Homo- and Heteroassociations Drive Activation of ErbB3

HER4 is a high-affinity dimerization partner for all EGFR/HER/ErbB family proteins

Human epidermal growth factor receptors (HER)-also known as EGFR or ErbB receptors-are a subfamily of receptor tyrosine kinases (RTKs) that play crucial roles in cell growth, division, and differentiation. HER4 (ErbB4) is the least studied member of this family, partly because its expression is lower in later stages of development. Recent work has suggested that HER4 can play a role in metastasis by regulating cell migration and invasiveness; however, unlike EGFR and HER2, the precise role that HER4 plays in tumorigenesis is still unresolved. Early work on HER family proteins suggested that there are direct interactions between the four members, but to date, there has been no single study of all four receptors in the same cell line with the same biophysical method. Here, we quantitatively measure the degree of association between HER4 and the other HER family proteins in live cells with a time-resolved fluorescence technique called pulsed interleaved excitation fluorescence cross-correlation spectroscopy (PIE-FCCS). PIE-FCCS is sensitive to the oligomerization state of membrane proteins in live cells, while simultaneously measuring single-cell protein expression levels and diffusion coefficients. Our PIE-FCCS results demonstrate that HER4 interacts directly with all HER family members in the cell plasma membrane. The interaction between HER4 and other HER family members intensified in the presence of a HER4-specific ligand. Our work suggests that HER4 is a preferred dimerization partner for all HER family proteins, even in the absence of ligands.

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.

Principles of Resonance Energy Transfer

This unit describes the basic principles of Förster resonance energy transfer (FRET). Beginning with a brief summary of the history of FRET applications, the theory of FRET is introduced in detail using figures to explain all the important parameters of the FRET process. After listing various approaches for measuring FRET efficiency, several pieces of advice are given on choosing the appropriate instrumentation. The unit concludes with a discussion of the limitations of FRET measurements followed by a few examples of the latest FRET applications, including new developments such as spectral flow cytometric FRET, single-molecule FRET, and combinations of FRET with super-resolution or lifetime imaging microscopy and with molecular dynamics simulations. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC.

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.

Dissociation of β2m from MHC class I Triggers formation of Noncovalent, transient heavy chain dimers

At the plasma membrane of mammalian cells, major histocompatibility complex class I molecules (MHC-I) present antigenic peptides to cytotoxic T cells. Following the loss of the peptide and the light chain beta-2 microglobulin (β2m), the resulting free heavy chains (FHCs) can associate into homotypic complexes in the plasma membrane. Here, we investigate the stoichiometry and dynamics of MHC-I FHCs assemblies by combining a micropattern assay with fluorescence recovery after photobleaching (FRAP) and with single molecule co-tracking. We identify non-covalent MHC-I FHC dimers mediated by the α3 domain as the prevalent species at the plasma membrane, leading a moderate decrease in the diffusion coefficient. MHC-I FHC dimers show increased tendency to cluster into higher order oligomers as concluded from an increased immobile fraction with higher single molecule co-localization. In vitro studies with isolated proteins in conjunction with molecular docking and dynamics simulations suggest that in the complexes, the α3 domain of one FHC binds to another FHC in a manner similar to the β2m light chain.

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.

Apoptosis-inducing anti-HER2 agents operate through oligomerization-induced receptor immobilization

Overexpression of the receptor tyrosine kinase HER2 plays a critical role in the development of various tumors. Biparatopic designed ankyrin repeat proteins (bipDARPins) potently induce apoptosis in HER2-addicted breast cancer cell lines. Here, we have investigated how the spatiotemporal receptor organization at the cell surface is modulated by these agents and is distinguished from other molecules, which do not elicit apoptosis. Binding of conventional antibodies is accompanied by moderate reduction of receptor mobility, in agreement with HER2 being dimerized by the bivalent IgG. In contrast, the most potent apoptosis-inducing bipDARPins lead to a dramatic arrest of HER2. Dual-color single-molecule tracking revealed that the HER2 "lockdown" by these bipDARPins is caused by the formation of HER2-DARPin oligomer chains, which are trapped in nanoscopic membrane domains. Our findings establish that efficient neutralization of receptor tyrosine kinase signaling can be achieved through intermolecular bipDARPin crosslinking alone, resulting in inactivated, locked-down bipDARPin-HER2 complexes.

Correlative Fluorescence- and Electron Microscopy of Whole Breast Cancer Cells Reveals Different Distribution of ErbB2 Dependent on Underlying Actin

Epidermal growth factor receptor 2 (ErbB2) is found overexpressed in several cancers, such as gastric, and breast cancer, and is, therefore, an important therapeutic target. ErbB2 plays a central role in cancer cell invasiveness, and is associated with cytoskeletal reorganization. In order to study the spatial correlation of single ErbB2 proteins and actin filaments, we applied correlative fluorescence microscopy (FM), and scanning transmission electron microscopy (STEM) to image specifically labeled SKBR3 breast cancer cells. The breast cancer cells were grown on microchips, transformed to express an actin-green fluorescent protein (GFP) fusion protein, and labeled with quantum dot (QD) nanoparticles attached to specific anti-ErbB2 Affibodies. FM was performed to identify cellular regions with spatially correlated actin and ErbB2 expression. For STEM of the intact plasma membrane of whole cells, the cells were fixed and covered with graphene. Spatial distribution patterns of ErbB2 in the actin rich ruffled membrane regions were examined, and compared to adjacent actin-low regions of the same cell, revealing an association of putative signaling active ErbB2 homodimers with actin-rich regions. ErbB2 homodimers were found absent from actin-low membrane regions, as well as after treatment of cells with Cytochalasin D, which breaks up larger actin filaments. In both latter data sets, a significant inter-label distance of 36 nm was identified, possibly indicating an indirect attachment to helical actin filaments via the formation of heterodimers of ErbB2 with epidermal growth factor receptor (EGFR). The possible attachment to actin filaments was further explored by identifying linear QD-chains in actin-rich regions, which also showed an inter-label distance of 36 nm.