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

Structural determinants for membrane binding of the EGFR juxtamembrane domain

Overactivation of the epidermal growth factor receptor (EGFR) is critical for the development of multiple cancers. Previous studies have shown that the cell membrane is a key regulator of EGFR kinase activity through its interaction with the EGFR juxtamembrane domain (JM). However, the lipid recognition specificity of EGFR-JM and its interaction details remain unclear. Using lipid strip and liposome pulldown assays, we showed that EGFR-JM could specifically interact with PI(4,5)P2-or phosphatidylserine-containing membranes. We further characterized the JM-membrane interaction using NMR-titration-based chemical shift perturbation and paramagnetic relaxation enhancement analyses, and found that residues I649 - L659 comprised the membrane-binding site. Furthermore, the membrane-binding region contains the predicted dimerization motif of JM, 655LRRLL659, suggesting that membrane binding may affect JM dimerization and, therefore, regulate kinase activation.

Trans-activating mutations of the pseudokinase ERBB3

Genetic changes in the ERBB family of receptor tyrosine kinases serve as oncogenic driver events and predictive biomarkers for ERBB inhibitor drugs. ERBB3 is a pseudokinase member of the family that, although lacking a fully active kinase domain, is well known for its potent signaling activity as a heterodimeric complex with ERBB2. Previous studies have identified few transforming ERBB3 mutations while the great majority of the hundreds of different somatic ERBB3 variants observed in different cancer types remain of unknown significance. Here, we describe an unbiased functional genetics screen of the transforming potential of thousands of ERBB3 mutations in parallel. The screen based on a previously described iSCREAM (in vitro screen of activating mutations) platform, and addressing ERBB3 pseudokinase signaling in a context of ERBB3/ERBB2 heterodimers, identified 18 hit mutations. Validation experiments in Ba/F3, NIH 3T3, and MCF10A cell backgrounds demonstrated the presence of both previously known and unknown transforming ERBB3 missense mutations functioning either as single variants or in cis as a pairwise combination. Drug sensitivity assays with trastuzumab, pertuzumab and neratinib indicated actionability of the transforming ERBB3 variants.

Molecular basis of VEGFR1 autoinhibition at the plasma membrane

Ligand-independent activation of VEGFRs is a hallmark of diabetes and several cancers. Like EGFR, VEGFR2 is activated spontaneously at high receptor concentrations. VEGFR1, on the other hand, remains constitutively inactive in the unligated state, making it an exception among VEGFRs. Ligand stimulation transiently phosphorylates VEGFR1 and induces weak kinase activation in endothelial cells. Recent studies, however, suggest that VEGFR1 signaling is indispensable in regulating various physiological or pathological events. The reason why VEGFR1 is regulated differently from other VEGFRs remains unknown. Here, we elucidate a mechanism of juxtamembrane inhibition that shifts the equilibrium of VEGFR1 towards the inactive state, rendering it an inefficient kinase. The juxtamembrane inhibition of VEGFR1 suppresses its basal phosphorylation even at high receptor concentrations and transiently stabilizes tyrosine phosphorylation after ligand stimulation. We conclude that a subtle imbalance in phosphatase activation or removing juxtamembrane inhibition is sufficient to induce ligand-independent activation of VEGFR1 and sustain tyrosine phosphorylation.

Amyloid Precursor Protein: A Regulatory Hub in Alzheimer's Disease

Decades of research have demonstrated an incontrovertible role of amyloid-β (Aβ) in the etiology of Alzheimer's disease (AD). However, the overemphasis on the pathological impacts of Aβ may obscure the role of its metabolic precursor, amyloid precursor protein (APP), as a significant hub in the occurrence and progression of AD. The complicated enzymatic processing, ubiquitous receptor-like properties, and abundant expression of APP in the brain, as well as its close links with systemic metabolism, mitochondrial function and neuroinflammation, imply that APP plays multifaceted roles in AD. In this review, we briefly describe the evolutionarily conserved biological characteristics of APP, including its structure, functions and enzymatic processing. We also discuss the possible involvement of APP and its enzymatic metabolites in AD, both detrimental and beneficial. Finally, we describe pharmacological agents or genetic approaches with the capability to reduce APP expression or inhibit its cellular internalization, which can ameliorate multiple aspects of AD pathologies and halt disease progression. These approaches provide a basis for further drug development to combat this terrible disease.

Distinct interactions stabilize EGFR dimers and higher-order oligomers in cell membranes

The epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase with important roles in many cellular processes as well as in cancer and other diseases. EGF binding promotes EGFR dimerization and autophosphorylation through interactions that are well understood structurally. How these dimers relate to higher-order EGFR oligomers seen in cell membranes, however, remains unclear. Here, we used single-particle tracking (SPT) and Förster resonance energy transfer imaging to examine how each domain of EGFR contributes to receptor oligomerization and the rate of receptor diffusion in the cell membrane. Although the extracellular region of EGFR is sufficient to drive receptor dimerization, we find that the EGF-induced EGFR slowdown seen by SPT requires higher-order oligomerization-mediated in part by the intracellular tyrosine kinase domain when it adopts an active conformation. Our data thus provide important insight into the interactions required for higher-order EGFR assemblies involved in EGF signaling.

Ebselen oxide and derivatives are new allosteric HER2 inhibitors for HER2-positive cancers

Human epidermal growth factor receptor 2 (ErbB2/HER2) is a tyrosine kinase receptor that is overexpressed in 25% of primary human breast cancers, as well as in multiple other cancers. HER2-targeted therapies improved progression-free and overall survival in patients with HER2+ breast cancers. However, associated resistance mechanisms and toxicity highlight the need for new therapeutic approaches for these cancers. We recently established that, in normal cells, HER2 is stabilized in a catalytically repressed state by direct interaction with members of the ezrin/radixin/moesin (ERM) family. In HER2-overexpressing tumors, the low expression of moesin contributes to the aberrant activation of HER2. Through a screen designed to find moesin-mimicking compounds, we identified ebselen oxide. We show that ebselen oxide, and some derivatives, conferred an efficient allosteric inhibition of overexpressed HER2, as well as mutated and truncated oncogenic forms of HER2, which are resistant to current therapies. Ebselen oxide selectively inhibited anchorage-dependent and -independent proliferation of HER2+ cancer cells and showed a significant benefit in combination with current anti-HER2 therapeutic agents. Finally, ebselen oxide significantly blocked HER2+ breast tumor progression in vivo. Collectively, these data provide evidence that ebselen oxide is a newly identified allosteric inhibitor of HER2 to be considered for therapeutic intervention on HER2+ cancers.

EGFR trafficking: effect of dimerization, dynamics, and mutation

Spontaneous dimerization of EGF receptors (EGFR) and dysregulation of EGFR signaling has been associated with the development of different cancers. Under normal physiological conditions and to maintain homeostatic cell growth, once EGFR signaling occurs, it needs to be attenuated. Activated EGFRs are rapidly internalized, sorted through early endosomes, and ultimately degraded in lysosomes by a process generally known as receptor down-regulation. Through alterations to EGFR trafficking, tumors develop resistance to current treatment strategies, thus highlighting the necessity for combination treatment strategies that target EGFR trafficking. This review covers EGFR structure, trafficking, and altered surface expression of EGFR receptors in cancer, with a focus on how therapy targeting EGFR trafficking may aid tyrosine kinase inhibitor treatment of cancer.

The combined action of the intracellular regions regulates FGFR2 kinase activity

Receptor tyrosine kinases (RTKs) are typically activated through a precise sequence of intracellular phosphorylation events starting with a tyrosine residue on the activation loop (A-loop) of the kinase domain (KD). From this point the mono-phosphorylated enzyme is active, but subject to stringent regulatory mechanisms which can vary dramatically across the different RTKs. In the absence of extracellular stimulation, fibroblast growth factor receptor 2 (FGFR2) exists in the mono-phosphorylated state in which catalytic activity is regulated to allow rapid response upon ligand binding, whilst restricting ligand-independent activation. Failure of this regulation is responsible for pathologic outcomes including cancer. Here we reveal the molecular mechanistic detail of KD control based on combinatorial interactions of the juxtamembrane (JM) and the C-terminal tail (CT) regions of the receptor. JM stabilizes the asymmetric dimeric KD required for substrate phosphorylation, whilst CT binding opposes dimerization, and down-regulates activity. Direct binding between JM and CT delays the recruitment of downstream effector proteins adding a further control step as the receptor proceeds to full activation. Our findings underscore the diversity in mechanisms of RTK oligomerisation and activation.

A perspective of fluorescence microscopy for cellular structural biology with EGFR as witness

The epidermal growth factor receptor (EGFR) is a poster child for the understanding of receptor behaviour, and of paramount importance to cell function and human health. Cloned almost forty years ago, the interest in EGFR's structure/function relationships remains unabated, not least because changes in oncogenic EGFR mutants are key drivers of the formation of lung and brain tumours. The structure of the assemblies formed by EGFR have been comprehensibly investigated by techniques such as high-resolution X-ray crystallography, NMR and all-atom molecular dynamics (MD) simulations. However, the complexity embedded in the portfolio of EGFR states that are only possible in the physiological environment of cells has often proved refractory to cell-free structural methods. Conversely, some key inroads made by quantitative fluorescence microscopy and super-resolution have depended on exploiting the wealth of structures available. Here, a brief personal perspective is provided on how quantitative fluorescence microscopy and super-resolution methods have cross-fertilised with cell-free-derived EGFR structural information. I primarily discuss areas in which my research group has made a contribution to fill gaps in EGFR's cellular structural biology and towards developing new tools to investigate macromolecular assemblies in cells.

Distinct interactions stabilize EGFR dimers and higher-order oligomers in cell membranes

The epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase (RTK) with important roles in many cellular processes as well as cancer and other diseases. EGF binding promotes EGFR dimerization and autophosphorylation through interactions that are well understood structurally. However, it is not clear how these dimers relate to higher-order EGFR oligomers detected at the cell surface. We used single-particle tracking (SPT) and Förster resonance energy transfer (FRET) imaging to examine how each domain within EGFR contributes to receptor dimerization and the rate of its diffusion in the cell membrane. We show that the EGFR extracellular region is sufficient to drive receptor dimerization, but that the EGF-induced EGFR slow-down seen by SPT requires formation of higher order oligomers, mediated in part by the intracellular tyrosine kinase domain - but only when in its active conformation. Our data thus provide important insight into higher-order EGFR interactions required for EGF signaling.

Regulation of ErbB Receptors by the Ca2+ Sensor Protein Calmodulin in Cancer

Overexpression and mutations of the epidermal growth factor receptor (EGFR/ErbB1/HER1) and other tyrosine kinase receptors of the ErbB family (ErbB2/HER2, ErbB3/HER3 and ErbB4/HER4) play an essential role in enhancing the proliferation, the migratory capacity and invasiveness of many tumor cells, leading to cancer progression and increased malignancy. To understand these cellular processes in detail is essential to understand at a molecular level the signaling pathways and regulatory mechanisms controlling these receptors. In this regard, calmodulin (CaM) is a Ca2+-sensor protein that directly interacts with and regulates ErbB receptors, as well as some CaM-dependent kinases that also regulate these receptors, particularly EGFR and ErbB2, adding an additional layer of CaM-dependent regulation to this system. In this short review, an update of recent advances in this area is presented, covering the direct action of Ca2+/CaM on the four ErbB family members mostly in tumor cells and the indirect action of Ca2+/CaM on the receptors via CaM-regulated kinases. It is expected that further understanding of the CaM-dependent mechanisms regulating the ErbB receptors in future studies could identify new therapeutic targets in these systems that could help to control or delay cancer progression.

Threonine phosphorylation regulates the molecular assembly and signaling of EGFR in cooperation with membrane lipids

The cytoplasmic domain of the receptor tyrosine kinases (RTKs) plays roles as a phosphorylation enzyme and a protein scaffold but the allocation of these two functions is not fully understood. We here analyzed assembly of the transmembrane (TM)-juxtamembrane (JM) region of EGFR, one of the best studied species of RTKs, by combining single-pair FRET imaging and a nanodisc technique. The JM domain of EGFR contains a threonine residue (Thr654) that is phosphorylated after ligand association. We observed that the TM-JM peptides of EGFR form anionic lipid-induced dimers and cholesterol-induced oligomers. The two forms involve distinct molecular interactions, with a bias towards oligomer formation upon threonine phosphorylation. We further analyzed the functions and oligomerization of whole EGFR molecules, with or without a substitution of Thr654 to alanine, in living cells. The results suggested an autoregulatory mechanism in which Thr654 phosphorylation causes a switch of the major function of EGFR from kinase activation dimers to scaffolding oligomers.

Ligand-induced transmembrane conformational coupling in monomeric EGFR

Single pass cell surface receptors regulate cellular processes by transmitting ligand-encoded signals across the plasma membrane via changes to their extracellular and intracellular conformations. This transmembrane signaling is generally initiated by ligand binding to the receptors in their monomeric form. While subsequent receptor-receptor interactions are established as key aspects of transmembrane signaling, the contribution of monomeric receptors has been challenging to isolate due to the complexity and ligand-dependence of these interactions. By combining membrane nanodiscs produced with cell-free expression, single-molecule Förster Resonance Energy Transfer measurements, and molecular dynamics simulations, we report that ligand binding induces intracellular conformational changes within monomeric, full-length epidermal growth factor receptor (EGFR). Our observations establish the existence of extracellular/intracellular conformational coupling within a single receptor molecule. We implicate a series of electrostatic interactions in the conformational coupling and find the coupling is inhibited by targeted therapeutics and mutations that also inhibit phosphorylation in cells. Collectively, these results introduce a facile mechanism to link the extracellular and intracellular regions through the single transmembrane helix of monomeric EGFR, and raise the possibility that intramolecular transmembrane conformational changes upon ligand binding are common to single-pass membrane proteins.

PDGFA-associated protein 1 is a novel target of c-Myc and contributes to colorectal cancer initiation and progression

BACKGROUND

The mechanism underlying colorectal cancer (CRC) initiation and progression remains elusive, and overall survival is far from satisfactory. Previous studies have shown that PDGFA-associated protein 1 (PDAP1) is upregulated in several cancers including CRC. Here, we aimed to identify the cause and consequence of PDAP1 dysregulation in CRC and evaluate its role as a potential therapeutic target.

METHODS

Multi-omics data analysis was performed to identify potential key players in CRC initiation and progression. Immunohistochemistry (IHC) staining was applied to determine the expression pattern of PDAP1 in CRC tissues. Pdap1 conditional knockout mice were used to establish colitis and CRC mouse models. RNA sequencing, a phosphoprotein antibody array, western blotting, histological analysis, 5-bromo-2'-deoxyuridine (BrdU) incorporation assay, and interactome analysis were applied to identify the underlying mechanisms of PDAP1. A human patient-derived xenograft (PDX) model was used to assess the potential of PDAP1 as a therapeutic target.

RESULTS

PDAP1 was identified as a potential key player in CRC development using multi-omics data analysis. PDAP1 was overexpressed in CRC cells and correlated with reduced overall survival. Further investigation showed that PDAP1 was critical for the regulation of cell proliferation, migration, invasion, and metastasis. Significantly, depletion of Pdap1 in intestinal epithelial cells impaired mucosal restitution in dextran sulfate sodium salt-induced colitis and inhibited tumor initiation and growth in colitis-associated cancers. Mechanistic studies showed that c-Myc directly transactivated PDAP1, which contributed to the high PDAP1 expression in CRC cells. PDAP1 interacted with the juxtamembrane domain of epidermal growth factor receptor (EGFR) and facilitated EGFR-mitogen-activated protein kinase (MAPK) signaling activation, which resulted in FOS-related antigen 1 (FRA-1) expression, thereby facilitating CRC progression. Notably, silencing of PDAP1 could hinder the growth of patient-derived xenografts that sustain high PDAP1 levels.

CONCLUSIONS

PDAP1 facilitates mucosal restitution and carcinogenesis in colitis-associated cancer. c-Myc-driven upregulation of PDAP1 promotes proliferation, migration, invasion, and metastasis of CRC cells via the EGFR-MAPK-FRA-1 signaling axis. These findings indicated that PDAP1 inhibition is warranted for CRC patients with PDAP1 overexpression.

Human Growth Hormone Fragment 176–191 Peptide Enhances the Toxicity of Doxorubicin-Loaded Chitosan Nanoparticles Against MCF-7 Breast Cancer Cells

Introduction

Numerous drugs with potent toxicity against cancer cells are available for treating malignancies, but therapeutic efficacies are limited due to their inefficient tumor targeting and deleterious effects on non-cancerous tissue. Therefore, two improvements are mandatory for improved chemotherapy 1) novel delivery techniques that can target cancer cells to deliver anticancer drugs and 2) methods to specifically enhance drug efficacy within tumors. The loading of inert drug carriers with anticancer agents and peptides which are able to bind (target) tumor-related proteins to enhance tumor drug accumulation and local cytotoxicity is a most promising approach.

Objective

To evaluate the anticancer efficacy of Chitosan nanoparticles loaded with human growth hormone hGH fragment 176–191 peptide plus the clinical chemotherapeutic doxorubicin in comparison with Chitosan loaded with doxorubicin alone.

Methods

Two sets of in silico experiments were performed using molecular docking simulations to determine the influence of hGH fragment 176–191 peptide on the anticancer efficacy of doxorubicin 1) the binding affinities of hGH fragment 176–191 peptide to the breast cancer receptors, 2) the effects of hGH fragment 176–191 peptide binding on doxorubicin binding to these same receptors. Further, the influence of hGH fragment 176–191 peptide on the anticancer efficacy of doxorubicin was validated using viability assay in Human MCF-7 breast cancer cells.

Results

In silico analysis suggested that addition of the hGH fragment to doxorubicin-loaded Chitosan nanoparticles can enhance doxorubicin binding to multiple breast cancer protein targets, while photon correlation spectroscopy revealed that the synthesized dual-loaded Chitosan nanoparticles possess clinically favorable particle size, polydispersity index, as well as zeta potential.

Conclusion

These dual-loaded Chitosan nanoparticles demonstrated greater anti-proliferative activity against a breast cancer cell line (MCF-7) than doxorubicin-loaded Chitosan. This dual-loading strategy may enhance the anticancer potency of doxorubicin and reduce the clinical side effects associated with non-target tissue exposure.

Origin of diverse phosphorylation patterns in the ERBB system

Intercellular signals induce various cellular responses, including growth, proliferation, and differentiation, via the dynamic processes of signal transduction pathways. For cell fate decisions, ligand-binding induces the phosphorylation of ERBB receptors, which in turn activate downstream molecules. The ERBB family includes four subtypes, which diverged through two gene duplications from a common ancestor. Differences in the expression patterns of the subtypes have been reported between different organs in the human body. However, how these different expression properties influence the diverse phosphorylation levels of ERBB proteins is not well understood. Here we study the origin of the phosphorylation responses by experimental and mathematical analyses. The experimental measurements clarified that the phosphorylation levels heavily depend on the ERBB expression profiles. We developed a mathematical model consisting of the four subtypes as monomers, homodimers, and heterodimers and estimated the rate constants governing the phosphorylation responses from the experimental data. To understand the origin of the diversity, we analyzed the effects of the expression levels and reaction rates of the ERBB subtypes on the diversity. The difference in phosphorylation rates between ERBB subtypes showed a much greater contribution to the diversity than did the dimerization rates. This result implies that divergent evolution in phosphorylation reactions rather than in dimerization reactions after whole genome duplications was essential for increasing the diversity of the phosphorylation responses.

Structural Insight and Development of EGFR Tyrosine Kinase Inhibitors

Lung cancer has a high prevalence, with a growing number of new cases and mortality every year. Furthermore, the survival rate of patients with non-small-cell lung carcinoma (NSCLC) is still quite low in the majority of cases. Despite the use of conventional therapy such as tyrosine kinase inhibitor for Epidermal Growth Factor Receptor (EGFR), which is highly expressed in most NSCLC cases, there was still no substantial improvement in patient survival. This is due to the drug’s ineffectiveness and high rate of resistance among individuals with mutant EGFR. Therefore, the development of new inhibitors is urgently needed. Understanding the EGFR structure, including its kinase domain and other parts of the protein, and its activation mechanism can accelerate the discovery of novel compounds targeting this protein. This study described the structure of the extracellular, transmembrane, and intracellular domains of EGFR. This was carried out along with identifying the binding pose of commercially available inhibitors in the ATP-binding and allosteric sites, thereby clarifying the research gaps that can be filled. The binding mechanism of inhibitors that have been used clinically was also explained, thereby aiding the structure-based development of new drugs.

EGFR-Targeted Photodynamic Therapy

The epidermal growth factor receptor (EGFR) plays a pivotal role in the proliferation and metastatization of cancer cells. Aberrancies in the expression and activation of EGFR are hallmarks of many human malignancies. As such, EGFR-targeted therapies hold significant potential for the cure of cancers. In recent years, photodynamic therapy (PDT) has gained increased interest as a non-invasive cancer treatment. In PDT, a photosensitizer is excited by light to produce reactive oxygen species, resulting in local cytotoxicity. One of the critical aspects of PDT is to selectively transport enough photosensitizers to the tumors environment. Accordingly, an increasing number of strategies have been devised to foster EGFR-targeted PDT. Herein, we review the recent nanobiotechnological advancements that combine the promise of PDT with EGFR-targeted molecular cancer therapy. We recapitulate the chemistry of the sensitizers and their modes of action in PDT, and summarize the advantages and pitfalls of different targeting moieties, highlighting future perspectives for EGFR-targeted photodynamic treatment of cancer.

Role of EGF Receptor Regulatory Networks in the Host Response to Viral Infections

In this review article, we will first provide a brief overview of EGF receptor (EGFR) structure and function, and its importance as a therapeutic target in epithelial carcinomas. We will then compare what is currently known about canonical EGFR trafficking pathways that are triggered by ligand binding, versus ligand-independent pathways activated by a variety of intrinsic and environmentally induced cellular stresses. Next, we will review the literature regarding the role of EGFR as a host factor with critical roles facilitating viral cell entry and replication. Here we will focus on pathogens exploiting virus-encoded and endogenous EGFR ligands, as well as EGFR-mediated trafficking and signaling pathways that have been co-opted by wild-type viruses and recombinant gene therapy vectors. We will also provide an overview of a recently discovered pathway regulating non-canonical EGFR trafficking and signaling that may be a common feature of viruses like human adenoviruses which signal through p38-mitogen activated protein kinase. We will conclude by discussing the emerging role of EGFR signaling in innate immunity to viral infections, and how viral evasion mechanisms are contributing to our understanding of fundamental EGFR biology.

Targeting the Non-Catalytic Functions: a New Paradigm for Kinase Drug Discovery?

Protein kinases have been highly fruitful targets for cancer drug discovery in the past two decades, while most of these drugs bind to the "adenosine triphosphate (ATP)-site" and inhibit kinase catalytic activity. Recently, accumulated evidence suggests that kinases possess functions beyond catalysis through their scaffolds, and the scaffolding functions could play critical roles in multiple cellular signaling and cell fate controls. Small molecules modulating the noncatalytic functions of kinases are rarely reported but emerge as new promising therapeutic strategies for various diseases. Herein, we summarize the characterized noncatalytic functions of kinases, and highlight the recent progress on developing small-molecule modulators of the noncatalytic functions of kinases. Mechanisms and characteristics of different kinds of modulators are also discussed. It is also speculated that targeting the noncatalytic functions would represent a new direction for kinase-based drug discovery.

A Review of HER4 (ErbB4) Kinase, Its Impact on Cancer, and Its Inhibitors

HER4 is a receptor tyrosine kinase that is required for the evolution of normal body systems such as cardiovascular, nervous, and endocrine systems, especially the mammary glands. It is activated through ligand binding and activates MAPKs and PI3K/AKT pathways. HER4 is commonly expressed in many human tissues, both adult and fetal. It is important to understand the role of HER4 in the treatment of many disorders. Many studies were also conducted on the role of HER4 in tumors and its tumor suppressor function. Mostly, overexpression of HER4 kinase results in cancer development. In the present article, we reviewed the structure, location, ligands, physiological functions of HER4, and its relationship to different cancer types. HER4 inhibitors reported mainly from 2016 to the present were reviewed as well.

Distinct interfacial ordering of liquid crystals observed by protein-lipid interactions that enabled the label-free sensing of cytoplasmic protein at the liquid crystal-aqueous interface

Interfaces formed between a lipid decorated liquid crystal (LC) film and an aqueous phase can mimic the bimolecular membrane where interfacially occurring biological phenomena (e.g., lipid-protein interactions, protein adsorption) can be visually monitored by observing the surface-sensitive orientations of LCs. The ordering behavior of LCs at different phospholipid-based LC interfaces (1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) and lysophosphatidic acid (LPA)) were investigated to determine the sensing of an important cytoplasmic protein (juxtamembrane of epidermal growth factor receptor (JM-EGFR)). At both DLPC and LPA decorated interfaces, the LC adopts homeotropic ordering, causing a dark optical appearance under crossed polarizers. Interestingly, upon the introduction of JM-EGFR to these LC-aqueous interfaces, the homeotropic orientation of the LC changed to planar (bright optical appearance), suggesting the potential of the designed system for JM-EGFR sensing. The use of different lipid decorated LC-aqueous interfaces results in the emergence of distinct optical patterns. For example, at a DLPC laden interface, elongated bright domains are observed, whereas a uniform bright texture is observed on an LPA laden interface. The DLPC decorated LC-aqueous interface is found to be highly selective for the sensing of JM-EGFR with a detection limit in the nanomolar concentration region (∼ 50 nM). When compared to spectroscopic and other conventional techniques, the LC-based design is simpler, and it allows the simple and label-free optical sensing of JM-EGFR at fluidic interfaces.

Expanding the Disorder-Function Paradigm in the C-Terminal Tails of Erbbs

ErbBs are receptor tyrosine kinases involved not only in development, but also in a wide variety of diseases, particularly cancer. Their extracellular, transmembrane, juxtamembrane, and kinase folded domains were described extensively over the past 20 years, structurally and functionally. However, their whole C-terminal tails (CTs) following the kinase domain were only described at atomic resolution in the last 4 years. They were shown to be intrinsically disordered. The CTs are known to be tyrosine-phosphorylated when the activated homo- or hetero-dimers of ErbBs are formed. Their phosphorylation triggers interaction with phosphotyrosine binding (PTB) or Src Homology 2 (SH2) domains and activates several signaling pathways controling cellular motility, proliferation, adhesion, and apoptosis. Beyond this passive role of phosphorylated domain and site display for partners, recent structural and function studies unveiled active roles in regulation of phosphorylation and interaction: the CT regulates activity of the kinase domain; different phosphorylation states have different compaction levels, potentially modulating the succession of phosphorylation events; and prolines have an important role in structure, dynamics, and possibly regulatory interactions. Here, we review both the canonical role of the disordered CT domains of ErbBs as phosphotyrosine display domains and the recent findings that expand the known range of their regulation functions linked to specific structural and dynamic features.

Allosteric Inhibition of HER2 by Moesin-Mimicking Compounds Targets HER2-Positive Cancers and Brain Metastases

Therapies targeting the tyrosine kinase receptor HER2 have significantly improved survival of patients with HER2⁺ cancer. However, both de novo and acquired resistance remain a challenge, particularly in the brain metastatic setting. Here we report that, unlike other HER tyrosine kinase receptors, HER2 possesses a binding motif in its cytosolic juxtamembrane region that allows interaction with members of the Ezrin/Radixin/Moesin (ERM) family. Under physiologic conditions, this interaction controls the localization of HER2 in ERM-enriched domains and stabilizes HER2 in a catalytically repressed state. In HER2⁺ breast cancers, low expression of Moesin correlated with increased HER2 expression. Restoring expression of ERM proteins in HER2⁺ breast cancer cells was sufficient to revert HER2 activation and inhibit HER2-dependent proliferation. A high-throughput assay recapitulating the HER2-ERM interaction allowed for screening of about 1,500 approved drugs. From this screen, we found Zuclopenthixol, an antipsychotic drug that behaved as a Moesin-mimicking compound, because it directly binds the juxtamembrane region of HER2 and specifically inhibits HER2 activation in HER2⁺ cancers, as well as activation of oncogenic mutated and truncated forms of HER2. Zuclopenthixol efficiently inhibited HER2⁺ breast tumor progression in vitro and in vivo and, more importantly, showed significant activity on HER2⁺ brain tumor progression. Collectively, these data reveal a novel class of allosteric HER2 inhibitors, increasing the number of approaches to consider for intervention on HER2⁺ breast cancers and brain metastases. SIGNIFICANCE: This study demonstrates the functional role of Moesin in maintaining HER2 in a catalytically repressed state and provides novel therapeutic approaches targeting HER2⁺ breast cancers and brain metastasis using Moesin-mimicking compounds.

Evolution and functional divergence of the ERBB receptor family

Aim: The ERBB gene family plays an important role in cell proliferation and differentiation, and aberrant activations could result in tumorigenesis, which makes this gene family an attractive drug target in the area of precision oncology. Materials & methods: Functional divergence analysis and conservation analysis were performed using ClustalW, MEGA7 and DIVERGE3 software. Results: One hundred and forty five functional divergence residues sites, 94 totally conserved sites and averagely 345 conserved sites of individual gene member were obtained. Some have been reported to play role in drug binding, tumorigenesis and drug resistance. Conclusion: Functional divergence residues with high posterior probabilities and conserved residues may possess certain functions, and aberrant alterations may confer drug resistance or contribute to tumorigenesis.

Galectins as modulators of receptor tyrosine kinases signaling in health and disease

Receptor tyrosine kinases (RTKs) constitute a large group of cell surface proteins that mediate communication of cells with extracellular environment. RTKs recognize external signals and transfer information to the cell interior, modulating key cellular activities, like metabolism, proliferation, motility, or death. To ensure balanced stream of signals the activity of RTKs is tightly regulated by numerous mechanisms, including receptor expression and degradation, ligand specificity and availability, engagement of co-receptors, cellular trafficking of the receptors or their post-translational modifications. One of the most widespread post-translational modifications of RTKs is glycosylation of their extracellular domains. The sugar chains attached to RTKs form a new layer of information, so called glyco-code that is read by galectins, carbohydrate binding proteins. Galectins are family of fifteen lectins implicated in immune response, inflammation, cell division, motility and death. The versatility of cellular activities attributed to galectins is a result of their high abundance and diversity of their cellular targets. A various sugar specificity of galectins and the differential ability of galectin family members to form oligomers affect the spatial distribution and the function of their cellular targets. Importantly, galectins and RTKs are tightly linked to the development, progression and metastasis of various cancers. A growing number of studies points on the close cooperation between RTKs and galectins in eliciting specific cellular responses. This review focuses on the identified complexes between galectins and RTK members and discusses their relevance for the cell physiology both in healthy tissues and in cancer.

Structure-function analysis of oncogenic EGFR Kinase Domain Duplication reveals insights into activation and a potential approach for therapeutic targeting

Mechanistic understanding of oncogenic variants facilitates the development and optimization of treatment strategies. We recently identified in-frame, tandem duplication of EGFR exons 18 - 25, which causes EGFR Kinase Domain Duplication (EGFR-KDD). Here, we characterize the prevalence of ERBB family KDDs across multiple human cancers and evaluate the functional biochemistry of EGFR-KDD as it relates to pathogenesis and potential therapeutic intervention. We provide computational and experimental evidence that EGFR-KDD functions by forming asymmetric EGF-independent intra-molecular and EGF-dependent inter-molecular dimers. Time-resolved fluorescence microscopy and co-immunoprecipitation reveals EGFR-KDD can form ligand-dependent inter-molecular homo- and hetero-dimers/multimers. Furthermore, we show that inhibition of EGFR-KDD activity is maximally achieved by blocking both intra- and inter-molecular dimerization. Collectively, our findings define a previously unrecognized model of EGFR dimerization, providing important insights for the understanding of EGFR activation mechanisms and informing personalized treatment of patients with tumors harboring EGFR-KDD. Finally, we establish ERBB KDDs as recurrent oncogenic events in multiple cancers.

An EGFR mutant with kinase domain duplication (EGFR-KDD) was previously identified in an index patient, but the functional and therapeutic implications remain unclear. Here, the authors show that KDD occurs in other ErbB receptors in multiple cancers, and characterize the mechanism and inhibition of EGFR-KDD.

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.

Allosteric Inhibition of the Epidermal Growth Factor Receptor

We previously reported a family of hydrocarbon-stapled peptides designed to interact with the epidermal growth factor receptor (EGFR) juxtamembrane (JM) segment, blocking its ability to form a coiled coil dimer that is essential for receptor activation. These hydrocarbon-stapled peptides, most notably E1^S, decreased the proliferation of cell lines that express wild-type EGFR (H2030 and A431) as well as those expressing the oncogenic mutants EGFR L858R (H3255) and L858R/T790M (H1975). Although our previous investigations provided evidence that E1^S interacted with EGFR directly, the location and details of these interactions were not established. Here we apply biochemical and cross-linking mass spectrometry tools to better define the interactions between E1^S and EGFR. Taken with previously reported structure-activity relationships, our results support a model in which E1^S interacts simultaneously with both the JM and the C-lobe of the activator kinase, effectively displacing the JM of the receiver kinase. Our results also reveal potential interactions between E1^S and the N-terminal region of the C-terminal tail. We propose a model in which E1^S inhibits EGFR by both mimicking and inhibiting JM coiled coil formation. This model could be used to design novel, allosteric (and perhaps nonpeptidic) EGFR inhibitors.

The role of the calmodulin-binding and calmodulin-like domains of the epidermal growth factor receptor in tyrosine kinase activation

The epidermal growth factor receptor (EGFR) harbors a calmodulin (CaM)-binding domain (CaM-BD) and a CaM-like domain (CaM-LD) upstream and downstream, respectively, of the tyrosine kinase (TK) domain. We demonstrate in this paper that deletion of the positively charged CaM-BD (EGFR/CaM-BD∆) inactivated the TK activity of the receptor. Moreover, deletion of the negatively charged CaM-LD (EGFR/CaM-LD∆), leaving a single negative residue (glutamate), reduced the activity of the receptor. In contrast, substituting the CaM-LD with a histidine/valine-rich peptide (EGFR/InvCaM-LD) caused full inactivation. We also demonstrated using confocal microscopy and flow cytometry that the chimera EGFR-green fluorescent protein (GFP)/CaM-BD∆, the EGFR/CaM-LD∆, and EGFR/InvCaM-LD mutants all bind tetramethylrhodamine-labelled EGF. These EGFR mutants were localized at the plasma membrane as the wild-type receptor does. However, only the EGFR/CaM-LD∆ and EGFR/InvCaM-LD mutants appear to undergo ligand-dependent internalization, while the EGFR-GFP/CaM-BD∆ mutant seems to be deficient in this regard. The obtained results and in silico modelling studies of the asymmetric structure of the EGFR kinase dimer support a role of a CaM-BD/CaM-LD electrostatic interaction in the allosteric activation of the EGFR TK.

Structural studies of full-length receptor tyrosine kinases and their implications for drug design

Receptor tyrosine kinases (RTKs) are important drug targets for cancer and immunological disorders. Crystal structures of individual RTK domains have contributed greatly to the structure-based drug design of clinically used drugs. Low-resolution structures from electron microscopy are now available for the RTKs, EGFR, PDGFR, and Kit. However, there are still no high-resolution structures of full-length RTKs due to the technical challenges of working with these complex, membrane proteins. Here, we review what has been learned from structural studies of these three RTKs regarding their mechanisms of ligand binding, activation, oligomerization, and inhibition. We discuss the implications for drug design. More structural data on full-length RTKs may facilitate the discovery of druggable sites and drugs with improved specificity and effectiveness against resistant mutants.

Autophosphorylation of EGFR at Y954 Facilitated Homodimerization and Enhanced Downstream Signals

Asymmetric dimer formation of epidermal growth factor receptor (EGFR) is crucial for EGF-induced receptor activation. Even though autophosphorylation is important for activation, its role remains elusive in the context of regulating dimers. In this study, employing overlapping time series analysis to raster image correlation spectroscopy (RICS), we observed time-dependent transient dynamics of EGFR dimerization and found EGFR kinase activity to be essential for dimerization. As a result of which, we hypothesized that phosphorylation could influence dimerization. Evaluating this point, we observed that one of the tyrosine residues (Y954) located in the C-terminal lobe of the activator kinase domain was important to potentiate dimerization. Functional imaging to monitor Ca²⁺ and ERK signals revealed a significant role of Y954 in influencing downstream signaling cascade. Crucial for stabilization of EGFR asymmetric dimer is a "latch" formed between kinase domains of the binding partners. Because Y954 is positioned adjacent to the latch binding region on the kinase domain, we propose that phosphorylation strengthened the latch interaction. On the contrary, we identified that threonine phosphorylation (T669) in the latch domain negatively regulated EGFR dimerization and the downstream signals. Overall, we have delineated the previously anonymous role of phosphorylation at the latch interface of kinase domains in regulating EGFR dimerization.

Direct stimulation of ERBB2 highlights a novel cytostatic signaling pathway driven by the receptor Thr701 phosphorylation

ERBB2 is a ligand-less tyrosine kinase receptor expressed at very low levels in normal tissues; when overexpressed, it is involved in malignant transformation and tumorigenesis in several carcinomas. In cancer cells, ERBB2 represents the preferred partner of other members of the ERBB receptor family, leading to stronger oncogenic signals, by promoting both ERK and AKT activation. The identification of the specific signaling downstream of ERBB2 has been impaired by the lack of a ligand and of an efficient way to selectively activate the receptor. In this paper, we found that antibodies (Abs) targeting different epitopes on the ERBB2 extracellular domain foster the activation of ERBB2 homodimers, and surprisingly induce a unique cytostatic signaling cascade promoting an ERK-dependent ERBB2 Thr⁷⁰¹ phosphorylation, leading to AKT de-phosphorylation, via PP2A Ser/Thr phosphatases. Furthermore, the immunophilin Cyclophilin A plays a crucial role in this pathway, acting as a negative modulator of AKT de-phosphorylation, possibly by competing with Ser/Thr phosphatases for binding to AKT. Altogether, our data show that Ab recognizing ERBB2 extracellular domain function as receptor agonists, promoting ERBB2 homodimer activation, leading to an anti-proliferative signaling. Thus, the ultimate outcome of ERBB2 activity might depend on the dimerization status: pro-oncogenic in the hetero-, and anti-oncogenic in the homo-dimeric form.

Canonical ErbB-2 isoform and ErbB-2 variant c located in the nucleus drive triple negative breast cancer growth

Triple negative breast cancer (TNBC) refers to tumors that do not express clinically significant levels of estrogen and progesterone receptors, and lack membrane overexpression or gene amplification of ErbB-2/HER2, a receptor tyrosine kinase. Transcriptome and proteome heterogeneity of TNBC poses a major challenge to precision medicine. Clinical biomarkers and targeted therapies for this disease remain elusive, so chemotherapy has been the standard of care for early and metastatic TNBC. Our present findings placed ErbB-2 in an unanticipated scenario: the nucleus of TNBC (NErbB-2). Our study on ErbB-2 alternative splicing events, using a PCR-sequencing approach combined with an RNA interference strategy, revealed that TNBC cells express either the canonical (wild-type) ErbB-2, encoded by transcript variant 1, or the non-canonical ErbB-2 isoform c, encoded by alternative variant 3 (RefSeq), or both. These ErbB-2 isoforms function in the nucleus as transcription factors. Evicting both from the nucleus or silencing isoform c only, blocks TN cell and tumor growth. This reveals not only NErbB-2 canonical and alternative isoforms role as targets of therapy in TNBC, but also isoform c dominant oncogenic potential. Furthermore, we validated our findings in the clinic and observed that NErbB-2 correlates with poor prognosis in primary TN tumors, disclosing NErbB-2 as a novel biomarker for TNBC. Our discoveries challenge the present scenario of drug development for personalized BC medicine that focuses on wild-type RefSeq proteins, which conserve the canonical domains and are located in their classical cellular compartments.

Single-molecule functional anatomy of endogenous HER2-HER3 heterodimers

Human epidermal growth factor receptors (HERs) are the primary targets of many directed cancer therapies. However, the reason a specific dimer of HERs generates a stronger proliferative signal than other permutations remains unclear. Here, we used single-molecule immunoprecipitation to develop a biochemical assay for endogenously-formed, entire HER2-HER3 heterodimers. We observed unexpected, large conformational fluctuations in juxta-membrane and kinase domains of the HER2-HER3 heterodimer. Nevertheless, the individual HER2-HER3 heterodimers catalyze tyrosine phosphorylation at an unusually high rate, while simultaneously interacting with multiple copies of downstream signaling effectors. Our results suggest that the high catalytic rate and multi-tasking capability make a concerted contribution to the strong signaling potency of the HER2-HER3 heterodimers.

Receptor tyrosine kinase activation: From the ligand perspective

Receptor tyrosine kinases (RTKs) are single-span transmembrane receptors in which relatively conserved intracellular kinase domains are coupled to divergent extracellular modules. The extracellular domains initiate receptor signaling upon binding to either soluble or membrane-embedded ligands. The diversity of extracellular domain structures allows for coupling of many unique signaling inputs to intracellular tyrosine phosphorylation. The combinatorial power of this receptor system is further increased by the fact that multiple ligands can typically interact with the same receptor. Such ligands often act as biased agonists and initiate distinct signaling responses via activation of the same receptor. Mechanisms behind such biased agonism are largely unknown for RTKs, especially at the level of receptor-ligand complex structure. Using recent progress in understanding the structures of active RTK signaling units, we discuss selected mechanisms by which ligands couple receptor activation to distinct signaling outputs.

Disruption of the EGFR-SQSTM1 interaction by a stapled peptide suppresses lung cancer via activating autophagy and inhibiting EGFR signaling

Despite the success of epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) in the treatment of non-small cell lung cancer (NSCLC) harboring EGFR-activating mutations, intrinsic or acquired resistance remains the major obstacle to long-term disease remission. Defective autophagy has been reported as an EGFR-TKI resistance mechanism. However, how EGFR regulate autophagic flux are still not fully understood. Here we found that EGFR-stimulated phosphorylation of SQSTM1 at tyrosine 433 induces dimerization of its UBA domain, which disturbs the sequestration function of SQSTM1 and causes autophagic flux blocking. SAH-EJ2, a staple optimized EGFR-derived peptide, showed enhanced in vitro and in vivo antitumor activity against NSCLC than the prototype regardless of EGFR mutation status. Mechanistically, SAH-EJ2 disrupts the EGFR-SQSTM1 interaction and protects against EGFR-induced SQSTM1 phosphorylation, which hinders the dimerization of the SQSTM1 UBA domains and restores SQSTM1 cargo function. Moreover, SAH-EJ2 suppresses EGFR activity by blocking its dimerization and reducing its protein stability, which reciprocally activates the core autophagy machinery. Our observations reveal that disturbing the EGFR-SQSTM1 interaction by SAH-EJ2 confers a potential strategy in the treatment of NSCLC through suppressing EGFR signalling and activating autophagy simultaneously.

Type II cGMP-dependent protein kinase phosphorylates EGFR at threonine 669 and thereby inhibits its activation

Our previous study demonstrated that type II cGMP-dependent protein kinase (PKG II) inhibited epidermal growth factor (EGF) induced tyrosine phosphorylation/activation of the EGF receptor (EGFR). This paper was designed to investigate the mechanism of the inhibition of PKG II on EGFR activation. Gastric cancer cells HGC-27 and AGS were infected with an adenoviral vector encoding the cDNA of PKG II (Ad-PKG II) to overexpress PKG II and treated with 8-(4-chlorophenylthio) guanosine-3',5'-cyclic monophosphate (8-pCPT-cGMP) to activate the kinase. Co-immunoprecipitation (Co-IP) and bimolecular fluorescence complementation (BiFC) assay were performed to detect the interaction between PKG II and EGFR. Western blotting, mass spectrometry (MS) and site mutagenesis were performed to detect the PKG II-specific phosphorylation site on EGFR. The results showed that in living COS-7 cells, which were infected with Ad-PKG II and treated with 8-pCPT-cGMP, there was an interaction between PKG II and EGFR. The results also showed that PKG II caused threonine 669 (T669) phosphorylation of EGFR in HGC-27 and AGS cells infected with Ad-PKG II and treated with 8-pCPT-cGMP, and then inhibited the activation of EGFR. When T669 of EGFR was mutated to alanine, the inhibitory effect of PKG II on the activation of EGFR was eradicated. These findings suggested a PKG II-specific phosphorylation site on EGFR, and might be beneficial to illuminate the anti-tumor role of PKG II.

Mechanisms of Receptor Tyrosine-Protein Kinase ErbB-3 (ERBB3) Action in Human Neoplasia

It is well established that the epidermal growth factor (EGF) receptor, receptor tyrosine-protein kinase erbB-2 (ERBB2)/human EGF receptor 2 (HER2), and, to a lesser extent, ERBB4/HER4, promote the pathogenesis of many types of human cancers. In contrast, the role that ERBB3/HER3, the fourth member of the ERBB family of receptor tyrosine kinases, plays in these diseases is poorly understood and, until recently, underappreciated. In large part, this was because early structural and functional studies suggested that ERBB3 had little, if any, intrinsic tyrosine kinase activity and, thus, was unlikely to be an important therapeutic target. Since then, however, numerous publications have demonstrated an important role for ERBB3 in carcinogenesis, metastasis, and acquired drug resistance. Furthermore, somatic ERBB3 mutations are frequently encountered in many types of human cancers. Dysregulation of ERBB3 trafficking as well as cooperation with other receptor tyrosine kinases further enhance ERBB3's role in tumorigenesis and drug resistance. As a result of these advances in our understanding of the structure and biochemistry of ERBB3, and a growing focus on the development of precision and combinatorial therapeutic regimens, ERBB3 is increasingly considered to be an important therapeutic target in human cancers. In this review, we discuss the unique structural and functional features of ERBB3 and how this information is being used to develop effective new therapeutic agents that target ERBB3 in human cancers.

Emerging Allosteric Mechanism of EGFR Activation in Physiological and Pathological Contexts

Protein kinases are key players in signal transduction pathways where they are crafted into two functional states. In response to growth factor binding stimulus, epidermal growth factor receptor (EGFR), which is physiologically populated in an autoinhibited inactive state, becomes active. Here, we outline a simple allostery scheme to clarify how an extracellular (ligand-dependent) binding event activates the intracellular EGFR kinase domain via (dimer-dependent) asymmetric dimerization, as well as how pathologically overexpressed EGFR or constitutively active mutants, leads to oncogenic pathway activation. Our underlying allosteric activation mechanism derives from a collection of inactive versus active EGFR structural, biochemical (negatively cooperative ligand binding), and biophysical (weak coupling between extracellular and intracellular kinase dimerization) data. The emerging structural insight reveals that ligand-dependent physiological activation is an outside-in allosteric activation with strong structural coupling across the membrane. In contrast, ligand-independent pathological activation is a weak inside-out activation mediated by intracellular kinase dimerization, which is structurally accommodated by additional extracellular dimers.

Revisiting a controversy: The effect of EGF on EGFR dimer stability

EGFR is a receptor tyrosine kinase that plays a critical role in cell proliferation, differentiation, survival and migration. Its activating ligand, EGF, has long been believed to stabilize the EGFR dimer. Two research studies aimed at quantitative measurements of EGFR dimerization, however, have led to contradicting conclusions and have questioned this view. Given the controversy, here we sought to measure the dimerization of EGFR in the absence and in the presence of saturating EGF concentrations, and to tease out the effect of ligand on dimer stability, using a FRET-based quantitative method. Our measurements show that the dissociation constant is decreased ~150 times due to ligand binding, indicative of significant dimer stabilization. In addition, our measurements demonstrate that EGF binding induces a conformational change in the EGFR dimer.

Deciphering the Allosteric Binding Mechanism of the Human Tropomyosin Receptor Kinase A ( hTrkA) Inhibitors

Access to cryptic binding pockets or allosteric sites on a kinase that present themselves when the enzyme is in a specific conformational state offers a paradigm shift in designing the next generation small molecule kinase inhibitors. The current work showcases an extensive and exhaustive array of in vitro biochemical and biophysical tools and techniques deployed along with structural biology efforts of inhibitor-bound kinase complexes to characterize and confirm the cryptic allosteric binding pocket and docking mode of the small molecule actives identified for hTrkA. Specifically, assays were designed and implemented to lock the kinase in a predominantly active or inactive conformation and the effect of the kinase inhibitor probed to understand the hTrkA binding and hTrkB selectivity. The current outcome suggests that inhibitors with a fast association rate take advantage of the inactive protein conformation and lock the kinase state by also exhibiting a slow off-rate. This in turn shifts the inactive/active state protein conformational equilibrium cycle, affecting the subsequent downstream signaling.

A low-molecular-weight compound exerts anticancer activity against breast and lung cancers by disrupting EGFR/Eps8 complex formation

BACKGROUND

Epidermal growth factor receptor (EGFR) and epidermal growth factor receptor pathway substrate 8 (Eps8) have been widely reported to be expressed in various tumors. Eps8 is an important active kinase substrate of EGFR that directly binds to the juxtamembrane (JXM) domain of EGFR to form an EGFR/Eps8 complex. The EGFR/Eps8 complex is involved in regulating cancer progression and might be an ideal target for antitumor therapy. This study focused on the screening of small-molecule inhibitors that target the EGFR/Eps8 complex in breast cancer and non-small cell lung cancer (NSCLC).

METHODS

In silico virtual screening was used to identify small-molecule EGFR/Eps8 complex inhibitors. These compounds were screened for the inhibition of A549 and BT549 cell viability. The direct interaction between EGFR and Eps8 was measured using coimmunoprecipitation (CoIP) and JXM domain replacement assays. The antitumor effects of the inhibitors were analyzed in cancer cells and xenograft models. An acute toxicity study of EE02 was performed in a mouse model. In addition, the effect of the EE02 inhibitor on the protein expression of elements downstream of the EGFR/Eps8 complex was determined by western blotting and protein chip assays.

RESULTS

In this study of nearly 390,000 compounds screened by virtual database screening, the top 29 compounds were identified as candidate small-molecule EGFR/Eps8 complex inhibitors and evaluated by using cell-based assays. The compound EE02 was identified as the best match to our selection criteria. Further investigation demonstrated that EE02 directly bound to the JXM domain of EGFR and disrupted EGFR/Eps8 complex formation. EE02 selectively suppressed growth and induced apoptosis in EGFR-positive and Eps8-positive breast cancer and NSCLC cells. More importantly, the PI3K/Akt/mTOR and MAPK/Erk pathways downstream of the EGFR/Eps8 complex were suppressed by EE02. In addition, the suppressive effect of EE02 on tumor growth in vivo was comparable to that of erlotinib at the same dose.

CONCLUSIONS

We identified EE02 as an EGFR/Eps8 complex inhibitor that demonstrated promising antitumor effects in breast cancer and NSCLC. Our data suggest that the EGFR/Eps8 complex offers a novel cancer drug target.

The RTK Interactome: Overview and Perspective on RTK Heterointeractions

Receptor tyrosine kinases (RTKs) play important roles in cell growth, motility, differentiation, and survival. These single-pass membrane proteins are grouped into subfamilies based on the similarity of their extracellular domains. They are generally thought to be activated by ligand binding, which promotes homodimerization and then autophosphorylation in trans. However, RTK interactions are more complicated, as RTKs can interact in the absence of ligand and heterodimerize within and across subfamilies. Here, we review the known cross-subfamily RTK heterointeractions and their possible biological implications, as well as the methodologies which have been used to study them. Moreover, we demonstrate how thermodynamic models can be used to study RTKs and to explain many of the complicated biological effects which have been described in the literature. Finally, we discuss the concept of the RTK interactome: a putative, extensive network of interactions between the RTKs. This RTK interactome can produce unique signaling outputs; can amplify, inhibit, and modify signaling; and can allow for signaling backups. The existence of the RTK interactome could provide an explanation for the irreproducibility of experimental data from different studies and for the failure of some RTK inhibitors to produce the desired therapeutic effects. We argue that a deeper knowledge of RTK interactome thermodynamics can lead to a better understanding of fundamental RTK signaling processes in health and disease. We further argue that there is a need for quantitative, thermodynamic studies that probe the strengths of the interactions between RTKs and their ligands and between different RTKs.

More than the sum of the parts: Toward full-length receptor tyrosine kinase structures

Intercellular communication governs complex physiological processes ranging from growth and development to the maintenance of cellular and organ homeostasis. In nearly all metazoans, receptor tyrosine kinases (RTKs) are central players in these diverse and fundamental signaling processes. Aberrant RTK signaling is at the root of many developmental diseases and cancers and it remains a key focus of targeted therapies, several of which have achieved considerable success in patients. These therapeutic advances in targeting RTKs have been propelled by numerous genetic, biochemical, and structural studies detailing the functions and molecular mechanisms of regulation and activation of RTKs. The latter in particular have proven to be instrumental for the development of new drugs, selective targeting of mutant forms of RTKs found in disease, and counteracting ensuing drug resistance. However, to this day, such studies have not yet yielded high-resolution structures of intact RTKs that encompass the extracellular and intracellular domains and the connecting membrane-spanning transmembrane domain. Technically challenging to obtain, these structures are instrumental to complete our understanding of the mechanisms by which RTKs are activated by extracellular ligands and of the effect of pathological mutations that do not directly reside in the catalytic sites of tyrosine kinase domains. In this review, we focus on the recent progress toward obtaining such structures and the insights already gained by structural studies of the subdomains of the receptors that belong to the epidermal growth factor receptor, insulin receptor, and platelet-derived growth factor receptor RTK families. © 2019 IUBMB Life, 71(6):706-720, 2019.