Epidermal growth factor receptor binding is not a simple one-step process

Theoretical Aspects of GPCR-Ligand Complex Pharmacology

Over the past 50 years in pharmacology, an understanding of seven transmembrane (7TMR) function has been gained from the comparison of experimental data to receptor models. These models have been constructed from building blocks composed of systems consisting of series and parallel mass action binding reactions. Basic functions such as the the isomerization of receptors upon ligand binding, the sequential binding of receptors to membrane coupling proteins, and the selection of multiple receptor conformations have been combined in various ways to build receptor systems such as the ternary complex, extended ternary complex, and cubic ternary complex models for 7TMR function. Separately, the Black/Leff operational model has furnished an extremely valuable method of quantifying drug agonism. In the past few years, incorporation of the basic allosteric nature of 7TMRs has led to additional useful models of functional receptor allosteric mechanisms; these models yield valuable methods for quantifying allosteric effects. Finally, molecular dynamics has provided yet another new set of models describing the probability of formation of multiple receptor states; these radically new models are extremely useful in the prediction of functionally selective drug effects.

Kinetics of epidermal growth factor/receptor binding on cells measured by total internal reflection/fluorescence recovery after photobleaching

Total internal reflection fluorescence (TIRF) microscopy is used to measure the dissociation kinetic rate of fluorescein-labeled epidermal growth factor from its specific receptors on the surface of intact but mildly fixed A431 human epidermoid cells in culture. Prior applications of TIRF microscopy have been limited to nonreceptor binding or to model membrane systems. The evanescent field excites fluorescence selectively at the surface of the cell proximal to the coverslip. "Prismless" epiillumination TIR is employed to avoid space limitations and is achieved by passing the excitation laser beam through a high (1.4)-aperture objective so that the light is incident at the glass/water interface beyond the critical angle. Long-term focus is maintained by a special feedback system. Of the possible effects that can influence the time course of the postbleach fluorescence recoveries-the EGF/receptor dissociation ratek 2, the bulk solution diffusion rate of EGF, and the cell surface motion of the receptors-we infer that the dissociation ratek 2 dominates. Several fitting schemes are compared and indicate the presence of a multiplicity of values fork 2, ranging from about 0.05 to 0.004 s(-1), with an average value of about 0.012 s(-1). These results compare well with values previously obtained by radiolabel/washing techniques. The significance of the results in terms of kinetic models and the advantages of the TIRF technique for these sorts of measurements are discussed.

Receptor theory

Receptor theory assigns mathematical rules to biological systems in order to quantify drug effects and define what biological systems can and cannot do, leading to the design of experiments that may further modify the model. Drug receptor theory also furnishes the tools for quantifying the activity of drugs in a system-independent manner, essential because drugs are almost always studied in test systems somewhat removed from the therapeutic system for which they are intended. Since biological systems operate at different set points in the body under different conditions, the ability to predict drug effects under a variety of circumstances is important. This unit provides a historical perspective of classical receptor theory and the currently used operational model of drug effects. The mechanism of drug receptor function is also described in terms of the various iterations of the ternary complex model, the two-state theory for ion channels, and a probabilistic model of multiple receptor conformations.

Endocytic Relay as a Potential Means for Enhancing Ligand Transport through Cellular Tissue Matrices: Analysis and Possible Implications for Drug Delivery

The transport of peptide ligands, such as cytokines, through tissue is complicated by resistances due to cell multilayers and holdup in extracellular matrix. To determine whether it is possible for receptor-mediated endocytic trafficking to enhance ligand transport, we have developed a mathematical model of ligand flux through tissue containing cells possessing complementary receptors. Tissue is considered as two phases: the cell phase and the matrix phase; thus tissue is modeled as analogous to a packed bed reactor. This model allows calculation of steady-state flux of intact and degraded peptide through a one-dimensional cell/tissue matrix. Both environmental and molecular parameters were considered in this study. Results predict that three quantities should have a major influence on growth factor flux: the ratio of matrix diffusivity to intracellular "diffusivity" (D(m)/D(i)), the extracellular matrix proteolysis rate constant (k (prot)), and the fraction of internalized growth factor degraded (f(1)). For basal levels of intracellular degradation (0 < f(1) >/= 0.05) but no extracellular proteolysis, significant enhancement is possible only for D(m)/D(i) >/=1. f(1) increases, enhancement is only possible up to f(1)= 0.07 even for D(m)/D(i) < 1. For significant levels of extracellular proteolysis (k (prot) > 0), the requirements for D(m)/D(i) and f(1) to permit transport enhancement encompass a broader range with the exact values dependent on k (prot). These insights may be helpful for delivery of ligands generated from controlled-release devices or genetically modified autocrine cells, and may also provide better understanding of cytokine transport in embryonic development.

Single-molecule imaging and fluorescence lifetime imaging microscopy show different structures for high- and low-affinity epidermal growth factor receptors in A431 cells

Epidermal growth factor (EGF) receptor (EGFR) modulates mitosis and apoptosis through signaling by its high-affinity (HA) and low-affinity (LA) EGF-binding states. The prevailing model of EGFR activation-derived from x-ray crystallography-involves the transition from tethered ectodomain monomers to extended back-to-back dimers and cannot explain these EGFR affinities or their different functions. Here, we use single-molecule Förster resonant energy transfer analysis in combination with ensemble fluorescence lifetime imaging microscopy to investigate the three-dimensional architecture of HA and LA EGFR-EGF complexes in cells by measuring the inter-EGF distances within discrete EGF pairs and the vertical distance from EGF to the plasma membrane. Our results show that EGFR ectodomains form interfaces resulting in two inter-EGF distances ( approximately 8 nm and < 5.5 nm), different from the back-to-back EGFR ectodomain interface ( approximately 11 nm). Distance measurements from EGF to the plasma membrane show that HA EGFR ectodomains are oriented flat on the membrane, whereas LA ectodomains stand proud from it. Their flat orientation confers on HA EGFR ectodomains the exclusive ability to interact via asymmetric interfaces, head-to-head with respect to the EGF-binding site, whereas LA EGFRs must interact only side-by-side. Our results support a structural model in which asymmetric EGFR head-to-head interfaces may be relevant for HA EGFR oligomerization.

A stopped-flow fluorescence anisotropy method for measuring hormone binding and dissociation kinetics with cell-surface receptors in living cells

We have developed a system for extending stopped-flow analysis to the kinetics of ligand capture and release by cell surface receptors in living cells. While most mammalian cell lines cannot survive the shear forces associated with turbulent, stopped-flow mixing, we determined that 32D cells, murine hematopoietic precursor cells, can survive rapid mixing, even at the high flow rates necessary to achieve dwell times as short as 10 msec. In addition, 32D cells do not express any member of the ErbB family of receptors, providing a null background for studying this receptor family. We have established a series of stable, monoclonal 32D-derived cell lines that express the epidermal growth factor (EGF) receptor, ErbB2, or a combination of both at different ratios. Using these cell lines and a homogeneous fluorescent derivative of H22Y-mEGF modified with fluorescein at the amino terminus (F-EGF), we have measured association and dissociation of F-EGF with its receptor. Association was measured by following the time-dependent changes in fluorescence anisotropy after rapidly mixing cells at various cell densities with F-EGF at 1-15nM. Dissociation was measured both by chase experiments in which unlabeled EGF was mixed with cells pre-equilibrated with F-EGF or by dilution of cells equilibrated with F-EGF. Comparison of these dissociation experiments demonstrated that little or no ligand-induced dissociation occurs in the chase dissociation experiments. For each cell line, data from a series of association experiments and dilution dissociation experiments were subjected to global analysis using a two independent receptor-class model. Our analysis is consistent with the presence of two distinct receptor populations, even in cells bearing only the EGF receptor. Increasing the relative expression of ErbB2 leads to an increase in the fraction of high affinity class receptors observed, without altering the total number of EGF binding sites.

Quantification of short term signaling by the epidermal growth factor receptor

During the past decade, our knowledge of molecular mechanisms involved in growth factor signaling has proliferated almost explosively. However, the kinetics and control of information transfer through signaling networks remain poorly understood. This paper combines experimental kinetic analysis and computational modeling of the short term pattern of cellular responses to epidermal growth factor (EGF) in isolated hepatocytes. The experimental data show transient tyrosine phosphorylation of the EGF receptor (EGFR) and transient or sustained response patterns in multiple signaling proteins targeted by EGFR. Transient responses exhibit pronounced maxima, reached within 15-30 s of EGF stimulation and followed by a decline to relatively low (quasi-steady-state) levels. In contrast to earlier suggestions, we demonstrate that the experimentally observed transients can be accounted for without requiring receptor-mediated activation of specific tyrosine phosphatases, following EGF stimulation. The kinetic model predicts how the cellular response is controlled by the relative levels and activity states of signaling proteins and under what conditions activation patterns are transient or sustained. EGFR signaling patterns appear to be robust with respect to variations in many elemental rate constants within the range of experimentally measured values. On the other hand, we specify which changes in the kinetic scheme, rate constants, and total amounts of molecular factors involved are incompatible with the experimentally observed kinetics of signal transfer. Quantitation of signaling network responses to growth factors allows us to assess how cells process information controlling their growth and differentiation.

Amphiregulin acts as an autocrine growth factor in two human polarizing colon cancer lines that exhibit domain selective EGF receptor mitogenesis

Colonic enterocytes, like many epithelial cells in vivo, are polarized with functionally distinct apical and basolateral membrane domains. The aims of this study were to characterize the endogenous epidermal growth factor (EGF)-like ligands expressed in two polarizing colon cancer cell lines, HCA-7 Colony 29 (HCA-7) and Caco-2, and to examine the effects of cell polarity on EGF receptor-mediated mitogenesis. HCA-7 and Caco-2 cells were grown on plastic, or as a polarized monolayer on Transwell filters. Cell proliferation was measured by 3H-thymidine incorporation and EGF receptor (EGFR) binding was assessed by Scatchard analysis. EGFR ligand expression was determined by Northern blot analysis, reverse transcription polymerase chain reaction, metabolic labelling and confocal microscopy. We found that amphiregulin (AR) was the most abundant EGFR ligand expressed in HCA-7 and Caco-2 cells. AR was localized to the basolateral surface and detected in basolateral-conditioned medium. Basolateral administration of neutralizing AR antibodies significantly reduced basal DNA replication. A single class of high-affinity EGFRs was detected in the basolateral compartment, whereas the apical compartment of polarized cells, and cells cultured on plastic, displayed two classes of receptor affinity. Basolateral administration of transforming growth factor alpha (TGF-alpha) or an EGFR neutralizing antibody also resulted in a dose-dependent stimulation or attenuation, respectively, of DNA replication. However, no mitogenic response was observed when these agents were added to the apical compartment or to confluent cells cultured on plastic. We conclude that amphiregulin acts as an autocrine growth factor in HCA-7 and Caco-2 cells, and EGFR ligand-induced proliferation is influenced by cellular polarity.

Monte Carlo simulations of membrane signal transduction events: effect of receptor blockers on G-protein activation

Cells have evolved elaborate strategies for sensing, responding to, and interacting with their environment. In many systems, interaction of cell surface receptors with extracellular ligand can activate cellular signal transduction pathways leading to G-protein activation and calcium mobilization. In BC3H1 smooth muscle-like cells, we find that the speed of calcium mobilization as well as the fraction of cells which mobilize calcium following phenylephrine stimulation is dependent upon receptor occupation. To determine whether receptor inactivation affects calcium mobilization, we use the receptor antagonist prazosin to block a fraction of cell surface receptors prior to phenylephrine stimulation. For cases of equal receptor occupation by agonist, cells with inactivated or blocked receptors show diminished calcium mobilization following phenylephrine stimulation as compared to cells without inactivated receptors. Ligand/receptor binding and two-dimensional diffusion of receptors and G-proteins in the cell membrane are studied using a Monte Carlo model. The model is used to determine if receptor inactivation affects G-protein activation and thus the following signaling events for cases of equal equilibrium receptor occupation by agonist. The model predicts that receptor inactivation by antagonist binding results in lower G-protein activation not only by reducing the number of receptors able to bind agonist but also by restricting the movement of agonist among free receptors. The latter process is important to increasing the access of bound receptors to G-proteins.

Dissociation and association of the HIV-1 protease dimer subunits: equilibria and rates

The kinetics and equilibrium properties were investigated for the interconversion between the active dimer of human immunodeficiency virus 1 (HIV-1) protease and its inactive monomeric subunits. The equilibrium dissociation constant (Kd) of the dimeric protease as well as the monomer association rate were obtained by monitoring the fluorescence change of an active-site-directed fluorescent probe (L-737244) upon its binding to the protease. The Kd of the HIV-1 protease is strongly pH dependent. At pH 5.5 where the enzyme is most active catalytically, the extrapolated values of Kd are 0.75 and 3.4 nM at 30 and 37 degrees C, respectively. The rate constant for HIV-1 monomer association, approximately 4 x 10(5) M-1 s-1, is within the range commonly observed for protein-protein interactions. Dimer dissociation was further scrutinized in the presence of an inactive, point mutant form of the enzyme. As a result of subunit exchange between the native and mutant enzymes and the formation of an inactive heterodimer, there was a time-dependent decrease in the activity of the native protease. Enzyme activity could be reinstated with the addition of an active-site-directed inhibitor (L-365862) which selectively binds active dimers. The rate of dimer dissociation was found to also decrease with pH. At pH 5.5 and 30 degrees C, the half-life for subunit dissociation is about 0.5 h. The slow dissociation, coupled with the high stability for dimer association, attests to the importance of allowing sufficient time for dimer-monomer equilibration in kinetic assays in order to avoid reaching erroneous conclusions in studies of dimer dissociation.

Human beta 2-adrenergic receptor produced in stably transformed insect cells is functionally coupled via endogenous GTP-binding protein to adenylyl cyclase

Spodoptera frugiperda insect cells (Sf9) containing the stably integrated human beta 2-adrenergic receptor gene under the control of the baculovirus IE1 promoter expressed up to 350,000 human receptors/cell. The number of receptors did not change with cell density or age of culture. The adrenergic receptors overexpressed in the insect cells were functional with respect to their ligand binding and signalling properties. Coupling of the receptors to endogenous GTP-binding proteins is demonstrated by hormone-dependent stimulation of GTPase and adenylyl cyclase activity in the transformed insect cells. Western-blot analysis revealed that the endogenous GTP-binding protein appears to be of the heterotrimeric type. Antibodies raised against the mammalian alpha subunit of stimulatory GTP-binding proteins cross-react with the insect alpha subunit of GTP-binding proteins, which also exhibits the same apparent molecular mass as its mammalian counterpart. The beta subunit of GTP-binding proteins from insect cells reacts with anti-peptide serum directed against the C-terminal amino acids of the mammalian beta subunit of GTP-binding proteins, but is about approximately 2 kDa larger than that of the beta subunit of GTP-binding proteins from bovine brain. Exposure of the transformed insect cells to L-isoproterenol rapidly induces uncoupling and internalization of 30% of the heterologously expressed receptors. In contrast to the situation in mammalian cells, prolonged exposure of the agonist (24 h) does not result in down regulation of the remaining 70% of the receptors.

pH sensitivity of epidermal growth factor receptor complexes

The association/dissociation binding kinetics of 125I-labeled mouse epidermal growth factor (EGF) to receptors on human fibroblast cells in monolayer culture have been measured at 4 degrees C as a function of extracellular pH from pH 5-9. At pH 8, steady-state total binding is maximal. As pH is lowered to 6.5, total binding monotonically decreases dramatically. It changes further only slightly between pH 6.5 and 5 to about 20% of the maximum binding value. Scatchard binding plots at pH 7.5 and above show the commonly observed concave-upward, non-linear curve; as pH is lowered, this plot becomes much more linear, indicating that the "high affinity" bound receptor population is greatly diminished. Application of our ternary complex binding model [Mayo et al., J Biol Chem 264:17838-17844, 1989], which hypothesizes complexation of the EGF-bound receptor with a cell surface interaction molecule, indicates that pH may have some direct effects on ternary complex formation, but the major effect is on EGF-receptor dissociation.

Analysis of the influences of the E5 transforming protein on kinetic parameters of epidermal growth factor binding and metabolism

The E5 protein of the bovine papillomavirus induces cellular transformation when transfected into NIH 3T3 cells, and the extent of focal transformation is enhanced by cotransfection with the epidermal growth factor (EGF) receptor (Martin et al., Cell 59:21-32, 1989). To determine whether E5 affects EGF:receptor interactions we analyzed the kinetics of 125I-EGF processing using a mathematical model that enabled us to evaluate rate constants for ligand association (ka), dissociation (kd), internalization (ke), recycling (kr), and degradation (kh). These rate constants were measured in NIH 3T3 cells transfected with the human EGF receptor (ER cells) and in cells transfected with both the EGF receptor and E5 (E5/ER cells). We found that the rate constant for 125I-EGF association ka was significantly decreased in E5/ER cells, but was apparently occupancy-independent in both cell lines. The 125I-EGF dissociation rate constant kd was significantly lower in E5 transformed cells, and increased with occupancy in both cell lines. This suggests that E5 alters the receptor before or during EGF binding so that ligand association is slower; however, once complexes are formed, EGF is bound more tightly to the receptor. Rate constants for internalization ke were also found to be occupancy-dependent, although at a given level of occupancy ke was similar for both cell lines. Also, there was no apparent effect of E5 on the recycling rate constant kr. The 125I-EGF degradation rate constant kh was 30% lower in E5 transformed cells, and was occupancy-independent. The overall effect of E5 is to stabilize intact EGF:receptor complexes which may alter mitogenic signaling of the receptor.

Antagonistic and synergistic peptide analogues of the tridecapeptide mating pheromone of Saccharomyces cerevisiae

Biologically inactive, truncated analogues of the Saccharomyces cerevisiae alpha-mating factor (WHWLQLKPGQPMY) either antagonized or synergized the activity of the native pheromone. An amino-terminal truncated pheromone [WLQLKPGQP(Nle)Y] had no activity by itself, but the analogue acted as an antagonist by competing with binding and activity of the mating factor. In contrast, a carboxyl-terminal truncated pheromone [WHWLQLKPGQP] was not active by itself nor did the peptide compete with alpha-factor for binding to the alpha-factor receptor, but it acted as a synergist by causing a marked increase in the activity of alpha-factor. The observation that residues near the amino terminus may be involved in signal transduction whereas those near the carboxyl terminus influence binding allows us to separate binding and signal transduction in the yeast pheromone response pathway. If found for other hormone-receptor systems, synergists may have potential as therapeutic compounds.

The effects of receptor density and cell shape on epidermal growth factor binding

In this paper we describe the effects of receptor density and cell shape on the binding of epidermal growth factor (EGF) to its receptor. Association kinetics of EGF binding to cells with a high receptor density was done using A431 cells. The association rate of EGF binding was apparently independent of the EGF concentration, most likely due to diffusion limited EGF binding as result of high receptor density. The effect of receptor density on EGF association rate was examined by reducing the number of functional EGF receptors on A431 cells. Preincubation of the cells with a monoclonal antibody directed against the EGF receptor and isolation of the cytoskeletons of A431 cells which both leaves only EGF binding to high affinity receptors revealed an EGF concentration dependent association rate. These results were confirmed in HeLa cells with 40 times less receptor numbers than A431 cells demonstrating the effect of receptor density on EGF binding. The influence of shape of the cell on EGF binding was examined by comparing the EGF association to monolayer cells with that of suspension cells. EGF association to suspended A431 cells was EGF concentration dependent. In conclusion we have shown that binding of EGF to A431 cells is dependent not only on the intrinsic rate constants but in addition on both receptor numbers per cell and the shape of cells. These results are in agreement with the hypothesis that EGF binding can be restricted by limited diffusion.

Preparation and characterization of spin-labeled derivatives of epidermal growth factor (EGF) for investigations of the interactions of EGF with its receptor by electron paramagnetic resonance spectroscopy

We prepared, purified, and characterized derivatives of epidermal growth factor (EGF) having a nitroxide spin-label attached covalently at the amino terminus. Characterization of these derivatives with regard to the positions of attachment of the spin-label was accomplished by a combination of peptide mapping, protein sequencing, and fast atom bombardment-mass spectrometry. One derivative was chosen for use in initial investigations by electron paramagnetic resonance (EPR) spectroscopy of receptor-bound EGF and its dissociation kinetics. This derivative was found to be equipotent with the native hormone in competitive binding assays, in activating the EGF receptor kinase, and in stimulating the formation of EGF receptor dimers in solubilized cell extracts. Upon binding to solubilized EGF receptor, the spin-labeled EGF derivative became immobilized, giving rise to a visually distinct slow-motion EPR spectrum. The resulting spectrum showed no detectable dipolar interaction between nitroxides, indicating that the nitroxide moieties of spin-labels reacted at the amino termini of receptor-bound spin-labeled EGF molecules are separated by a distance of at least 16 A. An EPR study of the kinetics of dissociation of spin-labeled EGF in the presence of excess unlabeled EGF revealed a rapid component with a k off approximately 2 x 10(-2) s-1 and a less well resolved slow component.

Biological activities of a synthetic peptide composed of two unlinked domains from a retroviral transmembrane protein sequence

We report several biological activities of a synthetic peptide whose sequence contains the highly conserved region of feline leukemia virus transmembrane protein (TM) synthetically linked to another short TM-derived sequence particularly rich in polar positive residues. This 29-amino-acid peptide blocked [3H]thymidine uptake 30 to 50% by concanavalin A-stimulated CD4(+)--but not CD8(+)-enriched murine splenocytes. Maximal suppression was detected at 12.5 micrograms (3 microM) to 75 micrograms (19 microM) per ml of growth medium; stimulation of [3H]thymidine uptake was observed at higher peptide concentrations. The synthetic peptide inhibited but did not stimulate [3H]thymidine uptake by mitogen-activated thymocytes and antibody production by splenocytes as determined in a liquid hemolytic plaque assay. Similarities are reported between a consensus sequence of diverse retroviral TMs and a region of alpha interferons shown by others to be important for antiviral and cytostatic properties. The TM sequence-derived synthetic peptide blocked in a nontoxic and sequence-specific manner the release of murine leukemia virus from two chronically infected cell lines. We suggest that some of the biological effects of retroviral TM are mediated through a common pathway shared with alpha interferons.