factor receptors (EGFRs). Among the four EGFRs, EGFR-HER2 and HER2-HER3 are well known in cancer. We have designed a small molecule that is targeted to modulate HER2-mediated signaling. Our approach is novel because the small molecule designed disrupts dimerization not only of EGFR-HER2, but also of HER2-HER3. In the present study we have shown, using surface plasmon resonance analysis, that a peptidomimetic, compound 5, binds specifically to HER2 protein extracellular domain and disrupts the dimerization of EGFRs. To evaluate the effect of compound 5 on HER2 signaling in vitro, Western blot and PathHunter assays were used. Results indicated that compound 5 inhibits the phosphorylation of HER2 kinase domain and inhibits the heterodimerization in a dose-dependent manner. Molecular modeling methods were used to model the PPI of HER2-HER3 heterodimer.

Structural and biological features of FOXP3 dimerization relevant to regulatory T cell function

FOXP3 is a key transcription factor for regulatory T cell function. We report the crystal structure of the FOXP3 coiled-coil domain, through which a loose or transient dimeric association is formed and modulated, accounting for the activity variations introduced by disease-causing mutations or posttranslational modifications. Structure-guided mutagenesis revealed that FOXP3 coiled-coil-mediated homodimerization is essential for Treg function in vitro and in vivo. In particular, we identified human FOXP3 K250 and K252 as key residues for the conformational change and stability of the FOXP3 dimer, which can be regulated by protein posttranslational modifications such as reversible lysine acetylation. These studies provide structural and mechanistic explanations for certain disease-causing mutations in the coiled-coil domain of FOXP3 that are commonly found in IPEX syndrome. Overall, the regulatory machinery involving homooligomerization, acetylation, and heteroassociation has been dissected, defining atomic insights into the biological and pathological characteristics of the FOXP3 complex.

Similar but different: ligand-induced activation of the insulin and epidermal growth factor receptor families

The insulin and epidermal growth factor receptor families are among the most intensively studied proteins in biology. They are closely related members of the receptor tyrosine kinase superfamily and deregulated signaling by members of either receptor family has been implicated in the progression of a variety of cancers. These receptors have thus emerged as validated therapeutic targets for the development of anti-tumour agents. Recent studies have revealed detail of the ligand-binding sites in the insulin receptor family, as well as detail of conformational change upon ligand binding in the epidermal growth factor receptor family. Taken together, these findings and further data relating to kinase activation highlight the fact that while the receptor families share common structural elements, the structural detail of their functioning is remarkably different.

Structure based antibody-like peptidomimetics

Biologics such as monoclonal antibodies (mAb) and soluble receptors represent new classes of therapeutic agents for treatment of several diseases. High affinity and high specificity biologics can be utilized for variety of clinical purposes. Monoclonal antibodies have been used as diagnostic agents when coupled with radionuclide, immune modulatory agents or in the treatment of cancers. Among other limitations of using large molecules for therapy the actual cost of biologics has become an issue. There is an effort among chemists and biologists to reduce the size of biologics which includes monoclonal antibodies and receptors without a reduction of biological efficacy. Single chain antibody, camel antibodies, Fv fragments are examples of this type of deconstructive process. Small high-affinity peptides have been identified using phage screening. Our laboratory used a structure-based approach to develop small-size peptidomimetics from the three-dimensional structure of proteins with immunoglobulin folds as exemplified by CD4 and antibodies. Peptides derived either from the receptor or their cognate ligand mimics the functions of the parental macromolecule. These constrained peptides not only provide a platform for developing small molecule drugs, but also provide insight into the atomic features of protein-protein interactions. A general overview of the reduction of monoclonal antibodies to small exocyclic peptide and its prospects as a useful diagnostic and as a drug in the treatment of cancer are discussed.

A conformationally constrained peptidomimetic binds to the extracellular region of HER2 protein

Human epidermal growth factor receptor 2 (HER2) is a member of the human epidermal growth factor receptor kinases (other members include EGFR or HER1, HER3, and HER4) that are involved in signaling cascades for cell growth and differentiation. It is well established that HER2-mediated heterodimerization has important implications in cancer. Deregulation of signaling pathways and overexpression of HER2 is known to occur in cancer cells, indicating a role of HER2 in tumorigenesis. Therefore, blocking HER2-mediated signaling has potential therapeutic value. We have designed several peptidomimetics to inhibit HER2-mediated signaling for cell growth. One of the compounds (HERP5, Arg-beta Naph-Phe) exhibited antiproliferative activity with IC(50) values in the micromolar-to-nanomolar range in breast cancer cell lines. Binding of fluorescently labeled HERP5 to HER2 protein was evaluated by fluorescence assay, microscopy, and circular dichroism spectroscopy. Results indicated that HERP5 binds to the extracellular region of the HER2 protein. Structure of the peptidomimetic HERP5 was studied by NMR and molecular dynamics simulations. Based on these results a model was proposed for HER2-EGFR dimerization and possible blocking by HERP5 peptidomimetic using a protein-protein docking method.

The tumor suppressor RECK interferes with HER-2/Neu dimerization and attenuates its oncogenic signaling

Our previous study demonstrates that HER-2/Neu oncogene inhibits a matrix metalloproteinase inhibitor and tumor metastasis suppressor RECK to promote metastasis. Conversely, the effect of RECK on the oncogenic function of HER-2/Neu is unknown. Ectopic expression of RECK in 293T cells and HER-2/Neu-overexpressing breast cancer cells shows that RECK and HER-2/Neu are co-localized and these two proteins can be co-immunoprecipitated. RECK inhibits HER-2/Neu receptor dimerization and autophosphorylation, which causes reduction of ERK and AKT kinase activity and down-regulation of HER-2/Neu target genes. RECK expression is reduced in 58.8% of breast cancer tissues and is associated with lymph node invasion supporting its anti-metastatic role. Collectively, we provide the first evidence that RECK can negatively regulate oncogenic activity of HER-2/Neu by inhibiting receptor dimerization.

Structure-activity relationship of conformationally constrained peptidomimetics for antiproliferative activity in HER2-overexpressing breast cancer cell lines

Human epidermal growth factor receptor 2 (HER2) is a member of the human epidermal growth factor receptor kinases and is involved in a signaling cascade for cell growth and differentiation. It is well established that HER2-mediated heterodimerization has important implications in cancer. Deregulation of signaling pathways and overexpression of HER2 is known to occur in cancer cells, indicating the role of HER2 in tumorigenesis. Therefore, blocking HER2-mediated signaling has potential therapeutic value. We have designed several peptidomimetics to inhibit HER2-mediated signaling for cell growth. One of the compounds (compound 5, Arg-[3-amino-3(1-napthyl)-propionic acid]-Phe) exhibited antiproliferative activity with IC(50) values in the nanomolar to micromolar range in breast cancer cell lines. To further investigate the structure-activity relationship of the compounds, various analogs of compound 5 were designed. Conformational constraints were initiated in the peptidomimetic with introduction of a Pro residue in the peptidomimetic sequence. Results of antiproliferative activity indicated that analogs of compound 5 with C-and N-terminal ends capped (compound 16) and compound 9 with Asp at the C-terminal exhibited antiproliferative activity in the lower micromolar range against breast cancer cell lines. Introduction of conformational constraints such as Pro residue in the sequence or cyclization did not enhance the activity of the peptidomimetic. Competitive binding studies were carried out to evaluate the binding of potent peptidomimetics to HER2-overexpressing cancer cell lines. Results indicated that compounds exhibiting antiproliferative activity in breast cancer cell lines bind to the cells that overexpress HER2 protein.

Targeting erbB receptors

Our work is concerned with the origins and therapy of human cancers. Members of the epidermal growth factor receptor (EGFR) family of tyrosine kinases, also known as erbB or HER receptors, are over expressed and/or activated in many types of human tumors and represent important therapeutic targets in cancer therapy. Studies from our laboratory identified targeted therapy as a way to treat cancer. Rational therapeutics targeting and disabling erbB receptors have been developed to reverse the malignant properties of tumors. Reversal of the malignant phenotype, best seen with disabling the HER2 receptors using monoclonal antibodies is a distinct process from that seen with blocking of ligand binding to cognate receptors as has been done for EGFr receptors. Here we review the mechanisms of action deduced from a number of approaches developed in our laboratory and elsewhere, including monoclonal antibodies, peptide mimetics, recombinant proteins and small molecules. The biochemical and biological principles which have been uncovered during these studies of disabling HER2 homomeric or HER2-EGFr heteromeric receptors will help the development of novel and more efficient therapeutics targeting erbB family receptors.

Phage-displayed combinatorial peptide libraries in fusion to beta-lactamase as reporter for an accelerated clone screening: Potential uses of selected enzyme-linked affinity reagents in downstream applications

Phage-display selection of combinatorial libraries is a powerful technique for identifying binding ligands against desired targets. Evaluation of target binding capacity of multiple clones recovered from phage display selection to a specific target is laborious, time-consuming, and a rate-limiting step. We constructed phage-display combinatorial peptide libraries in fusion with a beta-lactamase enzyme, which acts as a reporter. Linear dodecapeptide and cysteine-constrained decapeptide libraries were created at the amino-terminus of the Enterobacter cloacae P99 cephalosporinase molecule (P99 beta-lactamase). The overall and positional diversity of amino acids in both libraries was similar to other phage-display systems. The libraries were selected against the extracellular domain of ErbB2 receptor (ErbB2(ECD)). The target-selected clones were already conjugated to an enzyme reporter, therefore, did not require subcloning or any other post-panning modifications. We used beta-lactamase enzyme activity-based assays for sample normalizations and clone binding evaluation. Clones were identified that bound to purified ErbB2(ECD) and ErbB2-overexpressing cell-lines. The peptide sequences of the selected binding clones shared significant motifs with several rationally designed peptide mimetics and phage-display derived peptides that have been reported to bind ErbB2(ECD). beta-Lactamase fusion to peptides saved time and resources otherwise required by the phage-ELISA of a typical phage display screening protocol. The beta-lactamase enzyme assay protocols is a one-step process that does not require secondary proteins, several steps of lengthy incubations, or washings and can be finished in a few minutes instead of hours. The clone screening protocol can be adopted for a high throughput platform. Target-specific beta-lactamase-linked affinity reagents selected by this procedure can be produced in bulk, purified, and used, without any modification, for a variety of downstream applications, including targeted prodrug therapy.

Design, synthesis, and docking studies of peptidomimetics based on HER2-herceptin binding site with potential antiproliferative activity against breast cancer cell lines

Epidermal growth factor receptor kinase and the related human epidermal growth factor receptor-2 (HER2, ErbB2) are two growth factor receptors that have implications in cancer. The overexpression or activation of HER2 occurs frequently in breast, ovarian, and lung cancers, making it an important therapeutic target in the treatment of cancer. Blocking HER2-mediated signaling with antibodies or small molecules has been shown to be effective in inhibiting cell growth. After analyzing the crystal structure of the HER2-herceptin complex, several peptidomimetics (HERP5, 6, and 7) were designed to inhibit HER2-mediated signaling for cell growth. We have used an in silico screening method to investigate the chemical diversity of the designed compounds. autodock software was used to dock the different analogs of HERP5 and HERP7 with HER2 protein extracellular domain. A total of 53 compounds were docked to HER2 protein, and their binding modes were analyzed in terms of docking energy, hydrogen bonding, and hydrophobic interactions. Compounds that exhibited low-energy docked structures were chosen for chemical synthesis and biological activity. Two of the compounds (HERP5 and HERP7) exhibited antiproliferative activity, with IC(50) values of 0.396 microm and 0.143 microm, respectively, against SKBR-3 cell lines (breast cancer cell lines) that overexpress HER2 protein.

A SPR strategy for high-throughput ligand screenings based on synthetic peptides mimicking a selected subdomain of the target protein: a proof of concept on HER2 receptor

The discovery of pharmaceutical agents is a complex, lengthy and costly process, critically depending on the availability of rapid and efficient screening methods. In particular, when targets are large, multidomain proteins, their complexity may affect unfavorably technical feasibility, costs and unambiguity of binding test interpretation. A possible strategy to overcome these problems relies on molecular design of receptor fragments that are: sensible targets for ligand screenings, conformationally stable also as standalone domains, easily synthesized and immobilized on chip for Biacore experiments. An additional desirable feature for new ligands is the ability of selectively targeting alternative conformational states typical of many proteins. To test the feasibility of such approach on a case with potential applicative interest, we developed a surface plasmon resonance (SPR)-based screening method for drug candidates toward HER2, a Tyr-kinase receptor targeted in anticancer therapies. HER2 was mimicked by HER2-DIVMP, a modified fragment of it immobilized onto the sensor surface specifically modeling HER2 domain IV in its bounded form, designed by structural comparison of HER2 alone and in complex with Herceptin, a monoclonal therapeutic anti-HER2 antibody. This design and its implementation in SPR devices was validated by investigating Herceptin- HER2-DIVMP affinity, measuring its dissociation constant (K(D)=19.2 nM). An efficient synthetic procedure to prepare the HER2-DIVMP peptide was also developed. The HER2-DIVMP conformational stability suggested by experimental and computational results, makes it also a valuable candidate as a mold to design new molecules selectively targeting domain IV of HER2.

The role of HER2 in early breast cancer metastasis and the origins of resistance to HER2-targeted therapies

The HER2 gene encodes the receptor tyrosine kinase HER2 and is often over-expressed or amplified in breast cancer. Up-regulation of HER2 contributes to tumor progression. Many aspects of tumor growth are favorably affected through activation of HER2 signaling. Indeed, HER2 plays a role in increasing proliferation and survival of the primary tumor and distant lesions which upon completion of full transformation cause metastases. P185(HER2/neu) receptors and signaling from them and associated molecules increase motility of both intravasating and extravasating cells, decrease apoptosis, enhance signaling interactions with the microenvironment, regulate adhesion, as well as a multitude of other functions. Recent experimental and clinical evidence supports the view that the spread of incompletely transformed cells occurs at a very early stage in tumor progression. This review concerns the identification and characterization of HER2, the evolution of the metastasis model, and the more recent cancer stem cell model. In particular, we review the evidence for an emerging mechanism of HER2(+) breast cancer progression, whereby the untransformed HER2-expressing cell shows characteristics of stem/progenitor cell, metastasizes, and then completes its final transformation at the secondary site.

Humanized ADEPT comprised of an engineered human purine nucleoside phosphorylase and a tumor targeting peptide for treatment of cancer

Immunogenicity caused by the use of nonhuman enzymes in antibody-directed enzyme prodrug therapy has limited its clinical application. To overcome this problem, we have developed a mutant human purine nucleoside phosphorylase, which, unlike the wild-type enzyme, accepts (deoxy)adenosine-based prodrugs as substrates. Among the different mutants of human purine nucleoside phosphorylase tested, a double mutant with amino acid substitutions E201Q:N243D (hDM) is the most efficient in cleaving (deoxy)adenosine-based prodrugs. Although hDM is capable of using multiple prodrugs as substrates, it is most effective at cleaving 2-fluoro-2'-deoxyadenosine to a cytotoxic drug. To target hDM to the tumor site, the enzyme was fused to an anti-HER-2/neu peptide mimetic (AHNP). Treatment of HER-2/neu-expressing tumor cells with hDM-AHNP results in cellular localization of enzyme activity. As a consequence, harmless prodrug is converted to a cytotoxic drug in the vicinity of the tumor cells, resulting in tumor cell apoptosis. Unlike the nonhuman enzymes, the hDM should have minimal immunogenicity when used in antibody-directed enzyme prodrug therapy, thus providing a novel promising therapeutic agent for the treatment of tumors.

Ligand-induced ErbB receptor dimerization

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

Structure-based view of epidermal growth factor receptor regulation

High-resolution X-ray crystal structures determined in the past six years dramatically influence our view of ligand-induced activation of the epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases. Ligand binding to the extracellular region of EGFR promotes a major domain reorganization, plus local conformational changes, that are required to generate an entirely receptor-mediated dimer. In this activated complex the intracellular kinase domains associate to form an asymmetric dimer that supports the allosteric activation of one kinase. These models are discussed with emphasis on recent studies that add details or bolster the generality of this view of activation of this family of receptors. The EGFR family is implicated in several disease states, perhaps most notably in cancers. Activating tumor mutations have been identified in the intracellular and extracellular regions of EGFR. The impact of these tumor mutations on the understanding of EGFR activation and of its inhibition is discussed.

Noncovalent keystone interactions controlling biomembrane structure

There is a biomedical need to develop molecular recognition systems that selectively target the interfaces of protein and lipid aggregates in biomembranes. This is an extremely challenging problem in supramolecular chemistry because the biological membrane is a complex dynamic assembly of multifarious molecular components with local inhomogeneity. Two simplifying concepts are presented as a framework for basing molecular design strategies. The first generalization is that association of two binding partners in a biomembrane will be dominated by one type of non-covalent interaction which is referred to as the keystone interaction. Structural mutations in membrane proteins that alter the strength of this keystone interaction will likely have a major effect on biological activity and often will be associated with disease. The second generalization is to view the structure of a cell membrane as three spatial regions, that is, the polar membrane surface, the midpolar interfacial region and the non-polar membrane interior. Each region has a distinct dielectric, and the dominating keystone interaction between binding partners will be different. At the highly polar membrane surface, the keystone interactions between charged binding partners are ion-ion and ion-dipole interactions; whereas, ion-dipole and ionic hydrogen bonding are very influential at the mid-polar interfacial region. In the non-polar membrane interior, van der Waals forces and neutral hydrogen bonding are the keystone interactions that often drive molecular association. Selected examples of lipid and transmembrane protein association systems are described to illustrate how the association thermodynamics and kinetics are dominated by these keystone noncovalent interactions.

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

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

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

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

ErbB receptors: from oncogenes to targeted cancer therapies

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

Crystal structure of the major diabetes autoantigen insulinoma-associated protein 2 reveals distinctive immune epitopes

Insulinoma-associated protein-2 (IA-2) is a major autoantigen in type 1 diabetes that occurs through autoimmune-mediated beta-cell destruction. We present here the crystal structure of the protein tyrosine phosphatase (PTP)-like domain of human IA-2. The structure reveals a canonical PTP domain with the closed WPD loop over the active site pocket, explaining the lack of enzyme activity in the native protein. The structural interpretation of previous mutagenesis studies indicates that the B-cell epitopes are concentrated on two distinctive regions on peripheral loops of the central beta-sheet surrounding T-cell epitopes within the sheet. The detailed structural information on immune epitopes provides a framework for the future development of immune intervention strategies against diabetes.

The extracellular domain of p185(c-neu) induces density-dependent inhibition of cell growth in malignant mesothelioma cells and reduces growth of mesothelioma in vivo

EGFR is involved in the density-dependent inhibition of cell growth, while coexpression of EGFR with erbB2 can render normal cells transformed. In this study, we have examined the effect of a species of p185 that contains the transmembrane domain and the extracellular domain of p185(c-neu), on growth properties of a human malignant mesothelioma cell line that coexpresses EGFR and erbB2. The ectodomain form of p185(c-neu) enhanced density-dependent inhibition of cell growth and we found that p21 induction appeared to be responsible for this inhibitory effect. Previously, the extracellular domain species was shown to suppress the transforming abilities of EGFR and p185(c-neu/erbB2) in a dominant-negative manner. The ability of this subdomain to affect tumor growth is significant, as it reduced in vivo tumor growth. Unexpectedly, we found that the domain did not abrogate all of EGFR functions. We noted that EGFR-induced density-dependent inhibition of cell growth was retained. Tyrosine kinase inhibitors of EGFR did not cause density-dependent inhibition of cell growth of malignant mesothelioma cells. Therefore, simultaneously inhibiting the malignant phenotype and inducing density-dependent inhibition of cell growth in malignant mesothelioma cells by the extracellular domain of p185(c-neu) may represent an important therapeutic advance.

AHNP-streptavidin: a tetrameric bacterially produced antibody surrogate fusion protein against p185her2/neu

The anti-p185(her2/neu) peptidomimetic (AHNP) is a small exo-cyclic peptide derived from the anti-p185(her2/neu) rhumAb 4D5 (h4D5). AHNP mimics many but not all of the antitumor characteristics exhibited by h4D5. However, the pharmacokinetic profiles of AHNP are less than optimal for therapeutic or diagnostic purposes. To improve the binding affinity to p185(her2/neu) and the antitumor efficacy, we have engineered a fusion protein containing AHNP and a nonimmunoglobulin protein scaffold, streptavidin (SA). The recombinant protein, AHNP-SA (ASA) bound to p185(her2/neu) with high affinity, inhibited the proliferation of p185(her2/neu)-overexpressing cells, and reduced tumor growth induced by p185(her2/neu)-transformed cells. These data suggest that the bacterially produced tetrameric ASA can be used as an antibody-surrogate molecule. This class of molecule will play a role in the diagnosis and treatment of p185(her2/neu)-related tumors. Our studies establish a general principle by which a small biologically active synthetic exo-cyclic peptide can be engineered to enhance functional aspects by structured oligomerization and can be produced recombinantly using bacterial expression.

The membrane proximal disulfides of the EGF receptor extracellular domain are required for high affinity binding and signal transduction but do not play a role in the localization of the receptor to lipid rafts

The EGF receptor is a transmembrane receptor tyrosine kinase that is enriched in lipid rafts. Subdomains I, II and III of the extracellular domain of the EGF receptor participate in ligand binding and dimer formation. However, the function of the cysteine-rich subdomain IV has not been elucidated. In this study, we analyzed the role of the membrane-proximal portion of subdomain IV in EGF binding and signal transduction. A double Cys-->Ala mutation that breaks the most membrane-proximal disulfide bond (Cys600 to Cys612), ablated high affinity ligand binding and substantially reduced signal transduction. A similar mutation that breaks the overlapping Cys596 to Cys604 disulfide had little effect on receptor function. Mutation of residues within the Cys600 to Cys612 disulfide loop did not alter the ligand binding or signal transducing activities of the receptor. Despite the fact that the C600,612A EGF receptor was significantly impaired functionally, this receptor as well as all of the other receptors with mutations in the region of residues 596 to 612 localized normally to lipid rafts. These data suggest that the disulfide-bonded structure of the membrane-proximal portion of the EGF receptor, rather than its primary sequence, is important for EGF binding and signaling but is not involved in localizing the receptor to lipid rafts.

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

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

Functional Effects of Glycosylation at Asn-579 of the Epidermal Growth Factor Receptor

We have investigated functional effects of glycosylation at N(579) of the epidermal growth factor receptor (EGFR). Our previous study showed that the population of cell-surface expressed EGFRs in A431 cells, a human epidermoid carcinoma cell line, is composed of two subpopulations that differ by glycosylation at N(579) [Zhen et al. (2003) Biochemistry 42, 5478-5492]. To characterize the subpopulation of receptors not glycosylated at N(579), we established a 32D cell line expressing a point mutant of the EGFR (N579Q), which cannot be glycosylated at this position. Analysis of epitope accessibility suggests that the lack of glycosylation at N(579) weakens auto-inhibitory tether interactions, and cross-linking experiments suggest a somewhat elevated level of preformed N579Q-EGFR dimers in the absence of ligand relative to wild-type EGFR (WT-EGFR). However, ligand drives the majority of N579Q-EGFR dimerization, suggesting that untethering, while necessary, is not sufficient to drive dimerization. Ligand-binding experiments reveal a much greater fraction of N579Q-EGFRs in a high-affinity state compared to the fraction of WT-EGFRs in a high-affinity state. However, differences in the kinetic association and dissociation rates indicate that the high-affinity states of the WT and the N579Q receptors are distinct. EGF-stimulated phosphorylation in cells expressing N579Q-EGFRs results in notable differences in the pattern of tyrosine phosphorylated proteins compared with that obtained in cells expressing WT-EGFRs. Moreover, although WT-EGFRs confer cell survival in 32D cells in the absence of interleukin-3 and EGF, we found that receptors lacking glycosylation at N(579) do not. This is the first study of which we are aware to show that selective glycosylation of a specific N-glycosylation site can produce two functionally distinct receptors.

Disruption of protein-protein interactions: towards new targets for chemotherapy

Protein-protein interactions play a key role in various mechanisms of cellular growth and differentiation, and in the replication of pathogen organisms in host cells. Thus, inhibition of these interactions is a promising novel approach for rational drug design against a wide number of cellular and microbial targets. In the past few years, attempts to inhibit protein-protein interactions using antibodies, peptides, and synthetic or natural small molecules have met with varying degrees of success, and these will be the focus of this review.

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

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

HER2-mediated internalization of a targeted prodrug cytotoxic conjugate is dependent on the valency of the targeting ligand

HER2 is a validated therapeutic target for cancer. There are no natural ligands, but monoclonal antibodies and peptides that bind HER2 act as artificial ligands, selectively affecting HER2-overexpressing tumors. One reported mechanism for this effect is receptor downregulation, but the expected correlation of ligand-dependent HER2 internalization and tumor inhibition remain poorly characterized. Moreover, HER2 ligands have limited therapeutic efficacy and often they require adjuvant treatment with the chemotherapeutic Taxol. Here, we generated a series of HER2 ligands (Anti-HER2/neu peptide ligands, AHNPmonovalent and AHNPbivalent) with different valency and correlated their internalization-promoting ability to biological potency. Since AHNPbivalent (but not AHNPmonovalent) induces rapid receptor internalization, we exploited this feature to deliver cytotoxic conjugates coupling AHNPbivalent and Taxol (Taxol . AHNPbivalent). The prodrug conjugate releases Taxol after receptor-mediated internalization, and cytotoxicity can be used as a marker of internalization. Taxol . AHNPbivalent is significantly more cytotoxic than free Taxol + free AHNPbivalent. Hence, the Taxol x AHNP(bivalent) prodrug binds to HER2, induces receptor internalization and downregulation, and the subsequent release of free Taxol inside the targeted cell results in synergistic toxicity, The effect is selective towards HER2- expressing cells. This work links HER2 receptor internalization and growth arrest, and the chemical conjugation strategy may yield improved and HER2 selective therapeutics.