Identification of the high affinity binding site of transforming growth factor-alpha (TGF-alpha) for the chicken epidermal growth factor (EGF) receptor using EGF/TGF-alpha chimeras

Tenascin cytotactin epidermal growth factor-like repeat binds epidermal growth factor receptor with low affinity

Select epidermal growth factor (EGF)-like (EGFL) repeats of human tenascin cytotactin (tenascin C) can stimulate EGF receptor (EGFR) signaling, but activation requires micromolar concentrations of soluble EGFL repeats in contrast to subnanomolar concentrations of classical growth factors such as EGF. Using in silico homology modeling techniques, we generated a structure for one such repeat, the 14th EGFL repeat (Ten14). Ten14 assumes a tight EGF-like fold with truncated loops, consistent with circular dichroism studies. We generated bound structures for Ten14 with EGFR using two different approaches, resulting in two distinctly different conformations. Normal mode analysis of both structures indicated that the binding pocket of EGFR exhibits a significantly higher mobility in Ten14-EGFR complex compared to that of the EGF-EGFR complex; we hypothesized this may be attributed to loss of key high-affinity interactions within the Ten14-EGFR complex. We proved the efficacy of our in silico models by in vitro experiments. Surface plasmon resonance measurements yielded equilibrium constant K(D) of 74 microM for Ten14, approximately three orders of magnitude weaker than that of EGF. In accordance with our predicted bound models, Ten14 in monomeric form does not bind EGFR with sufficient stability so as to induce degradation of receptor, or undergo EGFR-mediated internalization over either the short (20 min) or long (48 h) term. This transient interaction with the receptor on the cell surface is in marked contrast to other EGFR ligands which cause EGFR transit through, and signaling from intracellular locales in addition to cell surface signaling.

Simulating the impact of a molecular 'decision-process' on cellular phenotype and multicellular patterns in brain tumors

Experimental evidence indicates that human brain cancer cells proliferate or migrate, yet do not display both phenotypes at the same time. Here, we present a novel computational model simulating this cellular decision-process leading up to either phenotype based on a molecular interaction network of genes and proteins. The model's regulatory network consists of the epidermal growth factor receptor (EGFR), its ligand transforming growth factor-alpha (TGF alpha), the downstream enzyme phospholipaseC-gamma (PLC gamma) and a mitosis-associated response pathway. This network is activated by autocrine TGF alpha secretion, and the EGFR-dependent downstream signaling this step triggers, as well as modulated by an extrinsic nutritive glucose gradient. Employing a framework of mass action kinetics within a multiscale agent-based environment, we analyse both the emergent multicellular behavior of tumor growth and the single-cell molecular profiles that change over time and space. Our results show that one can indeed simulate the dichotomy between cell migration and proliferation based solely on an EGFR decision network. It turns out that these behavioral decisions on the single cell level impact the spatial dynamics of the entire cancerous system. Furthermore, the simulation results yield intriguing experimentally testable hypotheses also on the sub-cellular level such as spatial cytosolic polarization of PLC gamma towards an extrinsic chemotactic gradient. Implications of these results for future works, both on the modeling and experimental side are discussed.

Solution structure of epiregulin and the effect of its C-terminal domain for receptor binding affinity

Epiregulin (EPR), a novel member of epidermal growth factor (EGF) family, is a ligand for ErbB-1 and ErbB-4 receptors. The binding affinity of EPR for the receptors is lower than those of other EGF-family ligands. The solution structure of EPR was determined using two-dimensional nuclear magnetic resonance spectroscopy. The secondary structure in the C-terminal domain of EPR is different from other EGF-family ligands because of the lack of hydrogen bonds. The structural difference in the C-terminal domain may provide an explanation for the reduced binding affinity of EPR to the ErbB receptors.

Activin and follicle-stimulating hormone signaling are required for long-term culture of functionally differentiated primary granulosa cells from the chicken ovary

Follicle-stimulating hormone, activin A, and transforming growth factor (TGF) alpha are important regulators of chicken granulosa cell (cGC) function. Hence, we aimed to test whether these growth factors are useful for establishing a suitable in vitro cell culture model system of primary cGC. Although cGC are easily isolated from distinct follicular stages, a long-term cGC culture system for in vitro studies has been unavailable. Here, we report a novel, long-term cell culture system that allows for cGC proliferation in vitro while maintaining the epithelial phenotype that granulosa cells exhibit in vivo. The cGC rapidly lose their epithelial morphology and acquire a mesenchymal or fibroblastoid phenotype when cultured in the absence of activin A. This process is strongly enhanced by TGFalpha, a well-known granulosa cell mitogen. However, FSH stimulates cGC proliferation without enhancing morphological changes and dedifferentiation. Interestingly, a combination of both activin A and FSH stimulates cGC proliferation and supports maintenance of differentiated epithelial morphology. Furthermore, activin A and FSH synergistically induce granulosa cell-specific differentiation markers such as inhibin alpha and chicken zona pellucida protein C, suggesting that cultured cGC resemble functionally differentiated granulosa cells. Our data demonstrate that activin signaling is necessary to sustain a morphologically differentiated phenotype of cGC in vitro. The results also suggest a pivotal importance of activin signaling for granulosa cell function in vivo.

Modeling the epidermal growth factor -- epidermal growth factor receptor l2 domain interaction: implications for the ligand binding process

Signaling from the epidermal growth factor (EGF) receptor is triggered by the binding of ligands such as EGF or transforming growth factor alpha (TGF-alpha) and subsequent receptor dimerization. An understanding of these processes has been hindered by the lack of structural information about the ligand-bound, dimerized EGF receptor. Using an NMR-derived structure of EGF and a homology model of the major ligand binding domain of the EGF receptor and experimental data, we modeled the binding of EGF to this EGF receptor fragment. In this low resolution model of the complex, EGF sits across the second face of the EGF receptor L2 domain and EGF residues 10-16, 36-37, 40-47 bind to this face. The structural model is largely consistent with previously published NMR data describing the residues of TGF-alpha which interact strongly with the EGF receptor. Other EGF residues implicated in receptor binding are accounted by our proposal that the ligand binding is a two-step process with the EGF binding to at least one other site of the receptor. This three-dimensional model is expected to be useful in the design of ligand-based antagonists of the receptor.

Structure and activity of the insect cytokine growth-blocking peptide. Essential regions for mitogenic and hemocyte-stimulating activities are separate

Growth-blocking peptide (GBP) is a 25-amino acid insect cytokine found in Lepidopteran insects that possesses diverse biological activities such as larval growth regulation, cell proliferation, and stimulation of immune cells (plasmatocytes). The tertiary structure of GBP consists of a structured core that contains a disulfide bridge and a short antiparallel beta-sheet (Tyr(11)-Arg(13) and Cys(19)-Pro(21)) and flexible N and C termini (Glu(1)-Gly(6) and Phe(23)-Gln(25)). In this study, deletion and point mutation analogs of GBP were synthesized to investigate the relationship between the structure of GBP and its mitogenic and plasmatocyte spreading activity. The results indicated that deletion of the N-terminal residue, Glu(1), eliminated all plasmatocyte spreading activity but did not reduce mitogenic activity. In contrast, deletion of Phe(23) along with the remainder of the C terminus destroyed all mitogenic activity but only slightly reduced plasmatocyte spreading activity. Therefore, the minimal structure of GBP containing mitogenic activity is 2-23 GBP, whereas that with plasmatocyte spreading activity is 1-22 GBP. NMR analysis indicated that these N- and C-terminal deletion mutants retained a similar core structure to wild-type GBP. Replacement of Asp(16) with either a Glu, Leu, or Asn residue similarly did not alter the core structure of GBP. However, these mutants had no mitogenic activity, although they retained about 50% of their plasmatocyte spreading activity. We conclude that specific residues in the unstructured and structured domains of GBP differentially affect the biological activities of GBP, which suggests the possibility that multifunctional properties of this peptide may be mediated by different forms of a GBP receptor.

Specific inhibition of the reaction between a tumor-inhibitory antibody and the ErbB-2 receptor by a mimotope derived from a phage display library

Overexpression of ErbB-2, a coreceptor for stroma-derived growth factors, is involved in malignancies of epithelial tissues, and a humanized antibody to ErbB-2 was shown to be therapeutic in a clinical setting. In an effort to understand and enhance immunotherapy, the laboratory has raised several tumor inhibitory monoclonal antibodies (mAb), including mAb L26 that blocks inter-receptor interactions. Here the application of the phage display methodology for the isolation of a phage clone that specifically recognizes mAb L26 is described. The isolated mimetic peptide (mimotope) specifically inhibited the binding of mAb L26 to ErbB-2 overexpressing cells. No sequence homology was found between the mimotope and ErbB-2. implying that it mimics a conformational structure of the receptor. Preliminary studies showed that the lead peptide can be truncated by removal of two to three amino acids from either the N- or C-terminal end without drastically affecting the inhibitory properties of the mimotope. A tryptophan'glycine residue at the C-terminus and a lysine at the N-terminus of the peptide seemed to play a role in its ability to compete with L26 antibody for binding to ErbB-2 overexpressing cells. These results highlight the potential of active immunization with conformation mimicking peptides in ErbB-2 overexpressing tumors.

Growth factor receptors: structure, mechanism, and drug discovery

The focus of this review is the relationship between the three-dimensional structure of ligands of the various members of the growth factor receptor tyrosine kinase superfamily and their interaction with the cognate receptor. Particular attention is given to the transforming growth factor-alpha, epidermal growth factor (EGF); nerve growth factor, neurotrophin; and insulin-like growth factor-1 (IGF-1), insulin systems since these have been extensively studied in recent years. The three receptor types, which bind these ligands, are the epidermal growth factor receptor family (erb B receptors), the neurotrophin or Trk receptor family, and IGF-1/insulin receptors, respectively, and represent three distinct members of the tyrosine kinase superfamily. For each of these, formation of the ligand-receptor complex initiates the signal transduction cascade through autophosphorylation by the intracellular tyrosine kinase domain. The extracellular portion of the receptor that contains the ligand binding domain in these systems varies significantly in organization in each case. For the EGF receptor system, ligand binding induces homo- and heterodimerization of the receptor leading to activation of the intracellular kinase. For the Trk receptor system, homodimerization of receptors has been shown to occur, although a second receptor, p75, is also required for high affinity binding of neurotrophins and for enhanced sensitivity of tyrosine kinase activation at low ligand concentrations. The IGF-1 and insulin receptors exist as covalent cross-linked dimers where each monomer is composed of two subunits. The aim of this review is also to discuss the mechanism of ligand-receptor interaction for each of these cases; however, since no structural information is yet available for the ligand-receptor complex, the discussion will largely be centered on the molecular requirements of ligand binding. As these receptors are activated through the ligand binding site on the extracellular domain, this represents a possible target for pharmacological intervention by inhibition or stimulation of this portion of the receptor. Thus from a drug design perspective, the focus of this review is to discuss progress in the development of agonists or antagonists of the ligand for these receptors.

The EGF domain: requirements for binding to receptors of the ErbB family

Epidermal growth factor (EGF) has been the prototype growth-stimulating peptide for many years. It has a characteristic structure with three disulfide bridges, which is essential for its activity. However, many other proteins, including both growth factors and proteins with unrelated functions, have similar EGF-like domains. This indicates that besides a characteristic conformation provided by the EGF-like domain, specific amino acids are required to provide specificity in protein functioning. Currently, more than 10 different growth factors with an EGF-like domain have been characterized which all exert their action by binding to the four members of the erbB family of receptors. In this review, studies are described on the structure-function relationship of these EGF-like growth factor molecules in an attempt to analyze the individual amino acids that determine their binding specificity to the individual members of the erbB family.

ErbB tyrosine kinases and the two neuregulin families constitute a ligand-receptor network

The recently isolated second family of neuregulins, NRG2, shares its primary receptors, ErbB-3 and ErbB-4, and induction of mammary cell differentiation with NRG1 isoforms, suggesting functional redundancy of the two growth factor families. To address this possibility, we analyzed receptor specificity of NRGs by using an engineered cellular system. The activity of isoform-specific but partly overlapping patterns of specificities that collectively activate all eight ligand-stimulatable ErbB dimers was revealed. Specifically, NRG2-alpha [corrected], like NRG1-beta [corrected], emerges as a narrow-specificity ligand, whereas NRG2-beta [corrected] is a pan-ErbB ligand that binds with different affinities to all receptor combinations, including those containing ErbB-1, but excluding homodimers of ErbB-2. The latter protein, however, displayed cooperativity with the direct NRG receptors. Apparently, signaling by all NRGs is funneled through the mitogen-activated protein kinase (MAPK). However, the duration and potency of MAPK activation depend on the identity of the stimulatory ligand-receptor ternary complex. We conclude that the NRG-ErbB network represents a complex and nonredundant machinery developed for fine-tuning of signal transduction.

Identification of the minimal requirements for binding to the human epidermal growth factor (EGF) receptor using chimeras of human EGF and an EGF repeat of Drosophila Notch

Many proteins contain so-called epidermal growth factor (EGF)-like domains that share the characteristic spacing of cysteines and glycines with members of the EGF family. They are, however, functionally unrelated, despite the fact that the three-dimensional structure of these EGF-like domains, also, is often very similar to that of the EGF receptor agonists. In the present study, we linked an EGF-like repeat from the Drosophila Notch protein to the N- and C-terminal linear tail sequences of human EGF (hEGF), and we showed that this chimera (E1N6E) is unable to bind or activate the hEGF receptor. This recombinant protein was then used as a basic construct for identifying the minimal requirements for high affinity EGF receptor binding and activation. We selectively reintroduced a limited number of important hEGF-derived residues, and by using this unique approach, we were able to make hEGF/Notch chimeras that, compared with wild type hEGF, showed nearly 100% binding affinity and mitogenic activity on HER-14 cells expressing the hEGF receptor.

Alternative intracellular routing of ErbB receptors may determine signaling potency

The ErbB signaling module consists of four receptor tyrosine kinases and several dozen ligands that activate specific homo- and heterodimeric complexes of ErbB proteins. Combinatorial ligand/receptor/effector interactions allow large potential for signal diversification. Here we addressed the possibility that turn-off mechanisms enhance the diversification potential. Concentrating on ErbB-1 and two of its ligands, epidermal growth factor (EGF) and transforming growth factor alpha (TGF-alpha), and the Neu differentiation factor (NDF/neuregulin) and one of its receptors, ErbB-3, we show that ligand binding variably accelerates endocytosis of the respective ligand-receptor complex. However, unlike the EGF-activated ErbB-1, which is destined primarily to degradation in lysosomes, NDF and TGF-alpha direct their receptors to recycling, probably because these ligands dissociate from their receptors earlier along the endocytic pathway. In the case of NDF, structural, as well as biochemical, analyses imply that ligand degradation occurs at a relatively late endosomal stage. Attenuation of receptor down-regulation by this mechanism apparently confers to both NDF and TGF-alpha more potent and prolonged signaling activity. In conclusion, alternative endocytic trafficking of ligand-ErbB complexes may tune and diversify signal transduction by EGF family ligands.

Thrombin aivation of the 9E3/CEF4 chemokine involves tyrosine kinases including c-src and the epidermal growth factor receptor

The 9E3/CEF4 gene codes for a chemokine that is highly homologous to human interleukin-8 and melanoma growth-stimulating activity/groalpha. These chemokines belong to a family of molecular mediators that are importantly involved in inflammation, wound healing, tumor development, and viral entry into cells. On the chorioallantoic membrane the 9E3 protein is chemotactic for monocyte/macrophages and lymphocytes and is angiogenic. In cultured chicken embryo fibroblasts, which have many of the properties of wound fibroblasts, the gene is stimulated by a variety of agents including oncogenes, growth factors, phorbol esters, and thrombin. The strong stimulation of 9E3 by thrombin in culture correlates well with the observation that in young chicks this gene is stimulated to very high levels in fibroblasts upon wounding and remains high throughout wound repair. Activation of 9E3 by thrombin: (i) occurs very rapidly, one minute exposure to thrombin is sufficient to initiate the signals necessary for gene activation; (ii) is independent of mitogenesis; (iii) operates through the proteolytically activated receptor for thrombin; (iv) is mediated by tyrosine kinases, including c-src and the epidermal growth factor (EGF) receptor, rather than Ser/Thr kinases such as protein kinase C and protein kinase A. Inhibition of either c-src or the EGF receptor tyrosine kinase inhibits the stimulation of 9E3 by thrombin. We show here for the first time that activation of the EGF receptor through a cell-surface receptor that does not have tyrosine kinase activity can lead to expression of an immediate early response gene which encodes for a secreted protein, a chemokine. This rapidly activated tyrosine kinase pathway may be a general stress response by which in vivo a localized cell population reacts to emergency situations such as viral infection, wounding, or tumor growth.

In vivo analysis of Argos structure-function. Sequence requirements for inhibition of the Drosophila epidermal growth factor receptor

The Drosophila Argos protein is the only known extracellular inhibitor of the epidermal growth factor receptor (EGFR). It is structurally related to the activating ligands, in that it is a secreted protein with a single epidermal growth factor (EGF) domain. To understand the mechanism of Argos inhibition, we have investigated which regions of the protein are essential. A series of deletions were made and tested in vivo; furthermore, by analyzing chimeric proteins between Argos and the activating ligand, Spitz (a transforming growth factor-alpha-like factor), we have examined what makes one inhibitory and the other activating. Our results reveal that Argos has structural requirements that differ from all known EGFR activating ligands; domains flanking the EGF domain are essential for its function. We have also defined the important regions of the atypical Argos EGF domain. The extended B-loop is necessary, whereas the C-loop can be replaced with the equivalent Spitz region without substantially affecting Argos function. Comparison of the argos genes from Drosophila melanogaster and the housefly, Musca domestica, supports our structure-function analysis. These studies are a prerequisite for understanding how Argos inhibits the Drosophila EGFR and provide a basis for designing mammalian EGFR inhibitors.

Mode matches and their locations in the hydrophobic free energy sequences of peptide ligands and their receptor eigenfunctions

Patterns in sequences of amino acid hydrophobic free energies predict secondary structures in proteins. In protein folding, matches in hydrophobic free energy statistical wavelengths appear to contribute to selective aggregation of secondary structures in "hydrophobic zippers." In a similar setting, the use of Fourier analysis to characterize the dominant statistical wavelengths of peptide ligands' and receptor proteins' hydrophobic modes to predict such matches has been limited by the aliasing and end effects of short peptide lengths, as well as the broad-band, mode multiplicity of many of their frequency (power) spectra. In addition, the sequence locations of the matching modes are lost in this transformation. We make new use of three techniques to address these difficulties: (i) eigenfunction construction from the linear decomposition of the lagged covariance matrices of the ligands and receptors as hydrophobic free energy sequences; (ii) maximum entropy, complex poles power spectra, which select the dominant modes of the hydrophobic free energy sequences or their eigenfunctions; and (iii) discrete, best bases, trigonometric wavelet transformations, which confirm the dominant spectral frequencies of the eigenfunctions and locate them as (absolute valued) moduli in the peptide or receptor sequence. The leading eigenfunction of the covariance matrix of a transmembrane receptor sequence locates the same transmembrane segments seen in n-block-averaged hydropathy plots while leaving the remaining hydrophobic modes unsmoothed and available for further analyses as secondary eigenfunctions. In these receptor eigenfunctions, we find a set of statistical wavelength matches between peptide ligands and their G-protein and tyrosine kinase coupled receptors, ranging across examples from 13.10 amino acids in acid fibroblast growth factor to 2.18 residues in corticotropin releasing factor. We find that the wavelet-located receptor modes in the extracellular loops are compatible with studies of receptor chimeric exchanges and point mutations. A nonbinding corticotropin-releasing factor receptor mutant is shown to have lost the signatory mode common to the normal receptor and its ligand. Hydrophobic free energy eigenfunctions and their transformations offer new quantitative physical homologies in database searches for peptide-receptor matches.

Expression of growth factors in chicken growth plate with special reference to tibial dyschondroplasia

Immunoreactive growth factors were identified in chick embryonic cartilage and bone, and in the growth plate of normal tibiotarsi and tibiotarsi affected with tibial dyschondroplasia (TD). A specific pattern of temporal and spatial expression was observed for each growth factor. Transforming growth factor beta and alpha (TGF beta and TGF alpha) and epidermal growth factor (EGF) were briefly expressed in chondrocytes of early chick embryos. Immunolabelling for TGF beta then gradually shifted into cartilaginous matrix and was not observed in cytoplasm of hypertrophic chondrocytes until the late embryonic and post-hatch stages. The distribution and intensity of TGF beta labelling was the same in chondrocytes of the TD and normal growth plate. Insulin-like growth factor I (IGF-I) labelling persisted from the early embryonic stage to the end of the mid-stage and then disappeared from chondrocytes. IGF-I appeared again in chondrocytes 1-2 days before hatching. After hatching, the labelling intensified in prehypertrophic and hypertrophic chondrocytes. TD lesions displayed IGF-I in the distal region, mainly in chondrocytes around small blood vessels. EGF reappeared in proliferative and hypertrophic chondrocytes of the mid-embryonic stage. By day 18 after hatching, EGF was present mainly in prehypertrophic and hypertrophic chondrocytes. EGF was demonstrated only in distal proliferative and early prehypertrophic chondrocytes of the dyschondroplastic growth plate. TGF alpha was identified in hypertrophic chondrocytes adjacent to the periosteum and in the distal tip of the mid-embryonic growth plate. With progressing ossification, TGF alpha labeling intensified in the embryonic hypertrophic chondrocytes. In the TD growth plate at day 18 after hatching, TGF alpha expression was limited to 1-3 concentric layers of chondrocytes surrounding blood vessels.

NMR study of the transforming growth factor-alpha (TGF-alpha)-epidermal growth factor receptor complex. Visualization of human TGF-alpha binding determinants through nuclear Overhauser enhancement analysis

The study of human transforming growth factor-alpha (TGF-alpha) in complex with the epidermal growth factor (EGF) receptor extracellular domain has been undertaken in order to generate information on the interactions of these molecules. Analysis of 1H NMR transferred nuclear Overhauser enhancement data for titration of the ligand with the receptor has yielded specific data on the residues of the growth factor involved in contact with the larger protein. Significant increases and decreases in nuclear Overhauser enhancement cross-peak intensity occur upon complexation, and interpretation of these changes indicates that residues of the A- and C-loops of TGF-alpha form the major binding interface, while the B-loop provides a structural scaffold for this site. These results corroborate the conclusions from NMR relaxation studies (Hoyt, D. W., Harkins, R. N., Debanne, M. T., O'Connor-McCourt, M., and Sykes, B. D. (1994) Biochemistry 33, 15283-15292), which suggest that the C-terminal residues of the polypeptide are immobilized upon receptor binding, while the N terminus of the molecule retains considerable flexibility, and are consistent with structure-function studies of the TGF-alpha/EGF system indicating a multidomain binding model. These results give a visualization, for the first time, of native TGF-alpha in complex with the EGF receptor and generate a picture of the ligand-binding site based upon the intact molecule. This will undoubtedly be of utility in the structure-based design of TGF-alpha/EGF agonists and/or antagonists.

The interaction of an epidermal growth factor/transforming growth factor alpha tail chimera with the human epidermal growth factor receptor reveals unexpected complexities

It has been assumed that substitution of homologous regions of transforming growth factor alpha (TGF-alpha) into epidermal growth factor (EGF) can be used to probe ligand-receptor recognition without detrimental effects on ligand characteristics for the human EGF receptor (EGFR). We show that a chimera of murine (m) EGF in which the carboxyl-terminal tail is substituted for that of TGF-alpha (mEGF/TGF-alpha44-50) results in complex features that belie this initial simplistic assumption. Comparison of EGF and mEGF/TGF-alpha44-50 in equilibrium binding assays showed that although the relative binding affinity of the chimera was reduced 80-200-fold, it was more potent than EGF in mitogenesis assays using NR6/HER cells. This superagonist activity could not be attributed to differences in ligand processing or to binding to other members of the c-erbB family. It appeared to be due, in part, to choice of an EGFR-overexpressing target cell where high receptor number compensated for the low affinity of the ligand; it also appeared to be related to the ability of the chimera to activate the EGFR tyrosine kinase. Thus, when EGFR autophosphorylation was measured, mEGF/TGF-alpha44-50 was more potent than EGF, despite its low affinity. When tested using chicken embryo fibroblasts, substitution of the TGF-alpha carboxyl-terminal tail into mEGF failed to enhance its binding affinity for chicken EGFRs; however, the chimera was intermediate in potency between TGF-alpha and mEGF in mitogenesis assays. Our results suggest a contextual requirement for EGFR recognition which is ligand-specific. Further, the unpredictable responses to chimeric ligands underline the complex nature of the processes of ligand recognition, receptor activation, and the ensuing cellular response.

Identification of residues of the epidermal growth factor receptor proximal to residue 45 of bound epidermal growth factor

A triple mutant of murine epidermal growth factor (mEGF), N1Q/H22Y/R45K-mEGF, was constructed by site-directed mutagenesis, expressed, purified, and characterized for use in an affinity cross-linking study to identify aminoacyl residues of the EGF receptor adjacent to a residue in the carboxyl-terminal domain of bound EGF thought to be important in distinguishing between EGF and transforming growth factor-alpha in their recognition by the receptor. Cyclization of Gln1 to form pyroglutamate (pE) limited the site of cross-linking in the mutant to Lys45, permitting identification of receptor residues that are proximal to this residue of bound EGF. The resulting N1pE/H22Y/R45K-mEGF was shown to be comparable to wild-type mEGF in receptor binding and stimulation of receptor autophosphorylation. 125I-Labeled N1pE/H22Y/R45K-mEGF was reacted with the heterobifunctional cross-linking reagent sulfo-N-succinimidyl-4-(fluorosulfonyl)benzoate, and the resulting modified EGF was incubated with A431 membrane vesicles bearing EGF receptors. Incubation resulted in specific cross-linking of the labeled N1pE/H22Y/R45K-mEGF to EGF receptors. The resulting cross-linked complex was then partially purified, denatured, reduced, and carboxyamidomethylated. Digestion with endoprotease LysC resulted in a unique radiolabeled peptide that could be immunoprecipitated using antibodies to mEGF. This immunoprecipitated fragment was purified by gel electrophoresis and subjected to microsequencing. The resulting sequence was matched to that of a LysC fragment of the receptor, which begins with Thr464 and is near the interface of receptor subdomains III and IV. Loss of signal at cycle 2 suggests that the point of attachment of cross-linked N1pE/H22Y/R45K is Lys465 of the receptor.

Rational design for the development of epidermal growth factor receptor antagonists

Epidermal growth factor (EGF) and transforming growth factor-alpha (TGF alpha) bind with similar high affinity to the human EGF receptor. Using a domain-exchange strategy we have shown that the C-terminal linear region of these molecules is involved in high affinity receptor binding. By further single amino acid substitution in this linear C-terminal region, a putative interaction site of these ligands with their receptor has been identified. This identification of a receptor binding domain in EGF/TGF alpha provides an important initial step in the development of EGF receptor antagonists with significant clinical potential.

Immunochemical study of a transforming growth factor-alpha-related protein in the chicken kidney

A number of polypeptides are involved in renal growth and physiology. Both transforming growth factor-alpha (TGF-alpha) protein and mRNA are expressed in kidney cells during embryonic and adult stages, and exert mitogenic activity on kidney cells in culture. We studied the immunolocalization of a TGF-alpha-related protein at the ultra-structural level and found it in the basolateral membranes of dark cells from distal tubules of the chicken kidney. By Western blotting techniques, we identified a protein complex composed of a least two TGF-alpha immunoreactive subunits of 40 and 88 kDa, respectively. Both subunits were sensitive to elastase digestion, and released TGF-alpha immunoreactive products. In addition, TGF-alpha immunoreaction was found in primary culture of chicken kidney cells. These findings suggest that the TGF-alpha-related protein complex plays a very specific role in proliferation and/or differentiation of kidney cells.

A single amino acid exchange, Arg-45 to Ala, generates an epidermal growth factor (EGF) mutant with high affinity for the chicken EGF receptor

The finding that human epidermal growth factor (hEGF) and human transforming growth factor (hTGF) alpha bind with similar affinity to the human EGF receptor but differ in their affinity for the chicken EGF receptor was used as a model system to study ligand-receptor interaction of EGF receptor agonists. We previously constructed domain-exchange mutants of hEGF and hTGF alpha and found that the region COOH-terminal of the sixth cysteine residue in hTGF alpha is important for high affinity binding to the chicken EGF receptor (Kramer, R. H., Lenferink, A. E. G., Lammerts van Bueren-Koornneef, I., van der Meer, A., van de Poll, M. L. M., and van Zoelen, E. J. J. (1994) J. Biol. Chem. 269, 8708-8711). To analyze this domain in more detail, we now constructed four additional chimeras in which either the region between the sixth cysteine residue and the highly conserved Leu-47 was exchanged or the region COOH-terminal of Leu-47. A mutant in which the latter region in hEGF was replaced by hTGF alpha (designated E6ET) showed intermediate binding affinity, whereas replacement of the former region in hEGF by hTGF alpha was sufficient to generate a mutant (designated E6TE) with a similar high affinity for the chicken EGF receptor as wild type hTGF alpha. Furthermore, a deletion mutant of hEGF lacking three COOH-terminal amino acids, EGF50, showed intermediate binding affinity for the chicken EGF receptor similar to E6ET, but upon additional deletions (EGF49 and EGF48), this initial gain in affinity was lost. A systematic analysis of the region between the sixth cysteine residue and Leu-47 showed that the low affinity of hEGF for the chicken EGF receptor is mainly due to the presence of Arg-45. Replacement of the positively charged Arg-45 by Ala, the corresponding amino acid in hTGF alpha, was sufficient to generate a mutant growth factor with high affinity for the chicken EGF receptor. This indicates that in hEGF Arg-45 may play an important role in receptor binding. A model is proposed in which positively charged amino acids close to or within the receptor recognition site of hEGF prohibit high affinity binding to the chicken EGF receptor due to electrostatic repulsion of positively charged amino acids in the putative ligand binding domain of the chicken EGF receptor.

Analysis of function and expression of the chick GPA receptor (GPAR alpha) suggests multiple roles in neuronal development

Growth promoting activity (GPA) is a chick growth factor with low homology to mammalian ciliary neurotrophic factor (CNTF) (47% sequence identity with rat CNTF) but displays similar biological effects on neuronal development. We have isolated a chick cDNA coding for GPA receptor (GPAR alpha), a GPI-anchored protein that is 70% identical to hCNTFR alpha. Functional analysis revealed that GPAR alpha mediates several biological effects of both GPA and CNTF. Soluble GPAR alpha supports GPA- and CNTF-dependent survival of human TF-1 cells. In sympathetic neurons, GPAR alpha mediates effects of both GPA and CNTF on the expression of vasoactive intestinal peptide (VIP) as shown by the inhibition of GPA- and CNTF-mediated VIP induction upon GPAR alpha antisense RNA expression. These results demonstrate that GPAR alpha is able to mediate effects of two neurokines that are only distantly related. GPAR alpha mRNA expression is largely restricted to the nervous system and was detected in all neurons that have been shown to respond to GPA or CNTF by increased survival or differentiation, i.e. ciliary, sympathetic, sensory dorsal root, motoneurons, retinal ganglion cells and amacrine cells. Interestingly, GPAR alpha mRNA was additionally found in neuronal populations and at developmental periods not known to be influenced by GPA or CNTF, suggesting novel functions for GPAR alpha and its ligands during neurogenesis and neuron differentiation.