The extracellular domains of ErbB3 retain high ligand binding affinity at endosome pH and in the locked conformation

Extensive conformational and physical plasticity protects HER2-HER3 tumorigenic signaling

Surface-targeting biotherapeutic agents have been successful in treating HER2-amplified cancers through immunostimulation or chemodelivery but have failed to produce effective inhibitors of constitutive HER2-HER3 signaling. We report an extensive structure-function analysis of this tumor driver, revealing complete uncoupling of intracellular signaling and tumorigenic function from regulation or constraints from their extracellular domains (ECDs). The canonical HER3 ECD conformational changes and exposure of the dimerization interface are nonessential, and the entire ECDs of HER2 and HER3 are redundant for tumorigenic signaling. Restricting the proximation of partner ECDs with bulk and steric clash through extremely disruptive receptor engineering leaves tumorigenic signaling unperturbed. This is likely due to considerable conformational flexibilities across the span of these receptor molecules and substantial undulations in the plane of the plasma membrane, none of which had been foreseen as impediments to targeting strategies. The massive overexpression of HER2 functionally and physically uncouples intracellular signaling from extracellular constraints.

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.

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.

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

Background

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

Methods

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

Results

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

Conclusion

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

Recruitment of the adaptor protein Grb2 to EGFR tetramers

Adaptor protein Grb2 binds phosphotyrosines in the epidermal growth factor (EGF) receptor (EGFR) and thereby links receptor activation to intracellular signaling cascades. Here, we investigated how recruitment of Grb2 to EGFR is affected by the spatial organization and quaternary state of activated EGFR. We used the techniques of image correlation spectroscopy (ICS) and lifetime-detected Förster resonance energy transfer (also known as FLIM-based FRET or FLIM–FRET) to measure ligand-induced receptor clustering and Grb2 binding to activated EGFR in BaF/3 cells. BaF/3 cells were stably transfected with fluorescently labeled forms of Grb2 (Grb2–mRFP) and EGFR (EGFR–eGFP). Following stimulation of the cells with EGF, we detected nanometer-scale association of Grb2–mRFP with EGFR–eGFP clusters, which contained, on average, 4 ± 1 copies of EGFR–eGFP per cluster. In contrast, the pool of EGFR–eGFP without Grb2–mRFP had an average cluster size of 1 ± 0.3 EGFR molecules per punctum. In the absence of EGF, there was no association between EGFR–eGFP and Grb2–mRFP. To interpret these data, we extended our recently developed model for EGFR activation, which considers EGFR oligomerization up to tetramers, to include recruitment of Grb2 to phosphorylated EGFR. The extended model, with adjustment of one new parameter (the ratio of the Grb2 and EGFR copy numbers), is consistent with a cluster size distribution where 2% of EGFR monomers, 5% of EGFR dimers, <1% of EGFR trimers, and 94% of EGFR tetramers are associated with Grb2. Together, our experimental and modeling results further implicate tetrameric EGFR as the key signaling unit and call into question the widely held view that dimeric EGFR is the predominant signaling unit.

An antibody that locks HER3 in the inactive conformation inhibits tumor growth driven by HER2 or neuregulin

HER2/HER3 dimerization resulting from overexpression of HER2 or neuregulin (NRG1) in cancer leads to HER3-mediated oncogenic activation of phosphoinositide 3-kinase (PI3K) signaling. Although ligand-blocking HER3 antibodies inhibit NRG1-driven tumor growth, they are ineffective against HER2-driven tumor growth because HER2 activates HER3 in a ligand-independent manner. In this study, we describe a novel HER3 monoclonal antibody (LJM716) that can neutralize multiple modes of HER3 activation, making it a superior candidate for clinical translation as a therapeutic candidate. LJM716 was a potent inhibitor of HER3/AKT phosphorylation and proliferation in HER2-amplified and NRG1-expressing cancer cells, and it displayed single-agent efficacy in tumor xenograft models. Combining LJM716 with agents that target HER2 or EGFR produced synergistic antitumor activity in vitro and in vivo. In particular, combining LJM716 with trastuzumab produced a more potent inhibition of signaling and cell proliferation than trastuzumab/pertuzumab combinations with similar activity in vivo. To elucidate its mechanism of action, we solved the structure of LJM716 bound to HER3, finding that LJM716 bound to an epitope, within domains 2 and 4, that traps HER3 in an inactive conformation. Taken together, our findings establish that LJM716 possesses a novel mechanism of action that, in combination with HER2- or EGFR-targeted agents, may leverage their clinical efficacy in ErbB-driven cancers.

Targeting ErbB3: the New RTK(id) on the Prostate Cancer Block

Most prostate cancers (PCa) are critically reliant on functional androgen receptor (AR) signaling. At its onset, PCa is androgen-dependent and although temporarily halted by surgically or pharmacologically blocking the AR (androgen ablation), the disease ultimately recurs as an aggressive, fatal castration resistant prostate cancer (CRPC). FDA-approved treatments like docetaxel, a chemotherapeutic agent, and Provenge, a cancer vaccine, extend survival by a scant 3 and 4 months, respectively. It is clear that more effective drugs targeting CRPC are urgently needed. The ErbB family (EGFR/ErbB1, ErbB2/HER2/neu, ErbB3/HER3 and ErbB4/HER4) of receptor tyrosine kinases (RTKs) have long been implicated in PCa initiation and progression, but inhibitors of ErbB1 and ErbB2 (prototypic family members) fared poorly in PCa clinical trials. Recent research suggests that another family member ErbB3 abets emergence of the castration-resistant phenotype. Considerable efforts are being directed towards understanding ErbB3-mediated molecular mechanisms of castration resistance and searching for novel ways of inhibiting ErbB3 activity via rational drug design. Antibody-based therapy that prevents ligand binding to ErbB3 appears promising and fully-humanized antibodies that inhibit ligand-induced phosphorylation of ErbB3 are currently in early development. Small molecule tyrosine kinase inhibitors are also being vigorously pursued, as are siRNA-based approaches and combination treatment strategies- the simultaneous suppression of ErbB3 and its signaling partners or downstream effectors - with the primary purpose of undermining the resiliency of ErbB3-mediated signal transduction. This review summarizes the existing literature and reinforces the importance of ErbB3 as a therapeutic target in the clinical management of prostate cancer.

Structural analysis of the catalytically inactive kinase domain of the human EGF receptor 3

The kinase domain of human epidermal growth factor receptor (HER) 3/ErbB3, a member of the EGF receptor (EGFR) family, lacks several residues that are critical for catalysis. Because catalytic activity in EGFR family members is switched on by an allosteric interaction between kinase domains in an asymmetric kinase domain dimer, HER3 might be specialized to serve as an activator of other EGFR family members. We have determined the crystal structure of the HER3 kinase domain and show that it appears to be locked into an inactive conformation that resembles that of EGFR and HER4. Although the crystal structure shows that the HER3 kinase domain binds ATP, we confirm that it is catalytically inactive but can serve as an activator of the EGFR kinase domain. The HER3 kinase domain forms a dimer in the crystal, mediated by hydrophobic contacts between the N-terminal lobes of the kinase domains. This N-lobe dimer closely resembles a dimer formed by inactive HER4 kinase domains in crystal structures determined previously, and molecular dynamics simulations suggest that the HER3 and HER4 N-lobe dimers are stable. The kinase domains of HER3 and HER4 form similar chains in their respective crystal lattices, in which N-lobe dimers are linked together by reciprocal exchange of C-terminal tails. The conservation of this tiling pattern in HER3 and HER4, which is the closest evolutionary homolog of HER3, might represent a general mechanism by which this branch of the HER receptors restricts ligand-independent formation of active heterodimers with other members of the EGFR family.

Rational optimization of a bispecific ligand trap targeting EGF receptor family ligands

The human epidermal growth factor (EGF) receptor (HER) family members cooperate in malignancy. Of this family, HER2 does not bind growth factors and HER3 does not encode an active tyrosine kinase. This diversity creates difficulty in creating pan-specific therapeutic HER family inhibitors. We have identified single amino acid changes in epidermal growth factor receptor (EGFR) and HER3 which create high affinity sequestration of the cognate ligands, and may be used as receptor decoys to downregulate aberrant HER family activity. In silico modeling and high throughput mutagenesis were utilized to identify receptor mutants with very high ligand binding activity. A single mutation (T15S; EGFR subdomain I) enhanced affinity for EGF (two-fold), TGF-alpha (twenty-six-fold), and heparin-binding (HB)-EGF (six-fold). This indicates that T15 is an important, previously undescribed, negative regulatory amino acid for EGFR ligand binding. Another mutation (Y246A; HER 3 subdomain II) enhanced neuregulin (NRG)1-beta binding eight-fold, probably by interfering with subdomain II-IV interactions. Further work revealed that the HER3 subunit of an EGFR:HER3 heterodimer suppresses EGFR ligand binding. Optimization required reversing this suppression by mutation of the EGFR tether domain (G564A; subdomain IV). This mutation resulted in enhanced ligand binding (EGF, ten-fold; TGF-alpha, thirty-four-fold; HB-EGF, seventeen-fold; NRG1-beta, thirty-one-fold). This increased ligand binding was reflected in improved inhibition of in vitro tumor cell proliferation and tumor suppression in a human non-small cell lung cancer xenograft model. In conclusion, amino acid substitutions were identified in the EGFR and HER3 ECDs that enhance ligand affinity, potentially enabling a pan-specific therapeutic approach for downregulating the HER family in cancer.

Higher-order association states of cellular ERBB3 probed with photo-cross-linkable aptamers

Nucleic acid aptamers are rapidly gaining prominence as diagnostic tools, targeting reagents, and potential therapeutics. To extend the use of aptamers into the biochemical analysis of protein interactions on the surface of live cells, we converted an enzymatically generated RNA aptamer into a photo-cross-linkable affinity tag through the replacement of all uracils with 4-thiouracil. Specifically, we converted a previously selected, inhibitory aptamer that binds the soluble extracellular domains of the ERBB3 receptor into a targeted and highly specific cross-linking reagent in a live cell setting. Since the photo-cross-linkable aptamer has two functionalities, targeted and highly selective as well as unspecific cross-linking capability, the attachment of this inhibitory aptamer converts ERBB3 into a passive and signaling incompetent probe of its immediate receptor environment. This approach detects receptor clustering of endogenous ERBB3 in the breast cancer cell line MCF7 at levels as low as 25000 receptors per cell and at aptamer concentrations as low as 20 nM. Our analysis also indicates that ERBB3 receptors are apparently segregated from ERBB2 receptors in their resting state, and both ligand-activated ERBB3 and ERBB2 do not share the same microenvironment as inactive ERBB3.

The ERBB3 receptor in cancer and cancer gene therapy

ERBB3, a member of the epidermal growth factor receptor (EGFR) family, is unique in that its tyrosine kinase domain is functionally defective. It is activated by neuregulins, by other ERBB and nonERBB receptors as well as by other kinases, and by novel mechanisms. Downstream it interacts prominently with the phosphoinositol 3-kinase/AKT survival/mitogenic pathway, but also with GRB, SHC, SRC, ABL, rasGAP, SYK and the transcription regulator EBP1. There are likely important but poorly understood roles for nuclear localization and for secreted isoforms. Studies of ERBB3 expression in primary cancers and of its mechanistic contributions in cultured cells have implicated it, with varying degrees of certainty, with causation or sustenance of cancers of the breast, ovary, prostate, certain brain cells, retina, melanocytes, colon, pancreas, stomach, oral cavity and lung. Recent results link high ERBB3 activity with escape from therapy targeting other ERBBs in lung and breast cancers. Thus a wide and centrally important role for ERBB3 in cancer is becoming increasingly apparent. Several approaches for targeting ERBB3 in cancers have been tested or proposed. Small inhibitory RNA (siRNA) to ERBB3 or AKT is showing promise as a therapeutic approach to treatment of lung adenocarcinoma.

Endocytic downregulation of ErbB receptors: mechanisms and relevance in cancer

ErbB receptors (EGFR (ErbB1), ErbB2, ErbB3, and ErbB4) are important regulators of normal growth and differentiation, and they are involved in the pathogenesis of cancer. Following ligand binding and receptor activation, EGFR is endocytosed and transported to lysosomes where the receptor is degraded. This downregulation of EGFR is a complex and tightly regulated process. The functions of ErbB2, ErbB3, and ErbB4 are also regulated by endocytosis to some extent, although the current knowledge of these processes is sparse. Impaired endocytic downregulation of signaling receptors is frequently associated with cancer, since it can lead to increased and uncontrolled receptor signaling. In this review we describe the current knowledge of ErbB receptor endocytic downregulation. In addition, we outline how ErbB receptors can escape endocytic downregulation in cancer, and we discuss how targeted anti-cancer therapy may induce endocytic downregulation of ErbB receptors.

HER2 therapy. HER2 (ERBB2): functional diversity from structurally conserved building blocks

EGFR-type receptor tyrosine kinases achieve a broad spectrum of cellular responses by utilizing a set of structurally conserved building blocks. Based on available crystal structures and biochemical information, significant new insights have emerged into modes of receptor control, its deregulation in cancer, and the nuances that differentiate the four human receptors. This review gives an overview of current models of the control of receptor activity with a special emphasis on HER2 and HER3.

Signaling through ERBB receptors: Multiple layers of diversity and control

The four known ERBB receptors in humans are involved in a broad range of cellular responses, and their deregulation is a significant aspect in a large number of disease states. However, their mechanism of action and modes of control are still poorly understood. This is largely due to the fact that the control of ERBB activity is a multilayered process with significant differences between the various ERBB members. In contrast to other receptor tyrosine kinases, the kinase domain of EGFR (ERBB1) does not require phosphorylation for activation. Consequently, the overall activation state of the receptor is controlled by constant balancing of activity favoring and activity suppressing actions within the receptor molecule. Influences of the membrane microenvironment and context dependent interactions with varying sets of signaling partners are superimposed on this system of intramolecular checks and balances. We will discuss current models of the control of ERBB signaling with an emphasis on the multilayered nature of activation control and aspects that give rise to diversity between ERBB receptors.

Enhanced insulin-like growth factor-I (IGF-I) cell association at reduced pH is dependent on IGF binding protein-3 (IGFBP-3) interaction

The cellular microenvironment impacts how signals are transduced by cells and plays a key role in tissue homeostasis. Although pH is generally well regulated, there are a number of situations where acidosis occurs and our work addresses how low pH impacts cell association of insulin-like growth factor-I (IGF-I) in the presence of IGF binding protein-3 (IGFBP-3). We have previously shown that IGF-I cell binding was enhanced in the presence of IGFBP-3 at low pH and now show that this binding is IGFBP-mediated as it is inhibited by Y60L-IGF-I, a mutant with reduced affinity for the IGF receptor (IGF-IR), and unaffected by insulin, which binds but not IGFBPs. Using surface plasmon resonance (SPR), we show that direct binding between IGF-I and IGFBP-3 is pH sensitive. Despite this, the key step in the process appears to be IGFBP-3 cell surface association as Long-R(3)-IGF-I, a mutant with reduced affinity for IGFBPs, shows a similar increase in cell association at pH 5.8 in the presence of IGFBP-3 but does not exhibit pH-dependent binding by SPR. Further, analysis indicates a large increase in low-affinity binding sites for IGF-I in the presence of IGFBP-3 and an elimination of IGF-I enhanced binding when a non-cell associating mutant of IGFBP-3 is added in place of IGFBP-3. That the IGFBP-3-mediated binding localizes IGF-I away from IGF-IR is suggested by triton-solubility testing and indicates additional complexities to IGF-I regulation by IGFBP-3. Identifying the pH-dependent binding partner(s) for IGFBP-3 is a necessary next step in deciphering this process.

Survey of the year 2005 commercial optical biosensor literature

We identified 1113 articles (103 reviews, 1010 primary research articles) published in 2005 that describe experiments performed using commercially available optical biosensors. While this number of publications is impressive, we find that the quality of the biosensor work in these articles is often pretty poor. It is a little disappointing that there appears to be only a small set of researchers who know how to properly perform, analyze, and present biosensor data. To help focus the field, we spotlight work published by 10 research groups that exemplify the quality of data one should expect to see from a biosensor experiment. Also, in an effort to raise awareness of the common problems in the biosensor field, we provide side-by-side examples of good and bad data sets from the 2005 literature.