Structure of the extracellular region of HER3 reveals an interdomain tether

HER3: Unmasking a twist in the tale of a previously unsuccessful therapeutic pursuit targeting a key cancer survival pathway

HER3, formally referred to as ERB-B2 receptor tyrosine kinase 3, is a member of the ErbB receptor tyrosine kinases (also known as EGFR) family. HER3 plays a significant pro-cancer role in various types of cancer due to its overexpression and abnormal activation, which initiates downstream signaling pathways crucial in cancer cell survival and progression. As a result, numerous monoclonal antibodies have been developed to block HER3 activation and subsequent signaling pathways. While pre-clinical investigations have effectively showcased significant anti-cancer effects of HER3-targeted therapies, these therapies have had little impact on cancer patient outcomes in the clinic, except for patients with rare NRG1 fusion mutations. This review offers a comprehensive description of the oncogenic functions of HER3, encompassing its structure and mediating signaling pathways. More importantly, it provides an in-depth exploration of past and ongoing clinical trials investigating HER3-targeted therapies for distinct types of cancer and discusses the tumor microenvironment and other critical determinants that may contribute to the observed suboptimal outcomes in most clinical studies using HER3-targeted therapies. Lastly, we suggest alternative approaches and the exploration of novel strategies to potentially improve the efficacy of targeting the pivotal oncogenic HER3 signaling pathway in future translational investigations.

Engineered-affibody conjugates contribute to the specific targeting and cellular retention of polyplexes in Erbb3 overexpressed lung cancer cells

Ligand-modified nanoparticles have shown the ability to specifically bind to tumor cells, improving retention in tumors after initial accumulation driven by the enhanced permeability and retention effect. These particles are typically engineered to bind to receptors overexpressed in cancer cells compared to healthy cells, such as the HER3 (Erbb3) receptor in lung cancer. In this study, we confirmed the overexpression of Erbb3 in various KRAS mutant lung cancer cell lines. An engineered affibody, well-established in previous research, was selected to target Erbb3 as a proof of concept. The affibody was integrated into the particle system via two distinct strategies. In the pre-functionalization approach, the affibody was conjugated to PEI or C14-PEI using SPDP as a linker. A spectral shift technique was then used to assess the affinity of the affibody and affibody conjugates toward Erbb3, allowing us to estimate the half-maximal effective concentration (EC50). Following synthesis and characterization, various polyplex formulations were prepared, including mRNA complexes with PEI-affibody, C14-PEI/PEI-affibody, and C14-PEI/C14-PEI-affibody. In the post-functionalization approach, polyplex formulations composed of different blends of C14-PEI and functionalized Azido-PEI were initially prepared and subsequently modified with DBCO-functionalized affibody via click chemistry. These formulations were prepared at various nitrogen to phosphate (N/P) ratios and characterized in terms of particle size, polydispersity index (PDI), and zeta potential. We also evaluated cellular uptake and eGFP mRNA expression to understand how the different formulations and conjugates influenced ligand-modified polyplex properties and delivery behavior. Our results demonstrated that affibody conjugates can specifically target Erbb3 and promote polyplex accumulation in KRAS-mutated lung cancer cells. We further analyzed the impact of conjugation methods and affibody density on polyplex design and performance. In conclusion, this study highlights the advantages of using specific targeting ligands. By optimizing formulation components, conjugation methods, and ligand density, various targeting ligands can be attached to polyplexes, enhancing cell-specific targeting, internalization, and retention. These findings provide valuable insights and a foundation for future targeted therapies and polyplex design.

Emerging importance of HER3 in tumorigenesis and cancer therapy

HER3 is a member of the HER/ErbB family of receptor tyrosine kinases, together with EGFR (HER1), HER2 and HER4. Despite having only weak intrinsic kinase activity, HER3 can contribute to oncogenic signalling via ligand-induced heterodimerization with other HER family members. Evidence indicates that HER3 is altered or aberrantly expressed across a variety of tumour types and can be associated with poor clinical outcomes. Whereas anticancer agents targeting EGFR and HER2 have been approved for decades, no drug targeting HER3 had been approved until very recently. Initial targeting of HER3 with monoclonal antibodies as single agents or in combination with other therapeutics produced disappointing clinical results. Subsequently, efforts have been made to target HER3 with novel agents such as antibody-drug conjugates and bispecific antibodies, with promising efficacy observed in several trials encompassing various tumour types. In December 2024, the HER3 × HER2 bispecific antibody zenocutuzumab was granted FDA Accelerated Approval for the treatment of non-small-cell lung cancers or pancreatic cancers harbouring fusions involving NRG1, the gene encoding the high-affinity HER3 ligand neuregulin 1. In this Review, we provide an essential guide to HER3 signalling and oncogenesis, HER3 expression in cancer and its prognostic implications, oncogenic HER3 somatic mutations as well as rare NRG1 fusions that might depend on HER3 signalling, and the roles of HER3 in resistance to cancer therapies. We also highlight efforts to target HER3 with diverse therapeutic strategies and the potential interplay between HER3 and the antitumour immune response.

Selective targeting of oncogenic hotspot mutations of the HER2 extracellular domain

Oncogenic mutations in the extracellular domain (ECD) of cell-surface receptors could serve as tumor-specific antigens that are accessible to antibody therapeutics. Such mutations have been identified in receptor tyrosine kinases including HER2. However, it is challenging to selectively target a point mutant, while sparing the wild-type protein. Here we developed antibodies selective to HER2 S310F and S310Y, the two most common oncogenic mutations in the HER2 ECD, via combinatorial library screening and structure-guided design. Cryogenic-electron microscopy structures of the HER2 S310F homodimer and an antibody bound to HER2 S310F revealed that these antibodies recognize the mutations in a manner that mimics the dimerization arm of HER2 and thus inhibit HER2 dimerization. These antibodies as T cell engagers selectively killed a HER2 S310F-driven cancer cell line in vitro, and in vivo as a xenograft. These results validate HER2 ECD mutations as actionable therapeutic targets and offer promising candidates toward clinical development.

HER3: Updates and current biology function, targeted therapy and pathologic detecting methods

Being a member of the EGFR tyrosine kinase family, HER3 has been shown to be overexpressed in a number of cancers, including breast cancer (BC). The kinase activity of HER3 is extremely low, and it forms heterodimers with partners, HER2 in particular, that promote biological processes like cell migration, survival, and proliferation by activating downstream carcinogenic signaling pathways. The overexpression of HER3 is also directly linked to tumor invasion, metastasis, and a poor prognosis. Despite the relatively low expression of HER3 compared to EGFR and HER2, a lot of targeted drugs are making their way into clinical trials and seem to have a bright further. This review aims to summarize the relationship between HER3 overexpression, mutations, and carcinogenicity and drug resistance, starting from the unique structure and kinase activity of HER3. Simultaneously, numerous approaches to HER3 targeted therapy are enumerated, and the clinical detection methods for HER3 that are commonly employed in pathology are sorted and contrasted to offer physicians a range of options. We think that a better knowledge of the mechanisms underlying HER3 in tumors and the advancement of targeted HER3 therapy will contribute to an improved prognosis for cancer patients and an increase in the efficacy of anticancer therapies.

Mechanisms of acquired resistance to HER2-Positive breast cancer therapies induced by HER3: A comprehensive review

Receptor tyrosine kinases (RTKs) are cell surface receptors with kinase activity that play a crucial role in diverse cellular processes. Among the RTK family members, Human epidermal growth factor receptor 2 (HER2) and HER3 are particularly relevant to breast cancer. The review delves into the complexities of receptor tyrosine kinase interactions, resistance mechanisms, and the potential of anti-HER3 drugs, offering valuable insights into the clinical implications and future directions in this field of study. It assesses the potential of anti-HER3 drugs, such as pertuzumab, in overcoming resistance observed in HER2-positive breast cancer therapies. The review also explores the resistance mechanisms associated with various drugs, including trastuzumab, lapatinib, and PI3K inhibitors, providing insights into the intricate molecular processes underlying resistance development. The review concludes by emphasizing the necessity for further clinical trials to assess the efficacy of HER3 inhibitors and the potential of developing safe and effective anti-HER3 treatments to improve treatment outcomes for patients with HER2-positive breast cancer.

HER3 V104 mutations regulate cell signaling, growth, and drug sensitivity in cancer

HER3 is mutated in ~2%-10% of cancers depending on the cancer type. We found the HER3-V104L mutation to be activating from patient-derived mutations introduced via lentiviral transduction in HER3KO HER2 + HCC1569 breast cancer cells in which endogenous HER3 was eliminated by CRISPR/Cas9. Cells expressing HER3-V104L showed higher p-HER3 and p-ERK1/2 expression versus cells expressing wild-type HER3 or HER3-V104M. Patients whose tumor expressed the HER3 V104L variant had a reduced probability of overall survival compared to patients lacking a HER3 mutation whereas we did not find a statistically significant difference in overall survival of various cancer patients with the HER3 V104M mutation. Our data showed that HER2 inhibitors suppressed cell growth of HCC1569HER3KO cells stably expressing the HER3-V104L mutation. Cancer cell lines (SNU407, UC15 and DV90) with endogenous HER3-V104M mutation showed reduced cell proliferation and p-HER2/p-ERK1/2 expression with HER2 inhibitor treatment. Knock down of HER3 abrogated cell proliferation in the above cell lines which were overall more sensitive to the ERK inhibitor SCH779284 versus PI3K inhibitors. HER3-V104L mutation stabilized HER3 protein expression in COS7 and SNUC5 cells. COS7 cells transiently transfected with the HER3-V104L mutation in the presence of HER binding partners showed higher expression of p-HER3, p-ERK1/2 versus HER3-WT in a NRG-independent manner without any change in AKT signaling. Overall, this study shows the clinical relevance of the HER3 V104L and the V104M mutations and its response to HER2, PI3K and ERK inhibitors.

HER2-CD3-Fc Bispecific Antibody-Encoding mRNA Delivered by Lipid Nanoparticles Suppresses HER2-Positive Tumor Growth

The human epidermal growth factor receptor 2 (HER2) is a transmembrane tyrosine kinase receptor and tumor-associated antigen abnormally expressed in various types of cancer, including breast, ovarian, and gastric cancer. HER2 overexpression is highly correlated with increased tumor aggressiveness, poorer prognosis, and shorter overall survival. Consequently, multiple HER2-targeted therapies have been developed and approved; however, only a subset of patients benefit from these treatments, and relapses are common. More potent and durable HER2-targeted therapies are desperately needed for patients with HER2-positive cancers. In this study, we developed a lipid nanoparticle (LNP)-based therapy formulated with mRNA encoding a novel HER2-CD3-Fc bispecific antibody (bsAb) for HER2-positive cancers. The LNPs efficiently transfected various types of cells, such as HEK293S, SKOV-3, and A1847, leading to robust and sustained secretion of the HER2-CD3-Fc bsAb with high binding affinity to both HER2 and CD3. The bsAb induced potent T-cell-directed cytotoxicity, along with secretion of IFN-λ, TNF-α, and granzyme B, against various types of HER2-positive tumor cells in vitro, including A549, NCI-H460, SKOV-3, A1847, SKBR3, and MDA-MB-231. The bsAb-mediated antitumor effect is highly specific and strictly dependent on its binding to HER2, as evidenced by the gained resistance of A549 and A1847 her2 knockout cells and the acquired sensitivity of mouse 4T1 cells overexpressing the human HER2 extracellular domain (ECD) or epitope-containing subdomain IV to the bsAb-induced T cell cytotoxicity. The bsAb also relies on its binding to CD3 for T-cell recruitment, as ablation of CD3 binding abolished the bsAb's ability to elicit antitumor activity. Importantly, intratumoral injection of the HER2-CD3-Fc mRNA-LNPs triggers a strong antitumor response and completely blocks HER2-positive tumor growth in a mouse xenograft model of human ovarian cancer. These results indicate that the novel HER2-CD3-Fc mRNA-LNP-based therapy has the potential to effectively treat HER2-positive cancer.

The Evolving Paradigm of Antibody-Drug Conjugates Targeting the ErbB/HER Family of Receptor Tyrosine Kinases

Current therapies targeting the human epidermal growth factor receptor (HER) family, including monoclonal antibodies (mAbs) and tyrosine kinase inhibitors (TKIs), are limited by drug resistance and systemic toxicities. Antibody-drug conjugates (ADCs) are one of the most rapidly expanding classes of anti-cancer therapeutics with 13 presently approved by the FDA. Importantly, ADCs represent a promising therapeutic option with the potential to overcome traditional HER-targeted therapy resistance by delivering highly potent cytotoxins specifically to HER-overexpressing cancer cells and exerting both mAb- and payload-mediated antitumor efficacy. The clinical utility of HER-targeted ADCs is exemplified by the immense success of HER2-targeted ADCs including trastuzumab emtansine and trastuzumab deruxtecan. Still, strategies to improve upon existing HER2-targeted ADCs as well as the development of ADCs against other HER family members, particularly EGFR and HER3, are of great interest. To date, no HER4-targeting ADCs have been reported. In this review, we extensively detail clinical-stage EGFR-, HER2-, and HER3-targeting monospecific ADCs as well as novel clinical and pre-clinical bispecific ADCs (bsADCs) directed against this receptor family. We close by discussing nascent trends in the development of HER-targeting ADCs, including novel ADC payloads and HER ligand-targeted ADCs.

Locally misfolded HER2 expressed on cancer cells is a promising target for development of cancer-specific antibodies

Overexpression of human epidermal growth factor receptor 2 (HER2) in breast and gastric cancers is associated with a poor prognosis, making it an important therapeutic target. Here, we establish a novel cancer-specific anti-HER2 antibody, H2Mab-214. H2Mab-214 reacts with HER2 on cancer cells, but unlike the therapeutic antibody trastuzumab, it does not react with HER2 on normal cells in flow cytometry measurements. A crystal structure suggests that H2Mab-214 recognizes a structurally disrupted region in the HER2 domain IV, which normally forms a β-sheet. We show that this misfolding is inducible by site-directed mutagenesis mimicking the disulfide bond defects that also may occur in cancer cells, indicating that the local misfolding in the Cys-rich domain IV governs the cancer-specificity of H2Mab-214. Furthermore, we show that H2Mab-214 effectively suppresses tumor growth in xenograft mouse models. Our findings offer a potential strategy for developing cancer-specific therapeutic antibodies that target partially misfolded cell surface receptors.

Site-specific glycosylation analysis of epidermal growth factor receptor 2 (ErbB2): exploring structure and function toward therapeutic targeting

Glycans found on receptor tyrosine kinases (RTKs) have emerged as promising targets for cancer chemotherapy, aiming to address issues such as drug resistance. However, to effectively select the target glycans, it is crucial to define the structure and function of candidate glycans in advance. Through mass spectrometric analysis, this study presents a "glycoform atlas" of epidermal growth factor receptor 2 (ErbB2), an RTK targeted for the treatment of ErbB2-positive cancers. Our analysis provides an in-depth and site-specific glycosylation profile, including both asparagine- and serine/threonine-linked glycosylation. Molecular dynamics simulations of N-glycosylated ErbB2 incorporating the identified glycan structures suggested that the N-glycan at N124 on the long flexible loop in the N-terminal region plays a role in stabilizing the ErbB2 structure. Based on the model structures obtained from the simulations, analysis employing an ErbB2 mutant deficient in N-glycosylation at N124 exhibited a significantly shorter intracellular half-life and suppressed autophosphorylation compared to wild-type ErbB2. Moreover, a structural comparison between the N-glycosylated forms of ErbB2 and its structurally homologous receptor, epidermal growth factor receptor (EGFR), demonstrated distinct variations in the distribution and density of N-glycans across these two molecules. These findings provide valuable insights into the structural and functional implications of ErbB2 glycosylation and will contribute to facilitating the establishment of glycan-targeted therapeutic strategies for ErbB2-positive cancers.

The chromosome-scale genome assembly of the yellowtail clownfish Amphiprion clarkii provides insights into the melanic pigmentation of anemonefish

Anemonefish are an emerging group of model organisms for studying genetic, ecological, evolutionary, and developmental traits of coral reef fish. The yellowtail clownfish Amphiprion clarkii possesses species-specific characteristics such as inter-species co-habitation, high intra-species color variation, no anemone specificity, and a broad geographic distribution, that can increase our understanding of anemonefish evolutionary history, behavioral strategies, fish-anemone symbiosis, and color pattern evolution. Despite its position as an emerging model species, the genome of A. clarkii is yet to be published. Using PacBio long-read sequencing and Hi-C chromatin capture technology, we generated a high-quality chromosome-scale genome assembly initially comprised of 1,840 contigs with an N50 of 1,203,211 bp. These contigs were successfully anchored into 24 chromosomes of 843,582,782 bp and annotated with 25,050 protein-coding genes encompassing 97.0% of conserved actinopterygian genes, making the quality and completeness of this genome the highest among all published anemonefish genomes to date. Transcriptomic analysis identified tissue-specific gene expression patterns, with the brain and optic lobe having the largest number of expressed genes. Further analyses revealed higher copy numbers of erbb3b (a gene involved in melanocyte development) in A. clarkii compared with other anemonefish, thus suggesting a possible link between erbb3b and the natural melanism polymorphism observed in A. clarkii. The publication of this high-quality genome, along with A. clarkii's many unique traits, position this species as an ideal model organism for addressing scientific questions across a range of disciplines.

CaRA - A multi-purpose phage display library for selection of calcium-regulated affinity proteins

Protein activity regulated by interactions with metal ions can be utilized for many different purposes, including biological therapies and bioprocessing, among others. Calcium ions are known to interact with the frequently occurring EF-hand motif, which can alter protein activity upon binding through an induced conformational change. The calcium-binding loop of the EF-hand motif has previously been introduced into a small protein domain derived from staphylococcal Protein A in a successful effort to render antibody binding dependent on calcium. Presented here, is a combinatorial library for calcium-regulated affinity, CaRA, based on this domain. CaRA is the first alternative scaffold library designed to achieve novel target specificities with metal-dependent binding. From this library, several calcium-dependent binders could be isolated through phage display campaigns towards a set of unrelated target proteins (IgE Cε3-Cε4, TNFα, IL23, scFv, tPA, PCSK9 and HER3) useful for distinct applications. Overall, these monomeric CaRA variants showed high stability and target affinities within the nanomolar range. They displayed considerably higher melting temperatures in the presence of 1 mM calcium compared to without calcium. Further, all discovered binders proved to be calcium-dependent, with the great majority showing complete lack of target binding in the absence of calcium. As demonstrated, the CaRA library is highly capable of providing protein-binding domains with calcium-dependent behavior, independent of the type of target protein. These binding domains could subsequently be of great use in gentle protein purification or as novel therapeutic modalities.

HER3 in cancer: from the bench to the bedside

The HER3 protein, that belongs to the ErbB/HER receptor tyrosine kinase (RTK) family, is expressed in several types of tumors. That fact, together with the role of HER3 in promoting cell proliferation, implicate that targeting HER3 may have therapeutic relevance. Furthermore, expression and activation of HER3 has been linked to resistance to drugs that target other HER receptors such as agents that act on EGFR or HER2. In addition, HER3 has been associated to resistance to some chemotherapeutic drugs. Because of those circumstances, efforts to develop and test agents targeting HER3 have been carried out. Two types of agents targeting HER3 have been developed. The most abundant are antibodies or engineered antibody derivatives that specifically recognize the extracellular region of HER3. In addition, the use of aptamers specifically interacting with HER3, vaccines or HER3-targeting siRNAs have also been developed. Here we discuss the state of the art of the preclinical and clinical development of drugs aimed at targeting HER3 with therapeutic purposes.

Cryo-electron Microscopic Analysis of Single-Pass Transmembrane Receptors

Single-pass transmembrane receptors (SPTMRs) represent a diverse group of integral membrane proteins that are involved in many essential cellular processes, including signal transduction, cell adhesion, and transmembrane transport of materials. Dysregulation of the SPTMRs is linked with many human diseases. Despite extensive efforts in past decades, the mechanisms of action of the SPTMRs remain incompletely understood. One major hurdle is the lack of structures of the full-length SPTMRs in different functional states. Such structural information is difficult to obtain by traditional structural biology methods such as X-ray crystallography and nuclear magnetic resonance (NMR). The recent rapid development of single-particle cryo-electron microscopy (cryo-EM) has led to an exponential surge in the number of high-resolution structures of integral membrane proteins, including SPTMRs. Cryo-EM structures of SPTMRs solved in the past few years have tremendously improved our understanding of how SPTMRs function. In this review, we will highlight these progresses in the structural studies of SPTMRs by single-particle cryo-EM, analyze important structural details of each protein involved, and discuss their implications on the underlying mechanisms. Finally, we also briefly discuss remaining challenges and exciting opportunities in the field.

PDGFR dimer-specific activation, trafficking and downstream signaling dynamics

Signaling through the platelet-derived growth factor receptors (PDGFRs) plays a critical role in multiple cellular processes during development. The two PDGFRs, PDGFRα and PDGFRβ, dimerize to form homodimers and/or heterodimers. Here, we overcome previous limitations in studying PDGFR dimer-specific dynamics by generating cell lines stably expressing C-terminal fusions of each PDGFR with bimolecular fluorescence complementation (BiFC) fragments corresponding to the N-terminal or C-terminal regions of the Venus fluorescent protein. We find that PDGFRβ receptors homodimerize more quickly than PDGFRα receptors in response to PDGF ligand, with increased levels of autophosphorylation. Furthermore, we demonstrate that PDGFRα homodimers are trafficked and degraded more quickly, whereas PDGFRβ homodimers are more likely to be recycled back to the cell membrane. We show that PDGFRβ homodimer activation results in a greater amplitude of phospho-ERK1/2 and phospho-AKT signaling, as well as increased proliferation and migration. Finally, we demonstrate that inhibition of clathrin-mediated endocytosis leads to changes in cellular trafficking and downstream signaling, particularly for PDGFRα homodimers. Collectively, our findings provide significant insight into how biological specificity is introduced to generate unique responses downstream of PDGFR engagement. This article has an associated First Person interview with the first author of the paper.

Regulation of Signaling from the Epidermal Growth Factor Family

The epidermal growth factor (EGF) system has allowed chemists, biologists, and clinicians to improve our understanding of cell production and cancer therapy. The discovery of EGF led to the recognition of cell surface receptors capable of controlling the proliferation and survival of cells. The detailed structures of the EGF-like ligand and the responses of their receptors (EGFR-family) has revealed the conformational and aggregation changes whereby ligands activate the intracellular kinase domains. Biophysical analysis has revealed the preformed clustering of different EGFR-family members and the processes which occur on ligand binding. Understanding these receptor activation processes and the consequential cytoplasmic signaling has allowed the development of inhibitors which are revolutionizing cancer therapy. This Review describes the recent progress in our understanding of the activation of the EGFR-family, the effects of signaling from the EGFR-family on cell proliferation, and the targeting of the EGFR-family in cancer treatment.

Critical roles for EGFR and EGFR-HER2 clusters in EGF binding of SW620 human carcinoma cells

Epidermal growth factor (EGF) signalling regulates normal epithelial and other cell growth, with EGF receptor (EGFR) overexpression reported in many cancers. However, the role of EGFR clusters in cancer and their dependence on EGF binding is unclear. We present novel single-molecule total internal reflection fluorescence microscopy of (i) EGF and EGFR in living cancer cells, (ii) the action of anti-cancer drugs that separately target EGFR and human EGFR2 (HER2) on these cells and (iii) EGFR–HER2 interactions. We selected human epithelial SW620 carcinoma cells for their low level of native EGFR expression, for stable transfection with fluorescent protein labelled EGFR, and imaged these using single-molecule localization microscopy to quantify receptor architectures and dynamics upon EGF binding. Prior to EGF binding, we observe pre-formed EGFR clusters. Unexpectedly, clusters likely contain both EGFR and HER2, consistent with co-diffusion of EGFR and HER2 observed in a different model CHO-K1 cell line, whose stoichiometry increases following EGF binding. We observe a mean EGFR : EGF stoichiometry of approximately 4 : 1 for plasma membrane-colocalized EGFR–EGF that we can explain using novel time-dependent kinetics modelling, indicating preferential ligand binding to monomers. Our results may inform future cancer drug developments.

Disentangling ERBB Signaling in Breast Cancer Subtypes-A Model-Based Analysis

Targeted therapies have shown striking success in the treatment of cancer over the last years. However, their specific effects on an individual tumor appear to be varying and difficult to predict. Using an integrative modeling approach that combines mechanistic and regression modeling, we gained insights into the response mechanisms of breast cancer cells due to different ligand-drug combinations. The multi-pathway model, capturing ERBB receptor signaling as well as downstream MAPK and PI3K pathways was calibrated on time-resolved data of the luminal breast cancer cell lines MCF7 and T47D across an array of four ligands and five drugs. The same model was then successfully applied to triple negative and HER2-positive breast cancer cell lines, requiring adjustments mostly for the respective receptor compositions within these cell lines. The additional relevance of cell-line-specific mutations in the MAPK and PI3K pathway components was identified via L1 regularization, where the impact of these mutations on pathway activation was uncovered. Finally, we predicted and experimentally validated the proliferation response of cells to drug co-treatments. We developed a unified mathematical model that can describe the ERBB receptor and downstream signaling in response to therapeutic drugs targeting this clinically relevant signaling network in cell line that represent three major subtypes of breast cancer. Our data and model suggest that alterations in this network could render anti-HER therapies relevant beyond the HER2-positive subtype.

Efficient expression, purification, and visualization by cryo-EM of unliganded near full-length HER3

Biochemical analyses of membrane receptor kinases have been limited by challenges in obtaining sufficient homogeneous receptor samples for downstream structural and biophysical characterization. Here, we report a suite of methods for the efficient expression, purification, and visualization by cryo-electron microscopy (cryo-EM) of near full-length Human Epidermal Growth Factor Receptor 3 (HER3), a receptor tyrosine pseudokinase, in the unliganded state. Through transient mammalian cell expression, a two-step purification with detergent exchange into lauryl maltose neopentyl glycol (LMNG), and freezing devoid of background detergent micelle, we obtained ~6Å reconstructions of the ~60kDa fully-glycosylated unliganded extracellular domain of HER3 from just 30mL of suspension culture. The reconstructions reveal previously unappreciated extracellular domain dynamics and glycosylation sites.

Small Molecule Compounds of Natural Origin Target Cellular Receptors to Inhibit Cancer Development and Progression

Receptors are macromolecules that transmit information regulating cell proliferation, differentiation, migration and apoptosis, play key roles in oncogenic processes and correlate with the prognoses of cancer patients. Thus, targeting receptors to constrain cancer development and progression has gained widespread interest. Small molecule compounds of natural origin have been widely used as drugs or adjuvant chemotherapeutic agents in cancer therapies due to their activities of selectively killing cancer cells, alleviating drug resistance and mitigating side effects. Meanwhile, many natural compounds, including those targeting receptors, are still under laboratory investigation for their anti-cancer activities and mechanisms. In this review, we classify the receptors by their structures and functions, illustrate the natural compounds targeting these receptors and discuss the mechanisms of their anti-cancer activities. We aim to provide primary knowledge of mechanistic regulation and clinical applications of cancer therapies through targeting deregulated receptors.

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

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

The Yin and Yang of ERBB4: Tumor Suppressor and Oncoprotein

ERBB4 (HER4) is a member of the ERBB family of receptor tyrosine kinases, a family that includes the epidermal growth factor receptor (EGFR/ERBB1/HER1), ERBB2 (Neu/HER2), and ERBB3 (HER3). EGFR and ERBB2 are oncoproteins and validated targets for therapeutic intervention in a variety of solid tumors. In contrast, the role that ERBB4 plays in human malignancies is ambiguous. Thus, here we review the literature regarding ERBB4 function in human malignancies. We review the mechanisms of ERBB4 signaling with an emphasis on mechanisms of signaling specificity. In the context of this signaling specificity, we discuss the hypothesis that ERBB4 appears to function as a tumor suppressor protein and as an oncoprotein. Next, we review the literature that describes the role of ERBB4 in tumors of the bladder, liver, prostate, brain, colon, stomach, lung, bone, ovary, thyroid, hematopoietic tissues, pancreas, breast, skin, head, and neck. Whenever possible, we discuss the possibility that ERBB4 mutants function as biomarkers in these tumors. Finally, we discuss the potential roles of ERBB4 mutants in the staging of human tumors and how ERBB4 function may dictate the treatment of human tumors. SIGNIFICANCE STATEMENT: This articles reviews ERBB4 function in the context of the mechanistic model that ERBB4 homodimers function as tumor suppressors, whereas ERBB4-EGFR or ERBB4-ERBB2 heterodimers act as oncogenes. Thus, this review serves as a mechanistic framework for clinicians and scientists to consider the role of ERBB4 and ERBB4 mutants in staging and treating human tumors.

Structures of the HER2-HER3-NRG1β complex reveal a dynamic dimer interface

Human epidermal growth factor receptor 2 (HER2) and HER3 form a potent pro-oncogenic heterocomplex^1-3 upon binding of growth factor neuregulin-1β (NRG1β). The mechanism by which HER2 and HER3 interact remains unknown in the absence of any structures of the complex. Here we isolated the NRG1β-bound near full-length HER2-HER3 dimer and, using cryo-electron microscopy, reconstructed the extracellulardomain module, revealing unexpected dynamics at the HER2-HER3 dimerization interface. We show that the dimerization arm of NRG1β-bound HER3 is unresolved because the apo HER2 monomer does not undergo a ligand-induced conformational change needed to establish a HER3 dimerization arm-binding pocket. In a structure of the oncogenic extracellular domain mutant HER2(S310F), we observe a compensatory interaction with the HER3 dimerization arm that stabilizes the dimerization interface. Both HER2-HER3 and HER2(S310F)-HER3 retain the capacity to bind to the HER2-directed therapeutic antibody trastuzumab, but the mutant complex does not bind to pertuzumab. Our structure of the HER2(S310F)-HER3-NRG1β-trastuzumab Fab complex reveals that the receptor dimer undergoes a conformational change to accommodate trastuzumab. Thus, similar to oncogenic mutations, therapeutic agents exploit the intrinsic dynamics of the HER2-HER3 heterodimer. The unique features of a singly liganded HER2-HER3 heterodimer underscore the allosteric sensing of ligand occupancy by the dimerization interface and explain why extracellular domains of HER2 do not homo-associate via a canonical active dimer interface.

Anti-EGFR Binding Nanobody Delivery System to Improve the Diagnosis and Treatment of Solid Tumours

BACKGROUND

Epidermal Growth Factor Receptor (EGFR) and members of its homologous protein family mediate transmembrane signal transduction by binding to a specific ligand, which leads to regulated cell growth, differentiation, proliferation and metastasis. With the development and application of Genetically Engineered Antibodies (GEAs), Nanobodies (Nbs) constitute a new research hot spot in many diseases. A Nb is characterized by its low molecular weight, deep tissue penetration, good solubility and high antigen-binding affinity, the anti-EGFR Nbs are of significance for the diagnosis and treatment of EGFR-positive tumours.

OBJECTIVE

This review aims to provide a comprehensive overview of the information about the molecular structure of EGFR and its transmembrane signal transduction mechanism, and discuss the anti-EGFR-Nbs influence on the diagnosis and treatment of solid tumours.

METHODS

Data were obtained from PubMed, Embase and Web of Science. All patents are searched from the following websites: the World Intellectual Property Organization (WIPO®), the United States Patent Trademark Office (USPTO®) and Google Patents.

RESULTS

EGFR is a key target for regulating transmembrane signaling. The anti-EGFR-Nbs for targeted drugs could effectively improve the diagnosis and treatment of solid tumours.

CONCLUSION

EGFR plays a role in transmembrane signal transduction. The Nbs, especially anti- EGFR-Nbs, have shown effectiveness in the diagnosis and treatment of solid tumours. How to increase the affinity of Nb and reduce its immunogenicity remain a great challenge.

The Radiolabeled HER3 Targeting Molecules for Tumor Imaging

The human epidermal growth factor receptor (HER) family plays pivotal roles in physiologic and pathologic conditions (such as tumor growth, proliferation, and progression in multiple epithelial malignancies). All the family members are considered tyrosine kinase, while HER3 as a member of this family shows no intrinsic tyrosine kinase. HER3 is called ‘pseudokinase’ because it undergoes heterodimerization and forms dimers such as HER2-HER3 and HER1 (EGFR)-HER3. The exact role of HER3 in cancer is still unclear; however, the overexpression of this receptor is involved in the poor prognosis of malignancies. To that end, different studies investigated the development of radiotracers for imaging of HER3. The main focus of this review is to gather all the studies on developing new radiotracers for imaging of HER3.

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.

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

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

Nanoscape, a data-driven 3D real-time interactive virtual cell environment

Our understanding of cellular and structural biology has reached unprecedented levels of detail, and computer visualisation techniques can be used to create three-dimensional (3D) representations of cells and their environment that are useful in both teaching and research. However, extracting and integrating the relevant scientific data, and then presenting them in an effective way, can pose substantial computational and aesthetic challenges. Here we report how computer artists, experts in computer graphics and cell biologists have collaborated to produce a tool called Nanoscape that allows users to explore and interact with 3D representations of cells and their environment that are both scientifically accurate and visually appealing. We believe that using Nanoscape as an immersive learning application will lead to an improved understanding of the complexities of cellular scales, densities and interactions compared with traditional learning modalities.

Role of Glycans on Key Cell Surface Receptors That Regulate Cell Proliferation and Cell Death

Cells undergo proliferation and apoptosis, migration and differentiation via a number of cell surface receptors, most of which are heavily glycosylated. This review discusses receptor glycosylation and the known roles of glycans on the functions of receptors expressed in diverse cell types. We included growth factor receptors that have an intracellular tyrosine kinase domain, growth factor receptors that have a serine/threonine kinase domain, and cell-death-inducing receptors. N- and O-glycans have a wide range of functions including roles in receptor conformation, ligand binding, oligomerization, and activation of signaling cascades. A better understanding of these functions will enable control of cell survival and cell death in diseases such as cancer and in immune responses.

PET and SPECT Imaging of the EGFR Family (RTK Class I) in Oncology

The human epidermal growth factor receptor family (EGFR-family, other designations: HER family, RTK Class I) is strongly linked to oncogenic transformation. Its members are frequently overexpressed in cancer and have become attractive targets for cancer therapy. To ensure effective patient care, potential responders to HER-targeted therapy need to be identified. Radionuclide molecular imaging can be a key asset for the detection of overexpression of EGFR-family members. It meets the need for repeatable whole-body assessment of the molecular disease profile, solving problems of heterogeneity and expression alterations over time. Tracer development is a multifactorial process. The optimal tracer design depends on the application and the particular challenges of the molecular target (target expression in tumors, endogenous expression in healthy tissue, accessibility). We have herein summarized the recent preclinical and clinical data on agents for Positron Emission Tomography (PET) and Single Photon Emission Tomography (SPECT) imaging of EGFR-family receptors in oncology. Antibody-based tracers are still extensively investigated. However, their dominance starts to be challenged by a number of tracers based on different classes of targeting proteins. Among these, engineered scaffold proteins (ESP) and single domain antibodies (sdAb) show highly encouraging results in clinical studies marking a noticeable trend towards the use of smaller sized agents for HER imaging.

A Novel Therapeutic Anti-ErbB3, ISU104 Exhibits Potent Antitumorigenic Activity by Inhibiting Ligand Binding and ErbB3 Heterodimerization

ErbB3, a member of the ErbB receptor family, is a potent mediator in the development and progression of cancer, and its activation plays pivotal roles in acquired resistance against anti-EGFR therapies and other standard-of-care therapies. Upon ligand (NRG1) binding, ErbB3 forms heterodimers with other ErbB proteins (i.e., EGFR and ErbB2), which allows activation of downstream PI3K/Akt signaling. In this study, we developed a fully human anti-ErbB3 antibody, named ISU104, as an anticancer agent. ISU104 binds potently and specifically to the domain 3 of ErbB3. The complex structure of ErbB3-domain 3::ISU104-Fab revealed that ISU104 binds to the NRG1 binding region of domain 3. The elucidated structure suggested that the binding of ISU104 to ErbB3 would hinder not only ligand binding but also the structural changes required for heterodimerization. Biochemical studies confirmed these predictions. ISU104 inhibited ligand binding, ligand-dependent heterodimerization and phosphorylation, and induced the internalization of ErbB3. As a result, downstream Akt phosphorylation and cell proliferation were inhibited. The anticancer efficacy of ISU104 was demonstrated in xenograft models of various cancers. In summary, a highly potent ErbB3 targeting antibody, ISU104, is suitable for clinical development.

A novel tumor inhibitory hybridoma monoclonal antibody with dual specificity for HER3 and HER2

BACKGROUND

The human epidermal growth factor receptor (HER/ErbB) family-targeted therapies result in a significant improvement in cancer immunotherapy. Monoclonal antibodies (MAb) against HER2 demonstrated a survival benefit for patients; however, drug resistance unavoidably occurs due to the overexpression of HER3, which leads to treatment failure. Effective inhibition of HER3 besides HER2 is thought to be required to overcome resistance and enhance therapeutic efficacy.

OBJECTIVE

The present study describes the production and characterization of a novel MAb, designated 1G5D2, which acts as a natural bispecific antibody targeting extracellular domains (ECD) of both HER2 and HER3.

METHODS

In this study, 1G5D2 was produced by hybridoma technology against HER3-ECD, and its structural and functional characteristics were studied by various methodologies, including enzyme linked-immunosorbent assays, flow cytometry, immunoblotting, cell signaling, and cell proliferation assays.

RESULTS

1G5D2 specifically binds to both HER2 (subdomain III + IV) and HER3 (subdomain I + II) expressed on tumor cells, and these receptors compete with each other for binding to this MAb. Competition flow cytometry experiments demonstrated that 1G5D2 does not compete with heregulin and recognizes an epitope out of HER3 ligand-binding site. Evaluation of 1G5D2 inhibitory effects in tumor cell lines co-expressing HER2 and HER3 showed that 1G5D2 synergizes with trastuzumab to inhibit both PI3K/AKT and MAPK/ERK pathways and potently downregulates the proliferation of these tumor cells more efficiently than each MAb alone.

CONCLUSION

1G5D2 is the first reported hybridoma antibody, which acts as a natural HER2/HER3 bispecific antibody. It might potentially be a suitable therapeutic candidate for HER2/HER3 overexpressing cancer types.

Thirty Years of HER3: From Basic Biology to Therapeutic Interventions

HER3 is a pseudokinase member of the EGFR family having a role in both tumor progression and drug resistance. Although HER3 was discovered more than 30 years ago, no therapeutic interventions have reached clinical approval to date. Because the evidence of the importance of HER3 is accumulating, increased amounts of preclinical and clinical trials with HER3-targeting agents are emerging. In this review article, we discuss the most recent HER3 biology in tumorigenic events and drug resistance and provide an overview of the current and emerging strategies to target HER3.

Trastuzumab Blocks the Receiver Function of HER2 Leading to the Population Shifts of HER2-Containing Homodimers and Heterodimers

HER2, a member of the Erythroblastosis Protein B/Human Epidermal Growth Factor Receptor (ErbB/HER) family of receptor tyrosine kinase, is overexpressed in 20~30% of human breast cancers. Trastuzumab, a HER2-targeted therapeutic monoclonal antibody, was developed to interfere with the homodimerization of HER2 in HER2-overexpressing breast cancer cells, which attenuates HER2-mediated signaling. Trastuzumab binds to the domain IV of the HER2 extracellular domain and does not directly block the dimerization interface of HER2-HER2 molecules. The three-dimensional structures of the tyrosine kinase domains of ErbB/HER family receptors show asymmetrical packing of the two monomers with distinct conformations. One monomer functions as an activator, whereas the other acts as a receiver. Once activated, the receiver monomer phosphorylates the activator or other proteins. Interestingly, in our previous work, we found that the binding of trastuzumab induced phosphorylation of HER2 with the phosphorylation pattern of HER2 that is different from that mediated by epidermal growth factor (EGF) in human epidermal growth factor receptor 2 (HER2)-positive breast cancer. Binding of trastuzumab to HER2 promoted an allosteric effect of HER2, in both tyrosine kinase domain and ectodomain of HER2 although details of allosteric regulation were missing. In this study, we utilized molecular dynamics (MD) simulations to model the allosteric consequences of trastuzumab binding to HER2 homodimers and heterodimers, along with the apo forms as controls. We focused on the conformational changes of HER2 in its monomeric and dimeric forms. The data indicated the apparent dual role of trastuzumab as an antagonist and an agonist. The molecular details of the simulation provide an atomic level description and molecular insight into the action of HER2-targeted antibody therapeutics.

Genotype Driven Therapy for Non-Small Cell Lung Cancer: Resistance, Pan Inhibitors and Immunotherapy

Eighty-five percent of patients with lung cancer present with Non-small Cell Lung Cancer (NSCLC). Targeted therapy approaches are promising treatments for lung cancer. However, despite the development of targeted therapies using Tyrosine Kinase Inhibitors (TKI) as well as monoclonal antibodies, the five-year relative survival rate for lung cancer patients is still only 18%, and patients inevitably become resistant to therapy. Mutations in Kirsten Ras Sarcoma viral homolog (KRAS) and epidermal growth factor receptor (EGFR) are the two most common genetic events in lung adenocarcinoma; they account for 25% and 20% of cases, respectively. Anaplastic Lymphoma Kinase (ALK) is a transmembrane receptor tyrosine kinase, and ALK rearrangements are responsible for 3-7% of NSCLC, predominantly of the adenocarcinoma subtype, and occur in a mutually exclusive manner with KRAS and EGFR mutations. Among drug-resistant NSCLC patients, nearly half exhibit the T790M mutation in exon 20 of EGFR. This review focuses on some basic aspects of molecules involved in NSCLC, the development of resistance to treatments in NSCLC, and advances in lung cancer therapy in the past ten years. Some recent developments such as PD-1-PD-L1 checkpoint-based immunotherapy for NSCLC are also covered.

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

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

Mechanistic insights into differential requirement of receptor dimerization for oncogenic activation of mutant EGFR and its clinical perspective

The epidermal growth factor receptor (EGFR), a member of the ErbB family (EGFR, ErbB2, ErbB3 and ErbB4), plays a crucial role in regulating various cellular responses such as proliferation, differentiation, and survival. As a result, aberrant activation of EGFR, mostly mediated through different classes of genomic alterations occurring within EGFR, is closely associated with the pathogenesis of numerous human cancers including lung adenocarcinoma, glioblastoma, and colorectal cancer. Thus, specific suppression of oncogenic activity of mutant EGFR with its targeted drugs has been routinely used in the clinic as a very effective anti-cancer strategy in treating a subset of tumors driven by such oncogenic EGFR mutants. However, the clinical efficacy of EGFR-targeted therapy does not last long due to several resistance mechanisms that emerge in the patients following the drug treatment. Thus, there is an urgent need for the development of novel therapeutic tactics specifically targeting mutant EGFR with the focus on the unique biological features of various mutant EGFR. Regarding this point, our review specifically emphasizes the recent findings about distinct requirements of receptor dimerization and autophosphorylation, which are critical steps for enzymatic activation of EGFR and signaling cascades, respectively, among wildtype and mutant EGFR and further discuss their clinical significance. In addition, the molecular mechanisms regulating EGFR dimerization and enzymatic activity by a key negative feedback inhibitor Mig6 as well as the clinical use for developing potential novel drugs targeting it are described in this review.

Structural Perspectives on Extracellular Recognition and Conformational Changes of Several Type-I Transmembrane Receptors

Type-I transmembrane proteins represent a large group of 1,412 proteins in humans with a multitude of functions in cells and tissues. They are characterized by an extracellular, or luminal, N-terminus followed by a single transmembrane helix and a cytosolic C-terminus. The domain composition and structures of the extracellular and intercellular segments differ substantially amongst its members. Most of the type-I transmembrane proteins have roles in cell signaling processes, as ligands or receptors, and in cellular adhesion. The extracellular segment often determines specificity and can control signaling and adhesion. Here we focus on recent structural understanding on how the extracellular segments of several diverse type-I transmembrane proteins engage in interactions and can undergo conformational changes for their function. Interactions at the extracellular side by proteins on the same cell or between cells are enhanced by the transmembrane setting. Extracellular conformational domain rearrangement and structural changes within domains alter the properties of the proteins and are used to regulate signaling events. The combination of structural properties and interactions can support the formation of larger-order assemblies on the membrane surface that are important for cellular adhesion and intercellular signaling.

EGFR forms ligand-independent oligomers that are distinct from the active state

The human epidermal growth factor receptor (EGFR/ERBB1) is a receptor tyrosine kinase (RTK) that forms activated oligomers in response to ligand. Much evidence indicates that EGFR/ERBB1 also forms oligomers in the absence of ligand, but the structure and physiological role of these ligand-independent oligomers remain unclear. To examine these features, we use fluorescence microscopy to measure the oligomer stability and FRET efficiency for homo- and hetero-oligomers of fluorescent protein-labeled forms of EGFR and its paralog, human epidermal growth factor receptor 2 (HER2/ERBB2) in vesicles derived from mammalian cell membranes. We observe that both receptors form ligand-independent oligomers at physiological plasma membrane concentrations. Mutations introduced in the kinase region at the active state asymmetric kinase dimer interface do not affect the stability of ligand-independent EGFR oligomers. These results indicate that ligand-independent EGFR oligomers form using interactions that are distinct from the EGFR active state.

Analysis of expression of candidate genes for polycystic ovary syndrome in adult and fetal human and fetal bovine ovaries†

Polycystic ovary syndrome (PCOS) appears to have a genetic predisposition and a fetal origin. We compared the expression levels of 25 PCOS candidate genes from adult control and PCOS human ovaries (n = 16) using microarrays. Only one gene was potentially statistically different. Using qRT-PCR, expression of PCOS candidate genes was examined in bovine fetal ovaries from early stages when they first developed stroma through to completion of development (n = 27; 60-270 days of gestation). The levels of ERBB3 mRNA negatively correlated with gestational age but positively with HMGA2, FBN3, TOX3, GATA4, and DENND1A.X1,2,3,4, previously identified as correlated with each other and expressed early. PLGRKT and ZBTB16, and less so IRF1, were also correlated with AMH, FSHR, AR, INSR, and TGFB1I1, previously identified as correlated with each other and expressed late. ARL14EP, FDFT1, NEIL2, and MAPRE1 were expressed across gestation and not correlated with gestational age as shown previously for THADA, ERBB4, RAD50, C8H9orf3, YAP1, RAB5B, SUOX, and KRR1. LHCGR, because of its unusual bimodal expression pattern, had some unusual correlations with other genes. In human ovaries (n = 15; <150 days of gestation), ERBB3.V1 and ERBB3.VS were expressed and correlated negatively with gestational age and positively with FBN3, HMGA2, DENND1A.V1,3,4, DENND1A.V1-7, GATA4, and FSHR, previously identified as correlated with each other and expressed early. Thus, the general lack of differential expression of candidate genes in adult ovaries contrasting with dynamic patterns of gene expression in fetal ovaries is consistent with a vulnerability to disturbance in the fetal ovary that may underpin development of PCOS.

Roles for receptor tyrosine kinases in tumor progression and implications for cancer treatment

Growth factors and their receptor tyrosine kinases (RTKs), a group of transmembrane molecules harboring cytoplasm-facing tyrosine-specific kinase functions, play essential roles in migration of multipotent cell populations and rapid proliferation of stem cells' descendants, transit amplifying cells, during embryogenesis and tissue repair. These intrinsic functions are aberrantly harnessed when cancer cells undergo intertwined phases of cell migration and proliferation during cancer progression. For example, by means of clonal expansion growth factors fixate the rarely occurring driver mutations, which initiate tumors. Likewise, autocrine and stromal growth factors propel angiogenesis and penetration into the newly sprouted vessels, which enable seeding micro-metastases at distant organs. We review genetic and other mechanisms that preempt ligand-mediated activation of RTKs, thereby supporting sustained cancer progression. The widespread occurrence of aberrant RTKs and downstream signaling pathways in cancer, identifies molecular targets suitable for pharmacological intervention. We list all clinically approved cancer drugs that specifically intercept oncogenic RTKs. These are mainly tyrosine kinase inhibitors and monoclonal antibodies, which can inhibit cancer but inevitably become progressively less effective due to adaptive rewiring processes or emergence of new mutations, processes we overview. Similarly important are patient treatments making use of radiation, chemotherapeutic agents and immune checkpoint inhibitors. The many interfaces linking RTK-targeted therapies and these systemic or local regimens are described in details because of the great promise offered by combining pharmacological modalities.

HER family in cancer progression: From discovery to 2020 and beyond

The human epidermal growth factor receptor (HER) family of receptor tyrosine kinases (RTKs) are among the first layer of molecules that receive, interpret, and transduce signals leading to distinct cancer cell phenotypes. Since the discovery of the tooth-lid factor-later characterized as the epidermal growth factor (EGF)-and its high-affinity binding EGF receptor, HER kinases have emerged as one of the commonly upregulated or hyperactivated or mutated kinases in epithelial tumors, thus allowing HER1-3 family members to regulate several hallmarks of cancer development and progression. Each member of the HER family exhibits shared and unique structural features to engage multiple receptor activation modes, leading to a range of overlapping and distinct phenotypes. EGFR, the founding HER family member, provided the roadmap for the development of the cell surface RTK-directed targeted cancer therapy by serving as a prototype/precursor for the currently used HER-directed cancer drugs. We herein provide a brief account of the discoveries, defining moments, and historical context of the HER family and guidepost advances in basic, translational, and clinical research that solidified a prominent position of the HER family in cancer research and treatment. We also discuss the significance of HER3 pseudokinase in cancer biology; its unique structural features that drive transregulation among HER1-3, leading to a superior proximal signaling response; and potential role of HER3 as a shared effector of acquired therapeutic resistance against diverse oncology drugs. Finally, we also narrate some of the current drawbacks of HER-directed therapies and provide insights into postulated advances in HER biology with extensive implications of these therapies in cancer research and treatment.

Insulin and epidermal growth factor receptor family members share parallel activation mechanisms

Insulin receptor (IR) and the epidermal growth factor receptor (EGFR) were the first receptor tyrosine kinases (RTKs) to be studied in detail. Both are important clinical targets-in diabetes and cancer, respectively. They have unique extracellular domain compositions among RTKs, but share a common module with two ligand-binding leucine-rich-repeat (LRR)-like domains connected by a flexible cysteine-rich (CR) domain (L1-CR-L2 in IR/domain, I-II-III in EGFR). This module is linked to the transmembrane region by three fibronectin type III domains in IR, and by a second CR in EGFR. Despite sharing this conserved ligand-binding module, IR and EGFR family members are considered mechanistically distinct-in part because IR is a disulfide-linked (αβ)₂ dimer regardless of ligand binding, whereas EGFR is a monomer that undergoes ligand-induced dimerization. Recent cryo-electron microscopy (cryo-EM) structures suggest a way of unifying IR and EGFR activation mechanisms and origins of negative cooperativity. In EGFR, ligand engages both LRRs in the ligand-binding module, "closing" this module to break intramolecular autoinhibitory interactions and expose new dimerization sites for receptor activation. How insulin binds the activated IR was less clear until now. Insulin was known to associate with one LRR (L1), but recent cryo-EM structures suggest that it also engages the second LRR (albeit indirectly) to "close" the L1-CR-L2 module, paralleling EGFR. This transition simultaneously breaks autoinhibitory interactions and creates new receptor-receptor contacts-remodeling the IR dimer (rather than inducing dimerization per se) to activate it. Here, we develop this view in detail, drawing mechanistic links between IR and EGFR.

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.

Inhibitory Effect of Polyclonal Antibodies Against HER3 Extracellular Subdomains on Breast Cancer Cell Lines

Objective:

Human epidermal growth factor receptor 3 (HER3) is a unique member of the tyrosine kinase receptors with an inactive kinase domain and is the preferable dimerization partner for HER2 which lead to potent tumorigenic signaling.

Methods:

In this study, the expression plasmids coding for the human HER3 subdomains were transfected into CHO-K1 cells. Produced proteins were characterized by ELISA and SDS-PAGE. Rabbits were immunized and produced polyclonal antibodies (pAbs) that were characterized by ELISA, Immunoblotting and flowcytometry and their inhibitory effects were assessed by XTT on BT-474 and JIMT-1 breast cancer cell lines.

Result:

The recombinant subdomains were highly immunogenic in rabbits. The pAbs reacted with the recombinant subdomains as well as commercial HER3 and the native receptor on tumor cell membranes and could significantly inhibit growth of Trastuzumab sensitive (BT-474) and resistant (JIMT-1) breast cancer cell lines in vitro.

Conclusion:

It seems that HER3 extra cellular domains (ECD) induce a strong anti-tumor antibody response and may prove to be potentially useful for immunotherapeutic applications.

Structural basis for adhesion G protein-coupled receptor Gpr126 function

Many drugs target the extracellular regions (ECRs) of cell-surface receptors. The large and alternatively-spliced ECRs of adhesion G protein-coupled receptors (aGPCRs) have key functions in diverse biological processes including neurodevelopment, embryogenesis, and tumorigenesis. However, their structures and mechanisms of action remain unclear, hampering drug development. The aGPCR Gpr126/Adgrg6 regulates Schwann cell myelination, ear canal formation, and heart development; and GPR126 mutations cause myelination defects in human. Here, we determine the structure of the complete zebrafish Gpr126 ECR and reveal five domains including a previously unknown domain. Strikingly, the Gpr126 ECR adopts a closed conformation that is stabilized by an alternatively spliced linker and a conserved calcium-binding site. Alternative splicing regulates ECR conformation and receptor signaling, while mutagenesis of the calcium-binding site abolishes Gpr126 function in vivo. These results demonstrate that Gpr126 ECR utilizes a multi-faceted dynamic approach to regulate receptor function and provide relevant insights for ECR-targeted drug design.

The extracellular regions (ECRs) of adhesion GPCRs have diverse biological functions, but their structures and mechanisms of action remain unclear. Here, the authors solve the ECR structure of the Gpr126 receptor and show that ECR conformation and signaling functions are regulated by alternative splicing.

10D1F, an Anti-HER3 Antibody that Uniquely Blocks the Receptor Heterodimerization Interface, Potently Inhibits Tumor Growth Across a Broad Panel of Tumor Models

In recent years, HER3 has increasingly been implicated in the progression of a variety of tumor types and in acquired resistance to EGFR and HER2 therapies. Whereas EGFR and HER2 primarily signal through the MAPK pathway, HER3, as a heterodimer with EGFR or HER2, potently activates the PI3K pathway. Despite its critical role, previous attempts to target HER3 with neutralizing antibodies have shown disappointing efficacy in the clinic, most likely due to suboptimal and indirect mechanisms of action that fail to completely block heterodimerization; for example, tumors can escape inhibition of ligand binding by upregulating ligand-independent mechanisms of HER3 activation. We therefore developed 10D1F, a picomolar affinity, highly specific anti-HER3 neutralizing antibody that binds the HER3 heterodimerization interface, a region that was hitherto challenging to raise antibodies against. We demonstrate that 10D1F potently inhibits both EGFR:HER3 and HER2:HER3 heterodimerization to durably suppress activation of the PI3K pathway in a broad panel of tumor models. Even as a monotherapy, 10D1F shows superior inhibition of tumor growth in the same cell lines both in vitro and in mouse xenograft experiments, when compared with other classes of anti-HER3 antibodies. This includes models demonstrating ligand-independent activation of heterodimerization as well as constitutively activating mutations in the MAPK pathway. Possessing favorable pharmacokinetic and toxicologic profiles, 10D1F uniquely represents a new class of anti-HER3 neutralizing antibodies with a novel mechanism of action that offers significant potential for broad clinical benefit.10D1F is a novel anti-HER3 antibody that uniquely binds the receptor dimerization interface to block ligand-dependent and independent heterodimerization with EGFR/HER2 and thus more potently inhibits tumor growth than existing anti-HER3 antibodies.