Two EGF molecules contribute additively to stabilization of the EGFR dimer

B3GNT2, a polylactosamine synthase, regulates glycosylation of EGFR in H7721 human hepatocellular carcinoma cells

The epidermal growth factor receptor (EGFR) is an important surface receptor with N-glycans in its extracellular domain, whose glycosylation is essential for its function, especially in tumor cells. Here, we demonstrated that polylactosamine is markedly increased in H7721 hepatocellular carcinoma cells after treatment with EGF, while it apparently declined after exposure to all-trans retinoic acid (ATRA). In the study of the enzymatic mechanism of this phenomenon, we explored changes in the expression of poly-N-acetyllactosamine (PLN) branching glycosyltransferases using RT-PCR. Among the four glycosyltransferases with altered expression, GnT-V was most elevated by EGF, while GnT-V and B3GNT2 were most declined by ATRA. Next, we conducted co-immunoprecipitation experiments to test whether B3GNT2 and EGFR associate with each other. We observed that EGFR is a B3GNT2-targeting protein in H7721 cells. Taken together, these findings indicated that the altered expression of B3GNT2 will remodel the PLN stucture of EGFR in H7721 cells, which may modify downstream signal transduction.

Characterization of membrane protein interactions in plasma membrane derived vesicles with quantitative imaging Förster resonance energy transfer

Here we describe an experimental tool, termed quantitative imaging Förster resonance energy transfer (QI-FRET), that enables the quantitative characterization of membrane protein interactions. The QI-FRET methodology allows us to acquire binding curves and calculate association constants for complex membrane proteins in the native plasma membrane environment. The method utilizes FRET detection, and thus requires that the proteins of interest are labeled with florescent proteins, either FRET donors or FRET acceptors. Since plasma membranes of cells have complex topologies precluding the acquisition of two-dimensional binding curves, the FRET measurements are performed in plasma membrane derived vesicles that bud off cells as a result of chemical or osmotic stress. The results overviewed here are acquired in vesicles produced with an osmotic vesiculation buffer developed in our laboratory, which does not utilize harsh chemicals. The concentrations of the donor-labeled and the acceptor-labeled proteins are determined, along with the FRET efficiencies, in each vesicle. The experiments utilize transient transfection, such that a wide variety of concentrations is sampled. Then, data from hundreds of vesicles are combined to yield dimerization curves. Here we discuss recent findings about the dimerization of receptor tyrosine kinases (RTKs), membrane proteins that control cell growth and differentiation via lateral dimerization in the plasma membrane. We focus on the dimerization of fibroblast growth factor receptor 3 (FGFR3), a RTK that plays a critically important role in skeletal development. We study the role of different FGFR3 domains in FGFR3 dimerization in the absence of ligand, and we show that FGFR3 extracellular domains inhibit unliganded dimerization, while contacts between the juxtamembrane domains, which connect the transmembrane domains to the kinase domains, stabilize the unliganded FGFR3 dimers. Since FGFR3 has been documented to harbor many pathogenic single amino acid mutations that cause skeletal and cranial dysplasias, as well as cancer, we also study the effects of these mutations on dimerization. First, we show that the A391E mutation, linked to Crouzon syndrome with acanthosis nigricans and to bladder cancer, significantly enhances FGFR3 dimerization in the absence of ligand and thus induces aberrant receptor interactions. Second, we present results about the effect of three cysteine mutations that cause thanatophoric dysplasia, a lethal phenotype. Such cysteine mutations have been hypothesized previously to cause constitutive dimerization, but we find instead that they have a surprisingly modest effect on dimerization. Most of the studied pathogenic mutations also altered FGFR3 dimer structure, suggesting that both increases in dimerization propensities and changes in dimer structure contribute to the pathological phenotypes. The results acquired with the QI-FRET method further our understanding of the interactions between FGFR3 molecules and RTK molecules in general. Since RTK dimerization regulates RTK signaling, our findings advance our knowledge of RTK activity in health and disease. The utility of the QI-FRET method is not restricted to RTKs, and we thus hope that in the future the QI-FRET method will be applied to other classes of membrane proteins, such as channels and G protein-coupled receptors.

Ligand regulation of a constitutively dimeric EGF receptor

Ligand-induced receptor dimerization has traditionally been viewed as the key event in transmembrane signalling by epidermal growth factor receptors (EGFRs). Here we show that the Caenorhabditis elegans EGFR orthologue LET-23 is constitutively dimeric, yet responds to its ligand LIN-3 without changing oligomerization state. SAXS and mutational analyses further reveal that the preformed dimer of the LET-23 extracellular region is mediated by its domain II dimerization arm and resembles other EGFR extracellular dimers seen in structural studies. Binding of LIN-3 induces only minor structural rearrangements in the LET-23 dimer to promote signalling. Our results therefore argue that EGFR can be regulated by allosteric changes within an existing receptor dimer--resembling signalling by insulin receptor family members, which share similar extracellular domain compositions but form covalent dimers.

EGFR Inhibition by Curcumin in Cancer Cells: A Dual Mode of Action

Epidermal Growth Factor Receptor (EGFR) is an important target of anticancer therapy. Nowadays, the search for new molecules inhibiting this receptor is turning toward natural substances. One of the most promising natural compounds that have shown an anti-EGFR activity is curcumin, a polyphenol found in turmeric. Its effect on the receptor kinase activity and on the receptor autophosphorylation has been already described, but the mechanism of how curcumin interacts with EGFR is not fully elucidated. We demonstrate that the mode of action of curcumin is dual. This polyphenol is able to inhibit directly but partially the enzymatic activity of the EGFR intracellular domain. The present work shows that curcumin also influences the cell membrane environment of EGFR. Using biomimetic membrane models, we show that curcumin insertion into the lipid bilayer leads to its rigidification. Single particle tracking analyses performed in the membrane of A431 cancer cells confirmed that this effect of curcumin on the membrane slows down the receptor diffusion. This is likely to affect the receptor dimerization and, in turn, its activation.

N-Glycosylation as determinant of epidermal growth factor receptor conformation in membranes

The epidermal growth factor receptor (EGFR) regulates several critical cellular processes and is an important target for cancer therapy. In lieu of a crystallographic structure of the complete receptor, atomistic molecular dynamics (MD) simulations have recently shown that they can excel in studies of the full-length receptor. Here we present atomistic MD simulations of the monomeric N-glycosylated human EGFR in biomimetic lipid bilayers that are, in parallel, also used for the reconstitution of full-length receptors. This combination enabled us to experimentally validate our simulations, using ligand binding assays and antibodies to monitor the conformational properties of the receptor reconstituted into membranes. We find that N-glycosylation is a critical determinant of EGFR conformation, and specifically the orientation of the EGFR ectodomain relative to the membrane. In the absence of a structure for full-length, posttranslationally modified membrane receptors, our approach offers new means to structurally define and experimentally validate functional properties of cell surface receptors in biomimetic membrane environments.

Design of a selenylsulfide-bridged EGFR dimerization arm mimic

The epidermal growth factor receptor (EGFR) dimerization arm is a key feature that stabilizes dimerization of the extracellular receptor, thereby mediating activation of the tyrosine kinase domain. Peptides mimicking this β-loop feature can disrupt dimer formation and kinase activation, yet these peptides lack structural constraints or contain redox sensitive disulfide bonds which may limit their stability in physiological environments. Selenylsulfide bonds are a promising alternative to disulfide bonds as they maintain much of the same structural and chemical behavior, yet they are inherently less prone to reduction. Herein, we describe the synthesis, stability and activity of selenylsulfide-bridged dimerization arm mimics. The synthesis was accomplished using an Fmoc-based strategy along with C-terminal labeling for improved overall yield. This selenylsulfide-bridged peptide displayed both proteolytic stability and structural stability even under reducing conditions, demonstrating the potential application of the selenylsulfide bond to generate redox stable β-loop peptides for disruption of protein-protein interactions.

Inhibiting EGFR dimerization using triazolyl-bridged dimerization arm mimics

The epidermal growth factor receptor (EGFR) is overexpressed in multiple carcinomas and is the focus of a variety of targeted therapies. Here we report the design of peptide-based compounds that mimic the EGFR dimerization arm and inhibit allosteric activation of EGFR. These peptides are modified to contain a triazolyl bridge between the peptide strands to constrain the EGFR dimerization arm β-loop. In this study, we demonstrate that these peptides have significantly improved proteolytic stability over the non-modified peptide sequence, and their inhibitory effects are dependent on the number of the methylene units and orientation of the introduced triazolyl bridge. We identified a peptide, EDA2, which downregulates receptor phosphorylation and dimerization and reduces cell viability. This is the first example of a biologically active triazolyl-bridged peptide targeting the EGFR dimerization interface that effectively downregulates EGFR activation.

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 FRET signatures of noninteracting proteins in membranes: simulations and experiments

Förster resonance energy transfer (FRET) experiments are often used to study interactions between integral membrane proteins in cellular membranes. However, in addition to the FRET of sequence-specific interactions, these experiments invariably record a contribution due to proximity FRET, which occurs when a donor and an acceptor approach each other by chance within distances of ∼100 Å. This effect does not reflect specific interactions in the membrane and is frequently unappreciated, despite the fact that its magnitude can be significant. Here we develop a computational description of proximity FRET, simulating the cases of proximity FRET when fluorescent proteins are used to tag monomeric, dimeric, trimeric, and tetrameric membrane proteins, as well as membrane proteins existing in monomer-dimer equilibria. We also perform rigorous experimental measurements of this effect, by identifying membrane receptors that do not associate in mammalian membranes. We measure the FRET efficiencies between yellow fluorescent protein and mCherry-tagged versions of these receptors in plasma-membrane-derived vesicles as a function of receptor concentration. Finally, we demonstrate that the experimental measurements are well described by our predictions. The work presented here brings additional rigor to FRET-based studies of membrane protein interactions, and should have broad utility in membrane biophysics research.

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.

Complex relationship between ligand binding and dimerization in the epidermal growth factor receptor

The epidermal growth factor receptor (EGFR) plays pivotal roles in development and is mutated or overexpressed in several cancers. Despite recent advances, the complex allosteric regulation of EGFR remains incompletely understood. Through efforts to understand why the negative cooperativity observed for intact EGFR is lost in studies of its isolated extracellular region (ECR), we uncovered unexpected relationships between ligand binding and receptor dimerization. The two processes appear to compete. Surprisingly, dimerization does not enhance ligand binding (although ligand binding promotes dimerization). We further show that simply forcing EGFR ECRs into preformed dimers without ligand yields ill-defined, heterogeneous structures. Finally, we demonstrate that extracellular EGFR-activating mutations in glioblastoma enhance ligand-binding affinity without directly promoting EGFR dimerization, suggesting that these oncogenic mutations alter the allosteric linkage between dimerization and ligand binding. Our findings have important implications for understanding how EGFR and its relatives are activated by specific ligands and pathological mutations.

Putting together structures of epidermal growth factor receptors

Numerous crystal structures have been reported for the isolated extracellular region and tyrosine kinase domain of the epidermal growth factor receptor (EGFR) and its relatives, in different states of activation and bound to a variety of inhibitors used in cancer therapy. The next challenge is to put these structures together accurately in functional models of the intact receptor in its membrane environment. The intact EGFR has been studied using electron microscopy, chemical biology methods, biochemically, and computationally. The distinct approaches yield different impressions about the structural modes of communication between extracellular and intracellular regions. They highlight possible differences between ligands, and also underline the need to understand how the receptor interacts with the membrane itself.

Structure-function relationships and supramolecular organization of the EGFR (epidermal growth factor receptor) on the cell surface

Dimerization and higher-order oligomerization are believed to play an important role in the activation of the EGFR (epidermal growth factor receptor). Understanding of the process has been limited by the lack of availability of suitable methods for the measurement in cells of distances in the range 10-100 nm, too short for imaging methods and too long for spectroscopic methods such as FRET. In the present article, we review the current state of our knowledge of EGFR oligomerization, and describe results from a new single-molecule localization method that has allowed the quantitative characterization of the distribution of EGFR-EGFR distances in cells. Recent data suggest the involvement of cortical actin in regulating the formation of EGFR complexes.

Mechanisms of activation of receptor tyrosine kinases: monomers or dimers

Receptor tyrosine kinases (RTKs) play essential roles in cellular processes, including metabolism, cell-cycle control, survival, proliferation, motility and differentiation. RTKs are all synthesized as single-pass transmembrane proteins and bind polypeptide ligands, mainly growth factors. It has long been thought that all RTKs, except for the insulin receptor (IR) family, are activated by ligand-induced dimerization of the receptors. An increasing number of diverse studies, however, indicate that RTKs, previously thought to exist as monomers, are present as pre-formed, yet inactive, dimers prior to ligand binding. The non-covalently associated dimeric structures are reminiscent of those of the IR family, which has a disulfide-linked dimeric structure. Furthermore, recent progress in structural studies has provided insight into the underpinnings of conformational changes during the activation of RTKs. In this review, I discuss two mutually exclusive models for the mechanisms of activation of the epidermal growth factor receptor, the neurotrophin receptor and IR families, based on these new insights.

Role of receptor tyrosine kinases and their ligands in glioblastoma

Glioblastoma multiforme is the most frequent, aggressive and fatal type of brain tumor. Glioblastomas are characterized by their infiltrating nature, high proliferation rate and resistance to chemotherapy and radiation. Recently, oncologic therapy experienced a rapid evolution towards “targeted therapy,” which is the employment of drugs directed against particular targets that play essential roles in proliferation, survival and invasiveness of cancer cells. A number of molecules involved in signal transduction pathways are used as molecular targets for the treatment of various tumors. In fact, inhibitors of these molecules have already entered the clinic or are undergoing clinical trials. Cellular receptors are clear examples of such targets and in the case of glioblastoma multiforme, some of these receptors and their ligands have become relevant. In this review, the importance of glioblastoma multiforme in signaling pathways initiated by extracellular tyrosine kinase receptors such as EGFR, PDGFR and IGF-1R will be discussed. We will describe their ligands, family members, structure, activation mechanism, downstream molecules, as well as the interaction among these pathways. Lastly, we will provide an up-to-date review of the current targeted therapies in cancer, in particular glioblastoma that employ inhibitors of these pathways and their benefits.

Structure-function relationships of ErbB RTKs in the plasma membrane of living cells

We review the states of the ErbB family of receptor tyrosine kinases (RTKs), primarily the EGF receptor (EGFR, ErbB1, HER1) and the orphan receptor ErbB2 as they exist in living mammalian cells, focusing on four main aspects: (1) aggregation state and distribution in the plasma membrane; (2) conformational features of the receptors situated in the plasma membrane, compared to the crystallographic structures of the isolated extracellular domains; (3) coupling of receptor disposition on filopodia with the transduction of signaling ligand gradients; and (4) ligand-independent receptor activation by application of a magnetic field.

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.

High-throughput de novo screening of receptor agonists with an automated single-cell analysis and isolation system

Reconstitution of signaling pathways involving single mammalian transmembrane receptors has not been accomplished in yeast cells. In this study, intact EGF receptor (EGFR) and a cell wall-anchored form of EGF were co-expressed on the yeast cell surface, which led to autophosphorylation of the EGFR in an EGF-dependent autocrine manner. After changing from EGF to a conformationally constrained peptide library, cells were fluorescently labeled with an anti-phospho-EGFR antibody. Each cell was subjected to an automated single-cell analysis and isolation system that analyzed the fluorescent intensity of each cell and automatically retrieved each cell with the highest fluorescence. In ~3.2 × 10⁶ peptide library, we isolated six novel peptides with agonistic activity of the EGFR in human squamous carcinoma A431 cells. The combination of yeast cells expressing mammalian receptors, a cell wall-anchored peptide library, and an automated single-cell analysis and isolation system might facilitate a rational approach for de novo drug screening.

EGFR signaling and autophagy dependence for growth, survival, and therapy resistance

The epidermal growth factor receptor (EGFR) is amplified or mutated in various human epithelial tumors. Its expression and activation leads to cell proliferation, differentiation, and survival. Consistently, EGFR amplification or expression of EGFR variant 3 (EGFRvIII) is associated with resistance to conventional cancer therapy through activation of pro-survival signaling and DNA-repair mechanisms. EGFR targeting has successfully been exploited as strategy to increase treatment efficacy. Nevertheless, these targeting strategies have only been proven effective in a limited percentage of human tumors. Recent knowledge indicates that EGFR deregulated tumors display differences in autophagy and dependence on autophagy for growth and survival and the use of autophagy to increase resistance to EGFR-targeting drugs. In this review the dependency on autophagy and its role in mediating resistance to EGFR-targeting agents will be discussed. Considering the current knowledge, autophagy inhibition could provide a novel strategy to enhance therapy efficacy in treatment of EGFR deregulated tumors.

Ligand binding and dynamics of the monomeric epidermal growth factor receptor ectodomain

The ectodomain of the human epidermal growth factor receptor (hEGFR) controls input to several cell signalling networks via binding with extracellular growth factors. To gain insight into the dynamics and ligand binding of the ectodomain, the hEGFR monomer was subjected to molecular dynamics simulation. The monomer was found to be substantially more flexible than the ectodomain dimer studied previously. Simulations where the endogeneous ligand EGF binds to either Subdomain I or Subdomain III, or where hEGFR is unbound, show significant differences in dynamics. The molecular mechanics Poisson-Boltzmann surface area method has been used to derive relative free energies of ligand binding, and we find that the ligand is capable of binding either subdomain with a slight preference for III. Alanine-scanning calculations for the effect of selected ligand mutants on binding reproduce the trends of affinity measurements. Taken together, these results emphasize the possible role of the ectodomain monomer in the initial step of ligand binding, and add details to the static picture obtained from crystal structures.

Epidermal growth factor receptor targeting in cancer: a review of trends and strategies

The epidermal growth factor receptor (EGFR) is a cell-surface receptor belonging to ErbB family of tyrosine kinase and it plays a vital role in the regulation of cell proliferation, survival and differentiation. However; EGFR is aberrantly activated by various mechanisms like receptor overexpression, mutation, ligand-dependent receptor dimerization, ligand-independent activation and is associated with development of variety of tumors. Therefore, specific EGFR inhibition is one of the key targets for cancer therapy. Two major approaches have been developed and demonstrated benefits in clinical trials for targeting EGFR; monoclonal antibodies (mAbs) and tyrosine kinase inhibitors (TKIs). EGFR inhibitors like, cetuximab, panitumumab, etc. (mAbs) and gefitinib, erlotinib, lapatinib, etc. (TKIs) are now commercially available for treatment of variety of cancers. Recently, many other agents like peptides, nanobodies, affibodies and antisense oligonucleotide have also shown better efficacy in targeting and inhibiting EGFR. Now a days, efforts are being focused to identify molecular markers that can predict patients more likely to respond to anti-EGFR therapy; to find out combinatorial approaches with EGFR inhibitors and to bring new therapeutic agents with clinical efficacy. In this review we have outlined the role of EGFR in cancer, different types of EGFR inhibitors, preclinical and clinical status of EGFR inhibitors as well as summarized the recent efforts made in the field of molecular EGFR targeting.

Molecular dynamics simulations of transitions for ECD epidermal growth factor receptors show key differences between human and drosophila forms of the receptors

Recent X-ray structural work on the Drosophila epidermal growth factor receptor (EFGR) has suggested an asymmetric dimer that rationalizes binding affinity measurements that go back decades (Alvarado et al., Cell 2010;142:568-579; Dawson et al., Structure 2007;15:942-954; Lemmon et al., Embo J 1997;16:281-294; Mattoon et al., Proc Natl Acad Sci USA 2004;101:923-928; Mayawala et al., Febs Lett 2005;579:3043-3047; Ozcan et al., Proc Natl Acad Sci USA 2006;103:5735-5740). This type of asymmetric structure has not been seen for the human EGF receptor family and it may or may not be important for function in that realm. We hypothesize that conformational changes in the Drosophila system have been optimized for the transition, whereas the barrier for the same transition is much higher in the human forms. To address our hypothesis we perform dynamic importance sampling (DIMS) (Perilla et al., J Comput Chem 2010;32:196-209) for barrier crossing transitions in both Drosophila and human EFGRs. For each set of transitions, we work from the hypothesis, based on results from the AdK system, that salt-bridge pairs making and breaking connections are central to the conformational change. To evaluate the effectiveness of the salt-bridges as drivers for the conformational change, we use the effective transfer entropy based on stable state MD calculations (Kamberaj and Der Vaart, Biophys J 2009;97:1747-1755) to define a reduced subset of degrees of freedom that seem to be important for driving the transition (Perilla and Woolf, J Chem Phys 2012;136:164101). Our results suggest that salt-bridge making and breaking is not the dominant factor in driving the symmetric to asymmetric transition, but that instead it is a result of more concerted and correlated functional motions within a subset of the dimer structures. Furthermore, the analysis suggests that the set of residues involved in the transitions from the Drosophila relative to the human forms differs and that this difference in substate distributions relates to why the asymmetric form may be more common to Drosophila than to the human forms. We close with a discussion about the residues that may be changed in the human and the Drosophila forms to potentially shift the kinetics of the symmetric to asymmetric transition.

Identification of potential glycan cancer markers with sialic acid attached to sialic acid and up-regulated fucosylated galactose structures in epidermal growth factor receptor secreted from A431 cell line

We have used powerful HPLC-mass spectrometric approaches to characterize the secreted form of epidermal growth factor receptor (sEGFR). We demonstrated that the amino acid sequence lacked the cytoplasmic domain and was consistent with the primary sequence reported for EGFR purified from a human plasma pool. One of the sEGFR forms, attributed to the alternative RNA splicing, was also confirmed by transcriptional analysis (RNA sequencing). Two unusual types of glycan structures were observed in sEGFR as compared with membrane-bound EGFR from the A431 cell line. The unusual glycan structures were di-sialylated glycans (sialic acid attached to sialic acid) at Asn-151 and N-acetylhexosamine attached to a branched fucosylated galactose with N-acetylglucosamine moieties (HexNAc-(Fuc)Gal-GlcNAc) at Asn-420. These unusual glycans at specific sites were either present at a much lower level or were not observable in membrane-bound EGFR present in the A431 cell lysate. The observation of these di-sialylated glycan structures was consistent with the observed expression of the corresponding α-N-acetylneuraminide α-2,8-sialyltransferase 2 (ST8SiA2) and α-N-acetylneuraminide α-2,8-sialyltransferase 4 (ST8SiA4), by quantitative real time RT-PCR. The connectivity present at the branched fucosylated galactose was also confirmed by methylation of the glycans followed by analysis with sequential fragmentation in mass spectrometry. We hypothesize that the presence of such glycan structures could promote secretion via anionic or steric repulsion mechanisms and thus facilitate the observation of these glycan forms in the secreted fractions. We plan to use this model system to facilitate the search for novel glycan structures present at specific sites in sEGFR as well as other secreted oncoproteins such as Erbb2 as markers of disease progression in blood samples from cancer patients.

Recombinant IgE antibody engineering to target EGFR

Monoclonal antibodies have become a mainstay for the targeted treatment of cancer today. Some of the most successful targets of monoclonal antibodies are constituted by the epidermal growth factor receptor family spearheaded by the epidermal growth factor receptor (EGFR). Prompted by studies indicating that IgE compared to IgG may harness alternate effector functions to eradicate malignant cells, we addressed the establishment, engineering, and the potential tumoricidal effects of recombinant anti-EGFR IgE. Therefore, two different therapeutic EGFR-specific antibodies, 225 and 425, were chosen for re-cloning into different chimeric IgE and IgG formats and produced in human cells. Simultaneous antibody binding to the sEGFR demonstrated accessibility of both epitopes for recombinant IgE. Proliferation and cytotoxicity assays demonstrated signal blocking and effector mediating capability of IgE isotypes. Pronounced degranulation in the presence of sEGFR upon activation exclusively with two IgE antibodies verified the epitope proximity and provides evidence that tumor-targeting by anti-EGFR IgE is safe with regard to soluble target structures. Degranulation mediated by tumor cells expressing EGFR could be demonstrated for singular and combined IgE antibodies; however, use of two IgE specificities was not superior to use of one IgE alone. The data suggest that the surface distribution of EGFR is optimally suited to mount a robust effector cell trigger and corroborate the potential and specificity of the IgE/IgE receptor network to react to xenobiotic or pathogenic patterns for targeting malignancies.

A pilot study of nimotuzumab combined with cisplatin and 5-FU in patients with advanced esophageal squamous cell carcinoma

OBJECTIVE

To observe the short-term effect and adverse reaction of Nimotuzumab in combination with chemotherapy on advanced esophageal squamous cell carcinoma (ESCC).

METHOD

19 patients were treated with the following protocol: Nimotuzumab 400mg/time/week in the 1(st) week, 200mg/time/week from the 2(nd) to 8(th) week, intravenous drip (IVD); Cisplatin 80 mg/m(2), IVD, 4 weeks a cycle and repeated again; 5-FU 750 mg/m(2), continuous 24-hours pump-in × 5 days, 4 weeks a cycle and repeated again.

RESULT

16 of all 19 patients can be evaluated. After treatment, RP is 42.1% (95% CI, 19.9-64.3%) and DCR is 68.4%; the main side effects include arrest of bone marrow, gastrointestinal reactions, asthenia, etc.

CONCLUSION

Nimotuzumab in combination with cisplatin/5-FU regimens in patients with advanced ESCC is safe and effective, which deserves a further expanded sample research.

Evaluation of dimerization-inhibitory activities of cyclic peptides containing a β-hairpin loop sequence of the EGF receptor

Structure-activity relationships of cyclic peptides mimicking the β-hairpin structure of the 'dimerization arm' at residues 242-259 of the EGF receptor are examined. Cyclic peptides containing the arm head of the β-hairpin loop showed inhibitory activity toward the EGF receptor's dimerization. Cyclic peptides containing a Retro-Inverso sequence of the dimerization arm showed clear inhibitory effects on the dimerization in vitro and efficiently suppressed the proliferation of A431 cells, which abundantly express the EGF receptor on their surface. The effects at a specific hydrophobic site of the loop structure were expected to enhance the interactions with the receptor.

Microtubule assembly-derived by dimerization of TPPP/p25. Evaluation of thermodynamic parameters for multiple equilibrium system from ITC data

BACKGROUND

The disordered Tubulin Polymerization Promoting Protein/p25 (TPPP/p25) modulates the dynamics and stability of the microtubule system. In this paper the role of dimerization in its microtubule-related functions is established, and an approach is proposed to evaluate thermodynamic constants for multiple equilibrium systems from ITC measurements.

METHODS

For structural studies size exclusion chromatography, SDS-PAGE, chemical cross-linking, circular dichroism, fluorescence spectroscopy and isothermal titration calorimetry were used; the functional effect was analyzed by tubulin polymerization assay. Numerical simulation of the multiple equilibrium was performed with Mathematica software.

RESULTS

The dimerization of TPPP/p25 is promoted by elevation of the protein concentration and by GTP addition. The dimeric form displaying enhanced tubulin polymerization promoting activity is stabilized by disulfide bond or chemical cross-linking. The GTP binding to the dimeric form (Kd-GTP=200 μM) is tighter with one order of magnitude than to the monomeric one leading to the enrichment of the dimers. A mathematical model elaborated for the multiple equilibrium of the TPPP/p25-GTP system was validated by fitting the GTP-dependent changes of ellipticity and fluorescence signal in the course of TPPP/p25 titrations. The evaluation of the equilibrium constants rendered it possible to determine the thermodynamic parameters of the association of different TPPP/p25 forms with GTP from ITC measurements.

CONCLUSIONS/GENERAL SIGNIFICANCE

The dimerization of TPPP/p25 with favorable physiological functional potency is proposed to play significant role in the fine tuning of TPPP/p25-mediated microtubule assembly; the unfolded monomers might be involved in the formation of pathological inclusions characteristic for Parkinson's disease and other synucleinopathies.

Regulation of integrin adhesions by varying the density of substrate-bound epidermal growth factor

Substrates coated with specific bioactive ligands are important for tissue engineering, enabling the local presentation of extracellular stimulants at controlled positions and densities. In this study, we examined the cross-talk between integrin and epidermal growth factor (EGF) receptors following their interaction with surface-immobilized Arg-Gly-Asp (RGD) and EGF ligands, respectively. Surfaces of glass coverslips, modified with biotinylated silane-polyethylene glycol, were functionalized by either biotinylated RGD or EGF (or both) via the biotin-NeutrAvidin interaction. Fluorescent labeling of the adhering A431 epidermoid carcinoma cells for zyxin or actin indicated that EGF had a dual effect on focal adhesions (FA) and stress fibers: at low concentrations (0.1; 1 ng/ml), it stimulated their growth; whereas at higher concentrations, on surfaces with low to intermediate RGD densities, it induced their disassembly, leading to cell detachment. The EGF-dependent dissociation of FAs was, however, attenuated on higher RGD density surfaces. Simultaneous stimulation by both immobilized RGD and EGF suggest a strong synergy between integrin and EGFR signaling, in FA induction and cell spreading. A critical threshold level of EGF was required to induce significant variation in cell adhesion; beyond this critical density, the immobilized molecule had a considerably stronger effect on cell adhesion than did soluble EGF. The mechanisms underlying this synergy between the adhesion ligand and EGF are discussed.

Investigating extracellular in situ EGFR structure and conformational changes using FRET microscopy

The crystallographic structures of functional fragments of ErbBs have provided excellent insights into the geometry of growth factor binding and receptor dimerization. By placing together receptor fragments to build structural models of entire receptors, we expect to understand how these enzymes are allosterically regulated; however, several predictions from these models are inconsistent with experimental evidence from cells. The opening of this gap underlines the need to investigate intact ErbBs by combining cellular and structural studies into a full picture.

In-depth biophysical analysis of interactions between therapeutic antibodies and the extracellular domain of the epidermal growth factor receptor

Targeting of the epidermal growth factor receptor (EGFR) with monoclonal antibodies has become an established antitumor strategy in clinical use or in late stages of drug development. The mAbs effector mechanisms have been widely analyzed based on in vivo or cell studies. Hereby we intend to complement these functional studies by investigating the mAb-EGFR interactions on a molecular level. Surface plasmon resonance, isothermal titration calorimetry, and static light scattering were employed to characterize the interactions of matuzumab, cetuximab, and panitumumab with the extracellular soluble form ecEGFR. The kinetic and thermodynamic determinants dissected the differences in mAbs binding mechanism toward ecEGFR. The quantitative stoichiometric data clearly demonstrated the bivalent binding of the mAbs to two ecEGFR molecules. Our results complement earlier studies on simultaneous binding of cetuximab and matuzumab. The antibodies retain their bivalent binding mode achieving a 1:2:1 complex formation. Interestingly the binding parameters remain nearly constant for the individual antibodies in this ternary assembly. In contrast the binding of panitumumab is almost exclusive either by directly blocking the accessibility for the second antibody or by negative allosteric modulation. Overall we provide a comprehensive biophysical dataset on binding parameters, the complex assembly, and relative epitope accessibility for therapeutic anti-EGFR antibodies.

ErbB1 dimerization is promoted by domain co-confinement and stabilized by ligand binding

The extent to which ligand occupancy and dimerization contribute to erbB1 signaling is controversial. To examine this, we utilized two-color Quantum Dot tracking for visualization of erbB1 homodimerization and quantification of the dimer off rate (k_off) on living cells. Kinetic parameters were extracted using a 3-state Hidden Markov Model to identify transition rates between free, co-confined, and dimerized states. We report that dimers composed of 2 ligand-bound receptors are long-lived and their k_off is independent of kinase activity. By comparison, unliganded dimers have >4-fold faster k_off. Transient co-confinement of receptors promotes repeated encounters and enhances dimer formation. Mobility decreases >6-fold when ligand-bound receptors dimerize. Blockade of erbB1 kinase activity or disruption of actin networks results in faster diffusion of receptor dimers. These results implicate both signal propagation and the cortical cytoskeleton in reduced mobility of signaling-competent erbB1 dimers.

Disconnecting the yin and yang relation of epidermal growth factor receptor (EGFR)-mediated delivery: a fully synthetic, EGFR-targeted gene transfer system avoiding receptor activation

The epidermal growth factor receptor (EGFR) is upregulated within a high percentage of solid tumors and hence is an attractive target for tumor-targeted therapies including gene therapy. The natural EGFR ligand epidermal growth factor (EGF) has been used for this purpose, despite the risk of mitogenic effects due to EGFR activation. We have developed a fully synthetic, EGFR-targeted gene delivery system based on PEGylated linear polyethylenimine (LPEI), allowing evaluation of different EGFR-binding peptides in terms of transfection efficiency and EGFR activation. Peptide sequences directly derived from the human EGF molecule enhanced transfection efficiency with concomitant EGFR activation. Only the EGFR-binding peptide GE11, which has been identified by phage display technique, showed specific enhancement of transfection on EGFR-overexpressing tumor cells including glioblastoma and hepatoma, but without EGFR activation. EGFR targeting led to high levels of cell association of fluorescently labeled polyplexes after only 30 min of incubation. EGF pretreatment of cells induced enhanced cellular internalization of all polyplex types tested, pointing at generally enhanced macropinocytosis. EGF polyplexes diminished cell surface expression of EGFR for up to 4 hr, whereas GE11 polyplexes did not. In a clinically relevant orthotopic prostate cancer model, intratumorally injected GE11 polyplexes were superior in inducing transgene expression when compared with untargeted polyplexes.

Differential and synergistic effects of epidermal growth factor receptor antibodies on unliganded ErbB dimers and oligomers

Antibodies directed against the epidermal growth factor receptor (EGFR) offer a potentially powerful therapeutic approach against cancers driven by the EGFR pathway. EGFR antibodies are believed to halt cell surface activation by blocking ligand-induced receptor tyrosine kinase activation, i.e., ligand binding, a change in conformation, or the monomer-dimer transition. In this work, we demonstrate that wild-type EGFR and the truncated de2-7-EGFR (tumor-associated mutant) formed unliganded homo-oligomers and examined the effects of two clinically relevant antibodies on the conformation and quaternary state of these ligand-free EGFR oligomers on the surface of cells. The EGFR antibodies were mAb528, a ligand-blocking antibody that binds domain III, and mAb806, a conformationally sensitive antibody that binds near the dimer interface in domain II. We used a model cellular system, BaF/3 cells, with GFP-tagged receptors in the absence of interference from secreted ligands or other erbB receptor members. Different antibody-mediated effects (conformational transition, receptor cross-linking, or receptor dissociation) were distinguished by combining two complementary biophysical techniques: image correlation spectroscopy (submicrometer scale clustering) and homo-Forster resonance energy transfer (association and/or conformation on a 1-10 nm scale). mAb528 cross-linked EGFR into an inactive EGFR dimer of dimers but had no effect when added to de2-7-EGFR oligomers. mAb806 had a minor effect on EGFR dimers as expected from its poor binding to a conformationally shielded epitope on wtEGFR but bound de2-7-EGFR oligomers, causing a conformational change in the intracellular C-terminal GFP-tagged tail. The combination of the two antibodies had synergistic effects, increasing the level of cross-linking of de2-7-EGFR, but did not lead to enhanced cross-linking of EGFR. The results reveal new modes of receptor-antibody interactions for EGFR and de2-7-EGFR.

Polymorphism of the epidermal growth factor receptor extracellular ligand binding domain: the dimer interface depends on domain stabilization

Epidermal growth factor receptors (EGFRs) and their cytoplasmic tyrosine kinases play important roles in cell proliferation and signaling. The EGFR extracellular domain (sEGFR) forms a dimer upon the binding of ligands, such as epidermal growth factor (EGF) and transforming growth factor α (TGFα). In this study, multiple molecular dynamics (MD) simulations of the 2:2 EGF·sEGFR3-512 dimer and the 2:2 TGFα·sEGFR3-512 dimer were performed in solvent and crystal environments. The simulations of systems comprising up to half a million atoms reveal part of the structural dynamics of which sEGFR dimers are capable. The solvent simulations consistently exhibited a prominent conformational relaxation from the initial crystal structures on the nanosecond time scale, leading to symmetry breaking and more extensive contacts between the two sEGFR monomers. In the crystal control simulation, this symmetry breaking and compaction was largely suppressed by crystal packing contacts. The simulations also provided evidence that the disordered domain IV of sEGFR may act as a stabilizing spacer in the dimer. Thus, the simulations suggest that the sEGFR dimer can take diverse configurations in solvent environments. These biologically relevant conformations of the EGFR signal transduction network can be controlled by contacts among the structural domains of sEGFR and its ligands.

Epidermal growth factor receptor mutation in combination with expression of MIG6 alters gefitinib sensitivity

BACKGROUND

Epidermal growth factor receptor (EGFR) signaling plays an important role in the regulation of cell proliferation, survival, metastasis, and invasion in various tumors. Earlier studies showed that the EGFR is frequently overexpressed in non-small-cell lung cancer (NSCLC) and EGFR mutations at specific amino acid residues in the kinase domain induce altered responsiveness to gefitinib, a small molecule EGFR tyrosine kinase inhibitor. However, the mechanism underlying the drug response modulated by EGFR mutation is still largely unknown. To elucidate drug response in EGFR signal transduction pathway in which complex dynamics of multiple molecules involved, a systematic approach is necessary. In this paper, we performed experimental and computational analyses to clarify the underlying mechanism of EGFR signaling and cell-specific gefitinib responsiveness in three H1299-derived NSCLC cell lines; H1299 wild type (H1299WT), H1299 with an overexpressed wild type EGFR (H1299EGFR-WT), and H1299 with an overexpressed mutant EGFR L858R (H1299L858R; gefitinib sensitive mutant).

RESULTS

We predicted and experimentally verified that Mig6, which is a known negative regulator of EGFR and specifically expressed in H1299L858R cells, synergized with gefitinib to suppress cellular growth. Computational analyses indicated that this inhibitory effect is amplified at the phosphorylation/dephosphorylation steps of MEK and ERK.

CONCLUSIONS

Thus, we showed that L858R receptor mutation in combination with expression of its negative regulator, Mig6, alters signaling outcomes and results in variable drug sensitivity.

The extracellular domain of fibroblast growth factor receptor 3 inhibits ligand-independent dimerization

Dysregulation of the ligand-independent dimerization of receptor tyrosine kinases (RTKs), which is the first step in the activation of RTKs, leads to various pathologies. A mechanistic understanding of the dimerization process is lacking, and this lack of basic knowledge is one bottleneck in the development of effective RTK-targeted therapies. For example, the roles and relative contributions of the different domains of RTKs to receptor dimerization are unknown. Here, we used quantitative imaging Förster resonance energy transfer (QI-FRET) to determine the contribution of the extracellular domain of fibroblast growth factor receptor 3 (FGFR3) to the dimerization of the receptor. We provide evidence that the contribution of the extracellular domain of FGFR3 to dimerization is repulsive in the absence of ligand and is on the order of ~1 kcal/mol. The repulsive contribution of the extracellular domain is similar in magnitude, but opposite in sign, to the contribution of pathogenic single-amino acid mutations to RTK signaling, and is therefore likely to be important for biological function. Together, these results highlight the fine balance in the domain interactions that regulate RTK dimerization and signaling.

The NMR solution structure of human epidermal growth factor (hEGF) at physiological pH and its interactions with suramin

Human epidermal growth factor (hEGF) induces the proliferation, differentiation and survival of various cell types including tumor-derived cells. Generally, hEGF performs its biological function by binding to a specific receptor (hEGFR) on the cell surface, thereby inducing signal transduction. Suramin, a polysulfonated naphthylurea that acts as a growth factor blocker, exhibits antiproliferative activity against non-small cell lung cancer (NSCLC) cells that overexpress EGFR on the cell surface. We determined the solution structure of hEGF under physiological conditions and investigated the interaction of suramin with hEGF using isothermal titration calorimetry and NMR spectroscopy techniques. The solution structure of hEGF presented in this paper is different from the bound form of hEGF present in the crystal structure of the 2:2 EGF-EGFR complex because its C-tail contains a hydrophobic core. This conformational difference supports the hypothesis that hEGF undergoes a conformational change when it binds to hEGFR and subsequently induces signal transduction. Based on the docking structure of the hEGF-suramin complex, we demonstrated how suramin blocks hEGF by binding to its receptor binding site (the C-terminal region around Arg45) and inhibits the crucial conformational change.

Structural basis for negative cooperativity in growth factor binding to an EGF receptor

Transmembrane signaling by the epidermal growth factor receptor (EGFR) involves ligand-induced dimerization and allosteric regulation of the intracellular tyrosine kinase domain. Crystallographic studies have shown how ligand binding induces dimerization of the EGFR extracellular region but cannot explain the "high-affinity" and "low-affinity" classes of cell-surface EGF-binding sites inferred from curved Scatchard plots. From a series of crystal structures of the Drosophila EGFR extracellular region, we show here how Scatchard plot curvature arises from negatively cooperative ligand binding. The first ligand-binding event induces formation of an asymmetric dimer with only one bound ligand. The unoccupied site in this dimer is structurally restrained, leading to reduced affinity for binding of the second ligand, and thus negative cooperativity. Our results explain the cell-surface binding characteristics of EGF receptors and suggest how individual EGFR ligands might stabilize distinct dimeric species with different signaling properties.

The cysteine pairs in CLV2 are not necessary for sensing the CLV3 peptide in shoot and root meristems

Receptor-like proteins (RLPs) are involved in both plant defense and developmental processes. Previous genetic and biochemical studies show that the leucine-rich repeat (LRR) receptor-like protein CLAVATA2 (CLV2) functions together with CLAVATA1 (CLV1) and CORYNE (CRN) in Arabidopsis to limit the stem cell number in shoot apical meristem, while in root it acts with CRN to trigger a premature differentiation of the stem cells after sensing the exogenously applied peptides of CLV3p, CLE19p or CLE40p. It has been proposed that disulfide bonds might be formed through two cysteine pairs in the extracellular LRR domains of CLV1 and CLV2 to stabilize the receptor complex. Here we tested the hypothesis by replacing these cysteines with alanines and showed that depletions of one or both of the cysteine pairs do not hamper the function of CLV2 in SAM maintenance. In vitro peptide assay also showed that removal of the cysteine pairs did not affect the perception of CLV3 peptides in roots. These observations allow us to conclude that the formation of disulfide bonds is not needed for the function of CLV2.

Transmembrane helix-helix interactions involved in ErbB receptor signaling

Among the many transmembrane receptor classes, the receptor tyrosine kinases represent an important superfamily, involved in many cellular processes like embryogenesis, development and cell division. Deregulation and dysfunctions of these receptors can lead to various forms of cancer and other diseases. Mostly, only fragmented knowledge exists about functioning of the entire receptors, and many studies have been performed on isolated receptor domains. In this review we focus on the function of the ErbB family of receptor tyrosine kinases with a special emphasis on the role of the transmembrane domain and on the mechanisms underlying regulated and deregulated signaling. Many general aspects of ErbB receptor structure and function have been analyzed and described. All human ErbBs appear to form homo- and heterodimers within cellular membranes and the single transmembrane domain of the receptors is involved in dimerization. Additionally, only defined structures of the transmembrane helix dimer allows signaling of ErbB receptors.

Reversible dimerization of EGFR revealed by single-molecule fluorescence imaging using quantum dots

The current work explores intermolecular interactions involved in the lateral propagation of cell-signaling by epidermal growth factor receptors (EGFRs). Activation of EGFRs by binding an EGF ligand in the extracellular domain of the EGFR and subsequent dimerization of the EGFR initiates cell-signaling. We investigated interactions between EGFRs in living cells by using single-molecule microscopy, Förster resonance energy transfer (FRET), and atomic force microscopy. By analyzing time-correlated intensity and propagation trajectories of quantum dot (QD)-labeled EGFR single molecules, we found that signaling dimers of EGFR [(EGF-EGFR)(2)] are continuously formed in cell membrane through reversible association of heterodimers [EGF(EGFR)(2)]. Also, we found that the lateral propagation of EGFR activation takes place through transient association of a heterodimer with predimers [(EGFR)(2)]. We varified the transient association between activated EGFR and predimers using FRET from QD-labeled heterodimers to Cy5-labeled predimers and correlated topography and fluorescence imaging. Without extended single-molecule fluorescence imaging and by using bio-conjugated QDs, reversible receptor dimerization in the lateral activation of EGFR remained obscured.

Oncologist's/haematologist's view on the roles of pathologists for molecular targeted cancer therapy

In the past two decades there has been a tremendous increase in the understanding of the molecular basis of human malignancies. In a variety of neoplasms, specific molecular markers became part of disease classifications and are now routinely used to define specific entities. Molecular analyses discriminate prognostic groups, guide differential treatment strategies and identify targets for molecular defined cancer therapy. A battery of new drugs has been developed to specifically inhibit oncogenic pathways. For an increasing number of solid and haematological malignancies, the availability of molecular targeted drugs has fundamentally changed treatment algorithms. However, the diagnostic, prognostic and therapeutic impact of selected molecular markers is still limited in many cases. After all, the success of a molecular targeted therapy is clearly determined by the significance of the targeted structure for the biology of cancer and the ability of the malignant cell to evade specific inhibition.

Cellular signalling: Peptide hormones and growth factors

Peptide hormones and growth factors initiate signalling by binding to and activating their cell surface receptors. The activated receptors interact with and modulate the activity of cell surface enzymes and adaptor proteins which entrain a series of reactions leading to metabolic and proliferative signals. Rapid internalization of ligand-receptor complexes into the endosomal system both prolongs and augments events initiated at the cell surface. In addition endocytosis brings activated receptors into contact with a wider range of substrates giving rise to unique signalling events critical for modulating proliferation and apoptosis. Within the endosomal system, receptor function is regulated by lowering vacuolar pH, augmenting ligand proteolysis and promoting receptor kinase dephosphorylation. Ubiquitination-deubiquitination plays a key role in regulating receptor traffic through the endosomal system resulting in either recycling to the cell surface or degradation in multivesicular-lysosomal elements. From a clinical perspective there are several studies showing that manipulating endosomal processes may constitute a new therapeutic strategy.

Modified epidermal growth factor receptor (EGFR)-bearing liposomes (MRBLs) are sensitive to EGF in solution

Cancers often overexpress EGF and other growth factors to promote cell replication and migration. Previous work has not produced targeted drug carriers sensitive to abnormal amounts of growth factors. This work demonstrates that liposomes bearing EGF receptors covalently crosslinked to p-toluic acid or methyl-PEO(4)-NHS ester (or, in short, MRBLs) exhibit an increased rate of release of encapsulated drug compounds when EGF is present in solution. Furthermore, the modified EGF receptors retain the abilities to form dimers in the presence of EGF and bind specifically to EGF. These results demonstrate that MRBLs are sensitive to EGF in solution and indicate that MRBL-reconstituted modified EGF receptors, in the presence of EGF in solution, form dimers which increase MRBL permeability to encapsulated compounds.

Energetics of ErbB1 transmembrane domain dimerization in lipid bilayers

One of the most extensively studied receptor tyrosine kinases is EGFR/ErbB1. Although our knowledge of the role of the extracellular domains and ligands in ErbB1 activation has increased dramatically based on solved domain structures, the exact mechanism of signal transduction across the membrane remains unknown. The transmembrane domains are expected to play an important role in the dimerization process, but the contribution of ErbB1 TM domain to dimer stability is not known, with published results contradicting one another. We address this controversy by showing that ErbB1 TM domain dimerizes in lipid bilayers and by calculating its contribution to stability as -2.5 kcal/mol. The stability calculations use two different methods based on Förster resonance energy transfer, which give the same result. The ErbB1 TM domain contribution to stability exceeds the change in receptor tyrosine kinases dimerization propensities that can convert normal signaling processes into pathogenic processes, and is thus likely important for biological function.

Cetuximab plus cisplatin-5-fluorouracil versus cisplatin-5-fluorouracil alone in first-line metastatic squamous cell carcinoma of the esophagus: a randomized phase II study of the Arbeitsgemeinschaft Internistische Onkologie

BACKGROUND

This study assessed the activity of the mAb cetuximab in combination with cisplatin and 5-fluorouracil (5-FU) in advanced esophageal squamous cell carcinoma.

PATIENTS AND METHODS

For a maximum of six 29-day cycles, patients received cisplatin 100 mg/m(2), day 1, plus 5-FU 1000 mg/m(2), days 1-5 (CF), either alone or in combination with cetuximab (CET-CF; 400 mg/m(2) initial dose followed by 250 mg/m(2) weekly thereafter). The primary end point was tumor response. Tumor material was obtained for analysis of KRAS mutation status.

RESULTS

Sixty-two eligible patients were included, 32 receiving CET-CF and 30 CF. Cetuximab did not exacerbate grade 3/4 toxicity, except for rash (6% versus 0%) and diarrhea (16% versus 0%). The overall response rate according to RECIST criteria was 19% and 13% and the disease control rate 75% and 57% for the CET-CF and CF arms, respectively. With a median follow-up of 21.5 months, the median progression-free survival was 5.9 and 3.6 months and median overall survival 9.5 and 5.5 months for CET-CF and CF, respectively. No KRAS codon 12/13 tumor mutations were identified in 37 evaluated samples.

CONCLUSION

Cetuximab can be safely combined with CF chemotherapy and may increase the efficacy of standard CF chemotherapy.

Mechanisms of tumor resistance to EGFR-targeted therapies

BACKGROUND

Much effort has been devoted to development of cancer therapies targeting EGFR, based on its role in regulating cell growth. Small-molecule and antibody EGFR inhibitors have clinical roles based on their efficacy in a subset of cancers, generally as components of combination therapies. Many cancers are either initially resistant to EGFR inhibitors or become resistant during treatment, limiting the efficacy of these reagents.

OBJECTIVE/METHODS

To review cellular resistance mechanisms to EGFR-targeted therapies.

RESULTS/CONCLUSIONS

The best validated of these mechanisms include activation of classic ATP-binding casette (ABC) multidrug transporters; activation or mutation of EGFR; and overexpression or activation of signaling proteins operating in relation to EGFR. We discuss current efforts and potential strategies to override these sources of resistance. We describe emerging systems-biology-based concepts of alternative resistance to EGFR-targeted therapies, and discuss their implications for use of EGFR-targeted and other targeted therapies.