Active and inactive conformations of the epidermal growth factor receptor

Development of 177Lu-Cetuximab-PAMAM dendrimeric nanosystem: a novel theranostic radioimmunoconjugate

PURPOSE

Epidermal growth factor receptors (EGFRs) are overexpressed in a wide range of tumors and are attractive candidates to target in targeted therapies. This study aimed to introduce a novel radiolabeled compound, 177Lu-cetuximab-PAMAM G4, for the treatment of EGFR-expressing tumors.

METHODS

In this study, the cetuximab mAb was bound to PAMAM G4 and labeled with 177Lu via DTPA-CHX chelator. The synthesized nanosystem was confirmed by different analyses such as DLS, FT-IR, TEM, and RT-LC. Cell viability of the radioimmunoconjugate was assessed over the EGFR-expressing cell line of SW480. The biodistribution of 177Lu-Cetuximab-PAMAMG4 was determined in different intervals after injection of the radiolabeled compound in normal and tumoral nude mice via scarification and SPECT images.

RESULTS

The average size of PAMAM G4 and PAMAM-Cetuximab-DTPA-CHX nanoparticles were 2 and 70 nm, respectively. 177Lu-Cetuximab-PAMAMG4 was prepared with radiochemical purity of more than 98%. The survival rates of SW480 cells at 24, 48, and 72 h post-treatment with177Lu-Cetuximab-PAMAMG4 (500 nM) were 18%, 15%, and 14%, respectively. The biodistribution studies showed a significant accumulation of 177Lu-Cetuximab-PAMAM in the EGFR-expressing tumor.

CONCLUSION

According to the results, this new agent can be considered as an efficient therapeutic complex for tumors expressing EGFR receptors.

Anti-EGFR antibody 528 binds to domain III of EGFR at a site shifted from the cetuximab epitope

Antibodies have been widely used for cancer therapy owing to their ability to distinguish cancer cells by recognizing cancer-specific antigens. Epidermal growth factor receptor (EGFR) is a promising target for the cancer therapeutics, against which several antibody clones have been developed and brought into therapeutic use. Another antibody clone, 528, is an antagonistic anti-EGFR antibody, which has been the focus of our antibody engineering studies to develop cancer drugs. In this study, we explored the interaction of 528 with the extracellular region of EGFR (sEGFR) via binding analyses and structural studies. Dot blotting experiments with heat treated sEGFR and surface plasmon resonance binding experiments revealed that 528 recognizes the tertiary structure of sEGFR and exhibits competitive binding to sEGFR with EGF and cetuximab. Single particle analysis of the sEGFR-528 Fab complex via electron microscopy clearly showed the binding of 528 to domain III of sEGFR, the domain to which EGF and cetuximab bind, explaining its antagonistic activity. Comparison between the two-dimensional class average and the cetuximab/sEGFR crystal structure revealed that 528 binds to a site that is shifted from, rather than identical to, the cetuximab epitope, and may exclude known drug-resistant EGFR mutations.

Bisphenol S and Epidermal Growth Factor Receptor Signaling in Human Placental Cytotrophoblasts

BACKGROUND

Bisphenol S (BPS) is an endocrine-disrupting chemical and the second most abundant bisphenol detected in humans. In vivo BPS exposure leads to reduced binucleate cell number in the ovine placenta. Binucleate cells form by cellular fusion, similar to the human placental syncytiotrophoblast layer. Given that human placental syncytialization can be stimulated through epidermal growth factor (EGF), we hypothesized that BPS would reduce human cytotrophoblast syncytialization through disruption of EGF receptor (EGFR) signaling.

OBJECTIVE

We tested whether BPS interferes EGFR signaling and disrupts human cytotrophoblast syncytialization.

METHODS

We first tested BPS competition for EGFR using an EGF/EGFR AlphaLISA assay. Using human primary term cytotrophoblast cells (hCTBs) and MDA-MD-231 cells, a breast cancer cell line with high EGFR expression, we evaluated EGFR downstream signaling and tested whether BPS could inhibit the EGF response by blocking EGFR activation. We also evaluated functional end points of EGFR signaling, including EGF endocytosis, cell proliferation, and syncytialization.

RESULTS

BPS blocked EGF binding in a dose-dependent manner and reduced EGF-mediated phosphorylated EGFR in both cell types. We further confirmed that BPS acted as an EGFR antagonist as shown by a reduction in EGF internalization in both hCTBs and MDA-MD-231 cells. Finally, we demonstrated that BPS interfered with EGF-mediated cell processes, such as cell proliferation in MDA-MD-231 cells and syncytialization in hCTBs. EGF-mediated, but not spontaneous, hCTB syncytialization was fully blocked by BPS (200 ng/mL), a dose within urinary BPS concentrations detected in humans.

CONCLUSIONS

Given the role of EGFR in trophoblast proliferation and differentiation during placental development, this study suggests that exposures to BPS at environmentally relevant concentrations may result in placenta dysfunction, affecting fetal growth and development. https://doi.org/10.1289/EHP7297.

The inhibitory effect of ECG and EGCG dimeric procyanidins on colorectal cancer cells growth is associated with their actions at lipid rafts and the inhibition of the epidermal growth factor receptor signaling

Colorectal cancer (CRC) is one of the most common cancers worldwide. Epidemiological studies indicate that consumption of fruits and vegetables containing procyanidins is associated with lower CRC risk. This study investigated the capacity of two dimeric procyanidins composed of epicatechin gallate (ECG) or epigallocatechin gallate (EGCG) isolated from persimmons, to inhibit CRC cell growth and promote apoptosis, characterizing the underlying mechanisms. ECG and EGCG dimers reduced the growth of five human CRC cell lines in a concentration (10-60 μM)- and time (24-72 h)-dependent manner, with a 72 h-IC50 value in Caco-2 cells of 10 and 30 μM, respectively. ECG and EGCG dimers inhibited Caco-2 cell proliferation by arresting the cell cycle in G2/M phase and by inducing apoptosis via the mitochondrial pathway. In addition, ECG and EGCG dimers inhibited cell migration, invasion, and adhesion, decreasing the activity of matrix metalloproteinases (MMP-2/9). Mechanistically, ECG and EGCG dimers inhibited the activation of lipid raft-associated epidermal growth factor (EGF) receptor (EGFR), without affecting its localization at lipid rafts. In particular, ECG and EGCG dimers reduced EGFR phosphorylation at Tyr1068 residue, prevented EGFR dimerization and activation upon stimulation, and induced EGFR internalization both in the absence and presence of EGF. Furthermore, ECG and EGCG dimers increased EGFR phosphorylation at Tyr1045 residue, providing a docking site for ubiquitin ligase c-Cbl and induced EGFR degradation by the proteasome. Downstream of EGFR, ECG and EGCG dimers inhibited the activation of the MEK/ERK1/2 and PI3K/AKT signaling pathways, downregulating proteins involved in the modulation of cell survival. In conclusion, ECG and EGCG dimers reduced CRC cell growth by inhibiting EGFR activation at multiple steps, including the disruption of lipid rafts integrity and promoting EGFR degradation. These results shed light on a potential molecular mechanism on how procyanidins-rich diets may lower CRC risk.

Dynamics fingerprints of active conformers of epidermal growth factor receptor kinase

Epidermal growth factor receptor (EGFR) is a prototypical cell-surface receptor that plays a key role in the regulation of cellular signaling, proliferation and differentiation. Mutations of its kinase domain have been associated with the development of a variety of cancers and, therefore, it has been the target of drug design. Single amino acid substitutions (SASs) in this domain have been proven to alter the equilibrium of pre-existing conformer populations. Despite the advances in structural descriptions of its so-called active and inactive conformations, the associated dynamics aspects that characterize them have not been thoroughly studied yet. As the dynamic behaviors and molecular motions of proteins are important for a complete understanding of their structure-function relationships we present a novel procedure, using (or based on) normal mode analysis, to identify the collective dynamics shared among different conformers in EGFR kinase. The method allows the comparison of patterns of low-frequency vibrational modes defining representative directions of motions. Our procedure is able to emphasize the main similarities and differences between the collective dynamics of different conformers. In the case of EGFR kinase, two representative directions of motions have been found as dynamics fingerprints of the active conformers. Protein motion along both directions reveals to have a significant impact on the cavity volume of the main pocket of the active site. Otherwise, the inactive conformers exhibit a more heterogeneous distribution of collective motions. © 2018 Wiley Periodicals, Inc.

Homo- and Heterodimerization of Proteins in Cell Signaling: Inhibition and Drug Design

Protein dimerization controls many physiological processes in the body. Proteins form homo-, hetero-, or oligomerization in the cellular environment to regulate the cellular processes. Any deregulation of these processes may result in a disease state. Protein-protein interactions (PPIs) can be inhibited by antibodies, small molecules, or peptides, and inhibition of PPI has therapeutic value. PPI drug discovery research has steadily increased in the last decade, and a few PPI inhibitors have already reached the pharmaceutical market. Several PPI inhibitors are in clinical trials. With advancements in structural and molecular biology methods, several methods are now available to study protein homo- and heterodimerization and their inhibition by drug-like molecules. Recently developed methods to study PPI such as proximity ligation assay and enzyme-fragment complementation assay that detect the PPI in the cellular environment are described with examples. At present, the methods used to design PPI inhibitors can be classified into three major groups: (1) structure-based drug design, (2) high-throughput screening, and (3) fragment-based drug design. In this chapter, we have described some of the experimental methods to study PPIs and their inhibition. Examples of homo- and heterodimers of proteins, their structural and functional aspects, and some of the inhibitors that have clinical importance are discussed. The design of PPI inhibitors of epidermal growth factor receptor heterodimers and CD2-CD58 is discussed in detail.

Protein Fluctuations and Cavity Changes Relationship

Protein cavities and tunnels are critical for function. Ligand recognition and binding, transport, and enzyme catalysis require cavities rearrangements. Therefore, the flexibility of cavities should be guaranteed by protein vibrational dynamics. Molecular dynamics simulations provide a framework to explore conformational plasticity of protein cavities. Herein, we present a novel procedure to characterize the dynamics of protein cavities in terms of their volume gradient vector. For this purpose, we make use of algorithms for calculation of the cavity volume that result robust for numerical differentiations. Volume gradient vector is expressed in terms of principal component analysis obtained from equilibrated molecular dynamics simulations. We analyze contributions of principal component modes to the volume gradient vector according to their frequency and degree of delocalization. In all our test cases, we find that low frequency modes play a critical role together with minor contributions of high frequency modes. These modes involve concerted motions of significant fractions of the total residues lining the cavities. We make use of variations of the potential energy of a protein in the direction of the volume gradient vector as a measure of flexibility of the cavity. We show that proteins whose collective low frequency fluctuations contribute the most to changes of cavity volume exhibit more flexible cavities.

Pockets as structural descriptors of EGFR kinase conformations

Epidermal Growth Factor Receptor (EGFR), a tyrosine kinase receptor, is one of the main tumor markers in different types of cancers. The kinase native state is mainly composed of two populations of conformers: active and inactive. Several sequence variations in EGFR kinase region promote the differential enrichment of conformers with higher activity. Some structural characteristics have been proposed to differentiate kinase conformations, but these considerations could lead to ambiguous classifications. We present a structural characterisation of EGFR kinase conformers, focused on active site pocket comparisons, and the mapping of known pathological sequence variations. A structural based clustering of this pocket accurately discriminates active from inactive, well-characterised conformations. Furthermore, this main pocket contains, or is in close contact with, ≈65% of cancer-related variation positions. Although the relevance of protein dynamics to explain biological function has been extensively recognised, the usage of the ensemble of conformations in dynamic equilibrium to represent the functional state of proteins and the importance of pockets, cavities and/or tunnels was often neglected in previous studies. These functional structures and the equilibrium between them could be structurally analysed in wild type as well as in sequence variants. Our results indicate that biologically important pockets, as well as their shape and dynamics, are central to understanding protein function in wild-type, polymorphic or disease-related variations.

Evaluating the effect of mutations and ligand binding on transthyretin homotetramer dynamics

Native transthyretin (TTR) homotetramer dissociation is the first step of the fibrils formation process in amyloid disease. A large number of specific point mutations that destabilize TTR quaternary structure have shown pro-amyloidogenic effects. Besides, several compounds have been proposed as drugs in the therapy of TTR amyloidosis due to their TTR tetramer binding affinities, and therefore, contribution to its integrity. In the present paper we have explored key positions sustaining TTR tetramer dynamical stability. We have identified positions whose mutations alter the most the TTR tetramer equilibrium dynamics based on normal mode analysis and their response to local perturbations. We have found that these positions are mostly localized at β-strands E and F and EF-loop. The monomer-monomer interface is pointed out as one of the most vulnerable regions to mutations that lead to significant changes in the TTR-tetramer equilibrium dynamics and, therefore, induces TTR amyloidosis. Besides, we have found that mutations on residues localized at the dimer-dimer interface and/or at the T4 hormone binding site destabilize the tetramer more than the average. Finally, we were able to compare several compounds according to their effect on vibrations associated to the ligand binding. Our ligand comparison is discussed and analyzed in terms of parameters and measurements associated to TTR-ligand binding affinities and the stabilization of its native state.

A peptidomimetic with a chiral switch is an inhibitor of epidermal growth factor receptor heterodimerization

Among different types of EGFR dimers, EGFR-HER2 and HER2-HER3 are well known in different types of cancers. Targeting dimerization of EGFR will have a significant impact on cancer therapies. A symmetric peptidomimetic was designed to inhibit the protein-protein interaction of EGFR. The peptidomimetic (Cyclo(1,10)PpR (R) Anapa-FDDF-(R)-Anapa)R, compound 18) was shown to exhibit antiproliferative activity with an IC₅₀ of 194 nM in HER2-expressing breast cancer cell lines and 18 nM in lung cancer cell lines. The peptidomimetic has a Pro-Pro sequence in the structure to stabilize the β-turn and a β-amino acid, amino napthyl propionic acid. To investigate the effect of the chirality of β-amino acid on the structure of the peptide and its antiproliferative activity, diastereoisomers of compound 18 were designed and synthesized. Structure-activity relationships of these compounds indicated that there is a chiral switch at β-amino acid in the designed compound. The peptidomimetic with R configuration at β-amino acid and with a L-Pro-D-Pro sequence was the most active compound (18). Using enzyme complement fragmentation assay and proximity ligation assay, we show that compound 18 inhibits HER2:HER3 and EGFR:HER2 dimerization. Surface plasmon resonance studies suggested that compound 18 binds to the HER2 extracellular domain and in particular to domain IV. The anticancer activity of compound 18 was evaluated using a xenograft model of breast cancer in mice; compound 18 suppressed the tumor growth in mice compared to control. Compound 18 was also shown to have a synergistic effect with erlotinib on EGFR mutated lung cancer cell lines.

SNAP-Tag Technology: A Useful Tool To Determine Affinity Constants and Other Functional Parameters of Novel Antibody Fragments

Antibody derivatives, such as the single chain fragment variable (scFv), can be developed as diagnostic and therapeutic tools in cancer research, especially in the form of fusion proteins. Such derivatives are easier to produce and modify than monoclonal antibodies (mAbs) and achieve better tissue/tumor penetration. The genetic modification of scFvs is also much more straightforward than the challenging chemical modification of mAbs. Therefore, we constructed two scFvs derived from the approved monoclonal antibodies cetuximab (scFv2112) and panitumumab (scFv1711), both of which are specific for the epidermal growth factor receptor (EGFR), a well-characterized solid tumor antigen. Both scFvs were genetically fused to the SNAP-tag, an engineered version of the human DNA repair enzyme O(6)-alkylguanine DNA alkyltransferase that allows the covalent coupling of benzylguanine (BG)-modified substrates such as fluorescent dyes. The SNAP-tag achieves controllable and irreversible protein modification and is an important tool for experimental studies in vitro and in vivo. The affinity constant of a scFv is a key functional parameter, especially in the context of a fusion protein. Therefore, we developed a method to define the affinity constants of scFv-SNAP fusion proteins by surface plasmon resonance (SPR) spectroscopy. We could confirm that both scFvs retained their functionality after fusion to the SNAP-tag in a variety of procedures and assays, including ELISA, flow cytometry, and confocal microscopy. The experimental procedures described herein, and the new protocol for affinity determination by SPR spectroscopy, are suitable for the preclinical evaluation of diverse antibody formats and derivatives.

Notch ligand delta-like1: X-ray crystal structure and binding affinity

This X-ray crystal structure of the ectodomain of delta-like ligand-1 (Dll-1) represents the largest fragment of any Notch ligand structure to date. Comparison with the Jagged1 structures reveals similarities and differences between the ligand families and potential receptor binding sites.

Effect of phosphorylation on EGFR dimer stability probed by single-molecule dynamics and FRET/FLIM

Deregulation of epidermal growth factor receptor (EGFR) signaling has been correlated with the development of a variety of human carcinomas. EGF-induced receptor dimerization and consequent trans- auto-phosphorylation are among the earliest events in signal transduction. Binding of EGF is thought to induce a conformational change that consequently unfolds an ectodomain loop required for dimerization indirectly. It may also induce important allosteric changes in the cytoplasmic domain. Despite extensive knowledge on the physiological activation of EGFR, the effect of targeted therapies on receptor conformation is not known and this particular aspect of receptor function, which can potentially be influenced by drug treatment, may in part explain the heterogeneous clinical response among cancer patients. Here, we used Förster resonance energy transfer/fluorescence lifetime imaging microscopy (FRET/FLIM) combined with two-color single-molecule tracking to study the effect of ATP-competitive small molecule tyrosine kinase inhibitors (TKIs) and phosphatase-based manipulation of EGFR phosphorylation on live cells. The distribution of dimer on-times was fitted to a monoexponential to extract dimer off-rates (koff). Our data show that pretreatment with gefitinib (active conformation binder) stabilizes the EGFR ligand-bound homodimer. Overexpression of EGFR-specific DEP-1 phosphatase was also found to have a stabilizing effect on the homodimer. No significant difference in the koff of the dimer could be detected when an anti-EGFR antibody (425 Snap single-chain variable fragment) that allows for dimerization of ligand-bound receptors, but not phosphorylation, was used. These results suggest that both the conformation of the extracellular domain and phosphorylation status of the receptor are involved in modulating the stability of the dimer. The relative fractions of these two EGFR subpopulations (interacting versus free) were obtained by a fractional-intensity analysis of ensemble FRET/FLIM images. Our combined imaging approach showed that both the fraction and affinity (surrogate of conformation at a single-molecule level) increased after gefitinib pretreatment or DEP-1 phosphatase overexpression. Using an EGFR mutation (I706Q, V948R) that perturbs the ability of EGFR to dimerize intracellularly, we showed that a modest drug-induced increase in the fraction/stability of the EGFR homodimer may have a significant biological impact on the tumor cell's proliferation potential.

The EGFR family: not so prototypical receptor tyrosine kinases

The epidermal growth factor receptor (EGFR) was among the first receptor tyrosine kinases (RTKs) for which ligand binding was studied and for which the importance of ligand-induced dimerization was established. As a result, EGFR and its relatives have frequently been termed "prototypical" RTKs. Many years of mechanistic studies, however, have revealed that--far from being prototypical--the EGFR family is quite unique. As we discuss in this review, the EGFR family uses a distinctive "receptor-mediated" dimerization mechanism, with ligand binding inducing a dramatic conformational change that exposes a dimerization arm. Intracellular kinase domain regulation in this family is also unique, being driven by allosteric changes induced by asymmetric dimer formation rather than the more typical activation-loop phosphorylation. EGFR family members also distinguish themselves from other RTKs in having an intracellular juxtamembrane (JM) domain that activates (rather than autoinhibits) the receptor and a very large carboxy-terminal tail that contains autophosphorylation sites and serves an autoregulatory function. We discuss recent advances in mechanistic aspects of all of these components of EGFR family members, attempting to integrate them into a view of how RTKs in this important class are regulated at the cell surface.

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

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

Passive immunotherapeutic strategies for the treatment of malignant gliomas

This review provides historical and recent perspectives related to passive immunotherapy for high-grade gliomas. The authors discuss approaches that use lymphokine-activated killer cells, cytotoxic T lymphocytes, and monoclonal antibodies.

Ligand binding induces a conformational change in epidermal growth factor receptor dimers

The activation of the epidermal growth factor receptor (EGFR) kinase requires ligand binding to the extracellular domain (ECD). Previous reports demonstrate that the EGFR-ECD can be crystallized in two conformations - a tethered monomer or, in the presence of ligand, an untethered back-to-back dimer. We use Biosensor analysis to demonstrate that even in the monomeric state different C-terminal extensions of both truncated (EGFR(1-501))-ECD and full-length EGFR(1-621)-ECD can change the conformation of the ligand-binding site. The binding of a monoclonal antibody mAb806, which recognizes the dimer interface, to the truncated EGFR(1-501)-Fc fusion protein is reduced in the presence of ligand, consistent with a change in conformation. On the cell surface, the presence of erythroblastosis B2 (erbB2) increases the binding of mAb806 to the EGFR. The conformation of the erbB2: EGFR heterodimer interface changes when the cells are treated with epidermal growth factor (EGF). We propose that ligand induces kinase-inactive, pre-formed EGFR dimers and heterodimers to change conformation leading to kinase-active tetramers, where kinase activation occurs via an asymmetric interaction between EGFR dimers.

Cetuximab and biomarkers in non-small-cell lung carcinoma

Cancer progression is a highly complex process that is driven by a constellation of deregulated signaling pathways and key molecular events. In non-small-cell lung cancer (NSCLC), as in several other cancer types, the epidermal growth factor receptor (EGFR) and its downstream signaling components represent a key axis that has been found not only to trigger cancer progression but also to support advanced disease leading to metastasis. Two major therapeutic approaches comprising monoclonal antibodies and small molecule tyrosine kinase inhibitors have so far been used to target this pathway, with a combination of positive, negative, and inconsequential results, as judged by patient survival indices. Since these drugs are expensive and not all patients derive benefits from taking them, it has become both pertinent and paramount to identify biomarkers that can predict not only beneficial response but also resistance. This review focuses on the chimeric monoclonal antibody, cetuximab, its application in the treatment of NSCLC, and the biomarkers that may guide its use in the clinical setting. A special emphasis is placed on the EGFR, including its structural and mechanistic attributes.

Inhibition of cell proliferation by an anti-EGFR aptamer

Aptamers continue to receive interest as potential therapeutic agents for the treatment of diseases, including cancer. In order to determine whether aptamers might eventually prove to be as useful as other clinical biopolymers, such as antibodies, we selected aptamers against an important clinical target, human epidermal growth factor receptor (hEGFR). The initial selection yielded only a single clone that could bind to hEGFR, but further mutation and optimization yielded a family of tight-binding aptamers. One of the selected aptamers, E07, bound tightly to the wild-type receptor (K(d) = 2.4 nM). This aptamer can compete with EGF for binding, binds to a novel epitope on EGFR, and also binds a deletion mutant, EGFRvIII, that is commonly found in breast and lung cancers, and especially in grade IV glioblastoma multiforme, a cancer which has for the most part proved unresponsive to current therapies. The aptamer binds to cells expressing EGFR, blocks receptor autophosphorylation, and prevents proliferation of tumor cells in three-dimensional matrices. In short, the aptamer is a promising candidate for further development as an anti-tumor therapeutic. In addition, Aptamer E07 is readily internalized into EGFR-expressing cells, raising the possibility that it might be used to escort other anti-tumor or contrast agents.

Pharmacodynamic trial of nimotuzumab in unresectable squamous cell carcinoma of the head and neck: a SENDO Foundation study

PURPOSE

To assess the pharmacodynamic effects of nimotuzumab, an anti-epidermal growth factor receptor (EGFR) monoclonal antibody with intermediate affinity for the receptor, in skin and tumor tissues from head and neck cancer patients.

EXPERIMENTAL DESIGN

Pharmacodynamic study in patients with advanced squamous cell carcinoma of the head and neck, unsuitable for chemoradiotherapy, enrolled in a single-center trial. Patients received 8 weekly infusions of nimotuzumab. The first nimotuzumab infusion was administered 1 week before starting radiation, whereas the remaining doses were administered concomitantly with irradiation. Paired biopsies were taken from skin and primary tumors, before (pretherapy) and 1 week (on single-agent therapy) after first infusion. Immunohistochemistry was conducted to assay the effects of nimotuzumab on total and phosphorylated EGFR, phosphorylated extracellular signal-regulated kinase 1/2 (p-ERK1/2), p-AKT, and proliferation (Ki-67).

RESULTS

Nimotuzumab was well tolerated and there was no evidence of skin rash. Objective response was achieved in 9 of 10 patients. The pharmacodynamic assays showed inhibition of p-EGFR in both skin and tumor (P = 0.042 in skin and P = 0.034 in tumor). No significant changes in p-ERK1/2, p-AKT, or Ki-67 were detected in skin. In addition, lymphocytic infiltrates, folliculitis, or perifolliculitis were not observed. In tumor samples, there was an upregulation of p-AKT (P = 0.043), a reduction in proliferation index (P = 0.012), and a nonsignificant trend toward a decrease of p-ERK1/2 (P = 0.091).

CONCLUSIONS

The pharmacodynamic data confirmed the ability of nimotuzumab to decrease EGFR phosphorylation. Downstream effects were observed in tumor cells but not in skin, a finding that may help to explain the lack of skin rash in patients treated with nimotuzumab.

Passive antibody-mediated immunotherapy for the treatment of malignant gliomas

Despite advances in understanding the molecular mechanisms of brain cancer, the outcome of patients with malignant gliomas treated according to the current standard of care remains poor. Novel therapies are needed, and immunotherapy has emerged with great promise. The diffuse infiltration of malignant gliomas is a major challenge to effective treatment; immunotherapy has the advantage of accessing the entire brain with specificity for tumor cells. Therapeutic immune approaches include cytokine therapy, passive immunotherapy, and active immunotherapy. Cytokine therapy involves the administration of immunomodulatory cytokines to activate the immune system. Active immunotherapy is the generation or augmentation of an immune response, typically by vaccination against tumor antigens. Passive immunotherapy connotes either adoptive therapy, in which tumor-specific immune cells are expanded ex vivo and reintroduced into the patient, or passive antibody-mediated therapy. In this article, the authors discuss the preclinical and clinical studies that have used passive antibody-mediated immunotherapy, otherwise known as serotherapy, for the treatment of malignant gliomas.

Nimotuzumab, an antitumor antibody that targets the epidermal growth factor receptor, blocks ligand binding while permitting the active receptor conformation

Overexpression of the epidermal growth factor (EGF) receptor (EGFR) in cancer cells correlates with tumor malignancy and poor prognosis for cancer patients. For this reason, the EGFR has become one of the main targets of anticancer therapies. Structural data obtained in the last few years have revealed the molecular mechanism for ligand-induced EGFR dimerization and subsequent signal transduction, and also how this signal is blocked by either monoclonal antibodies or small molecules. Nimotuzumab (also known as h-R3) is a humanized antibody that targets the EGFR and has been successful in the clinics. In this work, we report the crystal structure of the Fab fragment of Nimotuzumab, revealing some unique structural features in the heavy variable domain. Furthermore, competition assays show that Nimotuzumab binds to domain III of the extracellular region of the EGFR, within an area that overlaps with both the surface patch recognized by Cetuximab (another anti-EGFR antibody) and the binding site for EGF. A computer model of the Nimotuzumab-EGFR complex, constructed by docking and molecular dynamics simulations and supported by mutagenesis studies, unveils a novel mechanism of action, with Nimotuzumab blocking EGF binding while still allowing the receptor to adopt its active conformation, hence warranting a basal level of signaling.

Alpha6beta4 integrin crosslinking induces EGFR clustering and promotes EGF-mediated Rho activation in breast cancer

Background

The α6β4 integrin is overexpressed in the basal subtype of breast cancer and plays an important role in tumor cell motility and invasion. EGFR is also overexpressed in the basal subtype of breast cancer, and crosstalk between α6β4 integrin and EGFR appears to be important in tumor progression.

Methods

We evaluated the effects of α6β4 crosslinking on the distribution and function of EGFR in breast carcinoma cell line MDA-MB-231. Receptor distribution was evaluated by fluorescence microscopy and multispectral imaging flow cytometry, and ligand-mediated EGFR signaling was evaluated using Western blots and a Rho pull-down assay.

Results

Antibody-mediated crosslinking of α6β4 integrin was sufficient to induce cell-surface clustering of not only α6β4 but also EGFR in nonadherent cells. The induced clustering of EGFR was observed minimally after 5 min of integrin crosslinking but was more prominent after 15 min. EGFR clustering had minimal effect on the phosphorylation of Akt or Erk1,2 in response to EGF in suspended cells or in response to HB-EGF in adherent cells. However, EGFR clustering induced by crosslinking α6β4 had a marked effect on Rho activation in response to EGF.

Conclusion

Crosslinking α6β4 integrin in breast carcinoma cells induces EGFR clustering and preferentially promotes Rho activation in response to EGF. We hypothesize that this integrin-EGFR crosstalk may facilitate tumor cell cytoskeletal rearrangements important for tumor progression.

Structure-based view of epidermal growth factor receptor regulation

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

Structural basis for EGF receptor inhibition by the therapeutic antibody IMC-11F8

Therapeutic anticancer strategies that target and inactivate the epidermal growth factor receptor (EGFR) are under intense study in the clinic. Here we describe the mechanism of EGFR inhibition by an antibody drug IMC-11F8. IMC-11F8 is a fully human antibody that has similar antitumor potency as the chimeric cetuximab/Erbitux and might represent a safer therapeutic alternative. We report the X-ray crystal structure of the Fab fragment of IMC-11F8 (Fab11F8) in complex with the entire extracellular region and with isolated domain III of EGFR. We compare this to our previous study of the cetuximab/EGFR interaction. Fab11F8 interacts with a remarkably similar epitope, but through a completely different set of interactions. Both the similarities and differences in binding of these two antibodies have important implications for the development of inhibitors that could exploit this same mechanism of EGFR inhibition.

Matuzumab binding to EGFR prevents the conformational rearrangement required for dimerization

An increasing number of therapeutic antibodies targeting tumors that express the epidermal growth factor receptor (EGFR) are in clinical use or late stages of clinical development. Here we investigate the molecular basis for inhibition of EGFR activation by the therapeutic antibody matuzumab (EMD72000). We describe the X-ray crystal structure of the Fab fragment of matuzumab (Fab72000) in complex with isolated domain III from the extracellular region of EGFR. Fab72000 interacts with an epitope on EGFR that is distinct from the ligand-binding region on domain III and from the cetuximab/Erbitux epitope. Matuzumab blocks ligand-induced receptor activation indirectly by sterically preventing the domain rearrangement and local conformational changes that must occur for high-affinity ligand binding and receptor dimerization.

Effect of aging on EGF-induced proliferative response in primary cultured periportal and perivenous hepatocytes

BACKGROUND/AIMS

Aging relates to declined proliferative capacity of the liver, but the molecular mechanism is not well understood. We examined whether functional changes of epidermal growth factor (EGF) receptor (EGFR) are involved in age-related decline in EGF-induced DNA synthesis using hepatocytes isolated in periportal and perivenous regions of the liver, which differ in the proliferative capacity.

METHODS

Periportal hepatocytes (PPH) and perivenous hepatocytes (PVH) in 7-, 30-, and 90-week-old rats were isolated using the digitonin/collagenase perfusion technique. DNA synthesis was assessed by [methyl-(3)H]thymidine incorporation. EGFR binding affinity to EGF was analyzed by Scatchard analysis using [(125)I]EGF. EGFR dimerization and phosphorylation were determined by Western blot analysis.

RESULTS

EGF-induced DNA synthesis was greater in PPH than in PVH from rats of 7 weeks, but the zonal difference disappeared with aging. [(125)I]EGF binding studies indicated that high-affinity EGFR in both subpopulations also disappeared with aging. Furthermore, EGF-induced dimerization in both subpopulations was down-regulated with aging, and the pattern of EGFR phosphorylation was parallel to that of dimerization.

CONCLUSIONS

These data suggest that age-related decline in EGF-induced DNA synthesis of PPH and PVH is caused by down-regulation of EGFR dimerization through the decrease of high-affinity EGFR.

Novel antibodies as anticancer agents

In recent years antibodies, whether generated by traditional hybridoma technology or by recombinant DNA strategies, have evolved from Paul Ehrlich's 'magic bullets' to a modern age 'guided missile'. In the recent years of immunologic research, we are witnessing development in the fields of antigen screening and protein engineering in order to create specific anticancer remedies. The developments in the field of recombinant DNA, protein engineering and cancer biology have let us gain insight into many cancer-related mechanisms. Moreover, novel techniques have facilitated tools allowing unique distinction between malignantly transformed cells, and regular ones. This understanding has paved the way for the rational design of a new age of pharmaceuticals: monoclonal antibodies and their fragments. Antibodies can select antigens on both a specific and a high-affinity account, and further implementation of these qualities is used to target cancer cells by specifically identifying exogenous antigens of cancer cell populations. The structure of the antibody provides plasticity resonating from its functional sites. This review will screen some of the many novel antibodies and antibody-based approaches that are being currently developed for clinical applications as the new generation of anticancer agents.

The inhibitory effect of (-)-epigallocatechin gallate on activation of the epidermal growth factor receptor is associated with altered lipid order in HT29 colon cancer cells

(-)-Epigallocatechin gallate (EGCG), a major biologically active constituent of green tea, inhibits activation of the epidermal growth factor (EGF) receptor (EGFR) and downstream signaling pathways in several types of human cancer cells, but the precise mechanism is not known. Because several plasma membrane-associated receptor tyrosine kinases (RTK) including EGFR are localized in detergent-insoluble ordered membrane domains, so-called "lipid rafts," we examined whether the inhibitory effect of EGCG on activation of the EGFR is associated with changes in membrane lipid order in HT29 colon cancer cells. First, we did cold Triton X-100 solubility assays. Phosphorylated (activated) EGFR was found only in the Triton X-100-insoluble (lipid raft) fraction, whereas total cellular EGFR was present in the Triton X-100-soluble fraction. Pretreatment with EGCG inhibited the binding of Alexa Fluor 488-labeled EGF to the cells and also inhibited EGF-induced dimerization of the EGFR. To examine possible effects of EGCG on membrane lipid organization, we labeled the cells with the fluorescent lipid analogue 1, 1'-dihexadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate, which preferentially incorporates into ordered membrane domains in cells and found that subsequent treatment with EGCG caused a marked reduction in the Triton X-100-resistant membrane fraction. Polyphenon E, a mixture of green tea catechins, had a similar effect but (-)-epicatechin (EC), the biologically inactive compound, did not significantly alter the Triton X-100 solubility properties of the membrane. Furthermore, we found that EGCG but not EC caused dramatic changes in the function of bilayer-incorporated gramicidin channels. Taken together, these findings suggest that EGCG inhibits the binding of EGF to the EGFR and the subsequent dimerization and activation of the EGFR by altering membrane organization. These effects may also explain the ability of EGCG to inhibit activation of other membrane-associated RTKs, and they may play a critical role in the anticancer effects of this and related compounds.

Ligand release-independent transactivation of epidermal growth factor receptor by transforming growth factor-beta involves multiple signaling pathways

Many of the signaling responses induced by transforming growth factor-beta (TGF-beta) are mediated by Smad proteins, but there is evidence that it can also signal independently of Smads. Here, we provide evidence that multiple signal pathways induced by TGF-beta1-including Src family tyrosine kinases (SFKs), generation of reactive oxygen species (ROS), de novo protein synthesis and E-cadherin-dependent cell-cell interactions-transactivate the epidermal growth factor receptor (EGFR), which in turn regulates expression of c-Fos and c-Jun. Immunoprecipitation and immunofluorescence staining showed that EGFR was phosphorylated on tyrosine in response to TGF-beta1. EGFR transactivation required the activation of SFKs and the production of ROS via NADPH oxidase, but was not dependent on metalloproteases or the release of EGF-like ligands. In addition, the production of ROS was dependent on signaling by specific SFKs as well as de novo protein synthesis. Stable transfection of E-cadherin into MDA-MB-231 cells as well as E-cadherin-blocking assays revealed that E-cadherin-mediated cell-cell interactions were also essential for EGFR transactivation. Finally, EGFR transactivation was involved in the expression of c-Fos and c-Jun via the extracellular signal-regulated kinase signaling cascade. Taken together our data suggest that ligand release-independent transactivation of EGFR may diversify early TGF-beta signaling and represent a novel pathway leading to TGF-beta-mediated gene expression.

Rational bases for the development of EGFR inhibitors for cancer treatment

Growth factor receptors and their ligands not only regulate normal cell processes but have been also identified as key regulators of human cancer formation. The epidermal growth factor receptor (EGFR/ErbB1/HER1) belongs to the ErbB/HER-family of tyrosine kinase receptors (RTKs). These trans-membrane proteins are activated following binding with peptide growth factors of the EGF-family of proteins. Several evidences suggest that cooperation of multiple ErbB receptors and ligands is required for the induction of cell transformation. In this respect, EGFR, upon activation, sustains a complex and redundant network of signal transduction pathways with the contribution of other trans-membrane receptors. EGFR has been found to be expressed and altered in a variety of malignancies and clearly it plays a significant role in tumor development and progression, including cell proliferation, regulation of apoptotic cell death, angiogenesis and metastatic spread. Moreover, amplification of the EGFR gene and mutations in the EGFR tyrosine kinase domain have been recently reported in human carcinomas. As a result, investigators have developed approaches to inhibit the effects of EGFR activation, with the aim of blocking tumor growth and invasion. A number of agents targeting EGFR, including specific antibodies directed against its ligand-binding domain and small molecules inhibiting its tyrosine kinase activity are either in clinical trials or are already approved for clinical treatment. This article reviews the EGFR role in carcinogenesis and tumor progression as rational bases for the development of specific therapeutic inhibitors.

Membrane-permeable calmodulin inhibitors (e.g. W-7/W-13) bind to membranes, changing the electrostatic surface potential: dual effect of W-13 on epidermal growth factor receptor activation

Membrane-permeable calmodulin inhibitors, such as the napthalenesulfonamide derivatives W-7/W-13, trifluoperazine, and calmidazolium, are used widely to investigate the role of calcium/calmodulin (Ca2+/CaM) in living cells. If two chemically different inhibitors (e.g. W-7 and trifluoperazine) produce similar effects, investigators often assume the effects are due to CaM inhibition. Zeta potential measurements, however, show that these amphipathic weak bases bind to phospholipid vesicles at the same concentrations as they inhibit Ca2+/CaM; this suggests that they also bind to the inner leaflet of the plasma membrane, reducing its negative electrostatic surface potential. This change will cause electrostatically bound clusters of basic residues on peripheral (e.g. Src and K-Ras4B) and integral (e.g. epidermal growth factor receptor (EGFR)) proteins to translocate from the membrane to the cytoplasm. We measured inhibitor-mediated translocation of a simple basic peptide corresponding to the calmodulin-binding juxtamembrane region of the EGFR on model membranes; W-7/W-13 causes translocation of this peptide from membrane to solution, suggesting that caution must be exercised when interpreting the results obtained with these inhibitors in living cells. We present evidence that they exert dual effects on autophosphorylation of EGFR; W-13 inhibits epidermal growth factor-dependent EGFR autophosphorylation under different experimental conditions, but in the absence of epidermal growth factor, W-13 stimulates autophosphorylation of the receptor in four different cell types. Our interpretation is that the former effect is due to W-13 inhibition of Ca2+/CaM, but the latter results could be due to binding of W-13 to the plasma membrane.

Dimerization of protein tyrosine phosphatase sigma governs both ligand binding and isoform specificity

Signaling through receptor protein tyrosine phosphatases (RPTPs) can influence diverse processes, including axon development, lymphocyte activation, and cell motility. The molecular regulation of these enzymes, however, is still poorly understood. In particular, it is not known if, or how, the dimerization state of RPTPs is related to the binding of extracellular ligands. Protein tyrosine phosphatase sigma (PTPsigma) is an RPTP with major isoforms that differ in their complements of fibronectin type III domains and in their ligand-binding specificities. In this study, we show that PTPsigma forms homodimers in the cell, interacting at least in part through the transmembrane region. Using this knowledge, we provide the first evidence that PTPsigma ectodomains must be presented as dimers in order to bind heterophilic ligands. We also provide evidence of how alternative use of fibronectin type III domain complements in two major isoforms of PTPsigma can alter the ligand binding specificities of PTPsigma ectodomains. The data suggest that the alternative domains function largely to change the rotational conformations of the amino-terminal ligand binding sites of the ectodomain dimers, thus imparting novel ligand binding properties. These findings have important implications for our understanding of how heterophilic ligands interact with, and potentially regulate, RPTPs.

Structure-function analysis of delta trafficking, receptor binding and signaling in Drosophila

The transmembrane proteins Delta and Notch act as ligand and receptor in a conserved signaling pathway required for a variety of cell fate specification events in many organisms. Binding of Delta to Notch results in a proteolytic cascade that releases the Notch intracellular domain, allowing it to participate in transcriptional activation in the nucleus. Recent research has implicated the endocytic and ubiquitylation machinery as essential components of Delta-Notch signaling. Our analysis of chimeric and missense Delta variants has delineated a number of structural requirements for Delta trafficking, receptor binding, and signaling. We find that while the Delta N-terminal domain is necessary and sufficient for binding to Notch, the integrity of the epidermal-growth-factor-like repeat (ELR) 2 is also required for Notch binding. Screening of 117 Delta mutant lines for proteins that exhibit aberrant subcellular trafficking has led to the identification of 18 Delta alleles (DlTD alleles) that encode "trafficking-defective" Delta proteins. We find, unexpectedly, that many DlTD alleles contain missense mutations in ELRs within the Delta extracellular domain. Finally, we find that two DlTD alleles contain lysine missense mutations within the Delta intracellular domain (DeltaICD) that may identify residues important for DeltaICD mono-ubiquitylation and subsequent Delta endocytosis and signaling.

New and emerging radiosensitizers and radioprotectors

The combination of chemotherapy and radiation has led to clinical breakthroughs in several disease sites, and current work continues to define optimum combinations of proven chemotherapy as well as more recently available, noncytotoxic agents. Administration of systemic therapies allows modulation of radiation response to improve tumor control (radiosensitization) or to prevent normal tissue toxicity (radioprotection). Substantial progress has been made in identifying the targets of standard chemotherapeutic radiation sensitizers and protectors as well as in the introduction of a new generation of molecularly targeted therapies in combination with radiation. We have reviewed the most recent, predominantly early phase clinical trials combining systemic agents with radiation. Although the proof of an improved schedule ultimately needs to come from well-run Phase III trials, the search among schedules could be shortened by the use of surrogate endpoints such as presence of active drug metabolites in the tumor. This has been accomplished only in a few cases and needs to become a more standard part of radiation sensitizer and protector trials.

Neuregulins: Versatile growth and differentiation factors in nervous system development and human disease

The neuregulins are a family of growth and differentiation factors with a wide range of functions in the nervous system. The power and diversity of the neuregulin signaling system comes in part from a large number of alternatively-spliced forms of the NRG1 gene that can produce both soluble and membrane-bound forms. The soluble forms of neuregulin are unique from other factors in that they have a structurally distinct heparin-binding domain that targets and potentiates its actions. In addition, a finely tuned, bidirectional mechanism regulates when and where neuregulin is released from neurons in response to neurotrophic factors produced by both neuronal targets and supporting glial cells. Together, this produces a balanced intercellular signaling system that can be localized to distinct regions for both normal development and maintenance of the mature nervous system. Recent evidence suggests that neuregulin signaling plays important roles in many neurological disorders including multiple sclerosis, traumatic brain and spinal cord injury, peripheral neuropathy, and schizophrenia. Here, we review the basic biology of neuregulins and relate this to research suggesting their involvement with and potential therapeutic uses for neurological disorders.

Molecular biology of epidermal growth factor receptor inhibition for cancer therapy

Understanding the role of the epidermal growth factor receptor (EGFR) in cellular signalling processes underlying malignancy has enabled the development of rationally designed EGFR-targeted therapeutics. Strategies have been devised to interfere with the EGFR signalling at three different levels: at the extracellular level, competing with ligand binding; at the intracellular level, inhibiting the activation of the tyrosine kinase; or at the mRNA level, modulating the expression of the EGFR protein. Each of these strategies has proven to have an antitumour effect mediated by events such as inhibition of cell proliferation, induction of apoptosis, decrease of cellular invasion and migration; and/or inhibition of angiogenesis. Furthermore, the combination of these strategies with traditional chemotherapy or radiotherapy has generally resulted in enhanced antitumour effects. Likewise, the benefit of interfering simultaneously with different signalling pathways has been documented to improve tumour growth inhibition. These preclinical results have encouraged clinical studies that led to the FDA approval of three drugs. However, finding the perfect strategy for each individual patient appears to be a limiting factor, demanding further research to be able to generate relevant molecular expression profiles on a case-to-case basis. Taken together, a successful EGFR inhibition will require a better understanding of signalling pathways in combination with the development of rationally designed effective molecules.

Targeting EGFR and HER-2 receptor tyrosine kinases for cancer drug discovery and development

Conventional anticancer therapy using cytotoxic drugs lacks selectivity and is prone to toxicity and drug resistance. Anticancer therapies targeting aberrant growth factor receptor signaling are gaining interest. The erbB receptor family belongs to the type I, the receptor tyrosine kinases class, and comprises EGFR, HER-2, HER-3, and HER-4. It has been targeted for solid tumor therapy, including breast, ovarian, colon, head-and-neck, and non-small-cell lung cancers. This review summarizes structural aspects of this class of growth factor receptors, their oncogenic expression, and various pharmacological interventions including biological products and small molecules that inhibit these enzymes. We have also discussed various mutations that occur in EGFR and their consequences on anticancer therapy.

Plasma membrane phosphoinositide organization by protein electrostatics

Phosphatidylinositol 4,5-bisphosphate (PIP2), which comprises only about 1% of the phospholipids in the cytoplasmic leaflet of the plasma membrane, is the source of three second messengers, activates many ion channels and enzymes, is involved in both endocytosis and exocytosis, anchors proteins to the membrane through several structured domains and has other roles. How can a single lipid in a fluid bilayer regulate so many distinct physiological processes? Spatial organization might be the key to this. Recent studies suggest that membrane proteins concentrate PIP2 and, in response to local increases in intracellular calcium concentration, release it to interact with other biologically important molecules.

NMR Investigation of Main-Chain Dynamics of the H80E Mutant of Bovine Neurophysin-I: Demonstration of Dimerization-Induced Changes at the Hormone-Binding Site

Neurophysins are hormone-binding proteins composed of two partially homologous domains. Ligand-binding (localized to the amino domain) and dimerization (involves both domains) are cooperatively linked by an as yet undefined allosteric mechanism. To help define this mechanism, we investigated the backbone dynamics of the unliganded monomeric state of the H80E mutant of bovine neurophysin-I by (15)N NMR. Model-free analysis of the NMR relaxation parameters indicated significantly greater flexibility in the carboxyl domain than in the amino domain, particularly at their dimerization interface segments. Amino domain residues critical to hormone binding were highly structured, constraining potential allosteric mechanisms. Model-free analysis additionally demonstrated chemical exchange effects, manifest as R(ex) terms, in 16 residues, 14 of which are located in the amino domain at, or immediately adjacent to, either the dimerization interface or the hormone-binding site. The chemical exchange process was further characterized using relaxation-compensated CPMG measurements, the results allowing assignment of the process to monomer-dimer exchange and calculation of the exchange kinetics, which were slow on the NMR time scale. An apparently different concentration-dependent process, distinguished from normal dimerization by its fast exchange behavior and pH-independence, also principally involved a subset of residues at and immediately adjacent to either the hormone-binding site or the amino domain dimerization interface. The data represent the first direct demonstration of an effect of dimerization in the unliganded state on neurophysin's hormone-binding site, the effect particularly involving residues that interact with hormone residue 2, and specifically identify Ser25 and Ile26 as likely intermediaries between the sites of dimerization and of hormone binding. Consistent with recent views of the role of anchor residues in protein interactions, we propose that dimerization proceeds by a fast pH-independent association of the well-structured amino domain interface that is rapidly communicated to the binding site for hormone residue 2, followed by a rate-determining pH-dependent interaction of the less structured carboxyl domain interface.

An electrostatic engine model for autoinhibition and activation of the epidermal growth factor receptor (EGFR/ErbB) family

We propose a new mechanism to explain autoinhibition of the epidermal growth factor receptor (EGFR/ErbB) family of receptor tyrosine kinases based on a structural model that postulates both their juxtamembrane and protein tyrosine kinase domains bind electrostatically to acidic lipids in the plasma membrane, restricting access of the kinase domain to substrate tyrosines. Ligand-induced dimerization promotes partial trans autophosphorylation of ErbB1, leading to a rapid rise in intracellular [Ca²⁺] that can activate calmodulin. We postulate the Ca²⁺/calmodulin complex binds rapidly to residues 645–660 of the juxtamembrane domain, reversing its net charge from +8 to −8 and repelling it from the negatively charged inner leaflet of the membrane. The repulsion has two consequences: it releases electrostatically sequestered phosphatidylinositol 4,5-bisphosphate (PIP₂), and it disengages the kinase domain from the membrane, allowing it to become fully active and phosphorylate an adjacent ErbB molecule or other substrate. We tested various aspects of the model by measuring ErbB juxtamembrane peptide binding to phospholipid vesicles using both a centrifugation assay and fluorescence correlation spectroscopy; analyzing the kinetics of interactions between ErbB peptides, membranes, and Ca²⁺/calmodulin using fluorescence stop flow; assessing ErbB1 activation in Cos1 cells; measuring fluorescence resonance energy transfer between ErbB peptides and PIP₂; and making theoretical electrostatic calculations on atomic models of membranes and ErbB juxtamembrane and kinase domains.