Epidermal growth factor receptors associated to cytoskeletal elements of epidermoid carcinoma (A431) cells

Comprehensive Model for Epidermal Growth Factor Receptor Ligand Binding Involving Conformational States of the Extracellular and the Kinase Domains

The epidermal growth factor (EGF) receptor (EGFR) undergoes ligand-dependent dimerization to initiate transmembrane signaling. Although crystallographic structures of the extracellular and kinase domains are available, ligand binding has not been quantitatively analyzed taking the influence of both domains into account. Here, we developed a model explicitly accounting for conformational changes of the kinase and extracellular domains, their dimerizations and ligand binding to monomeric and dimeric receptor species. The model was fitted to ligand binding data of suspended cells expressing receptors with active or inactive kinase conformations. Receptor dimers with inactive, symmetric configuration of the kinase domains exhibit positive cooperativity and very weak binding affinity for the first ligand, whereas dimers with active, asymmetric kinase dimers are characterized by negative cooperativity and subnanomolar binding affinity for the first ligand. The homodimerization propensity of EGFR monomers with active kinase domains is ∼100-times higher than that of dimers with inactive kinase domains. Despite this fact, constitutive, ligand-independent dimers are mainly generated from monomers with inactive kinase domains due to the excess of such monomers in the membrane. The experimental finding of increased positive cooperativity at high expression levels of EGFR was recapitulated by the model. Quantitative prediction of ligand binding to different receptor species revealed that EGF binds to receptor monomers and dimers in an expression-level dependent manner without significant recruitment of monomers to dimers upon EGF stimulation below the phase transition temperature of the membrane. Results of the fitting offer unique insight into the workings of the EGFR.

Assessing metastatic potential of breast cancer cells based on EGFR dynamics

Derailed transmembrane receptor trafficking could be a hallmark of tumorigenesis and increased tumor invasiveness, but receptor dynamics have not been used to differentiate metastatic cancer cells from less invasive ones. Using single-particle tracking techniques, we  developed a phenotyping asssay named Transmembrane Receptor Dynamics (TReD), studied the dynamics of epidermal growth factor receptor (EGFR) in seven breast epithelial cell lines and developed a phenotyping assay named Transmembrane Receptor Dynamics (TReD). Here we show a clear evidence that increased EGFR diffusivity and enlarged EGFR confinement size in the plasma membrane (PM) are correlated with the enhanced metastatic potential in these cell lines. By comparing the TReD results with the gene expression profiles, we found a clear negative correlation between the EGFR diffusivities and the breast cancer luminal differentiation scores (r = -0.75). Upon the induction of epithelial-mesenchymal transition (EMT), EGFR diffusivity significantly increased for the non-tumorigenic MCF10A (99%) and the non-invasive MCF7 (56%) cells, but not for the highly metastatic MDA-MB-231 cell. We believe that the reorganization of actin filaments during EMT modified the PM structures, causing the receptor dynamics to change. TReD can thus serve as a new biophysical marker to probe the metastatic potential of cancer cells and even to monitor the transition of metastasis.

Strong Epidermal Growth Factor Receptor Expression But Not HER2/neu Expression Correlates with Cell Proliferation in Anal Canal Carcinomas

Twenty-eight anal carcinomas were analyzed for their proliferative status and immunoreactivity to epidermal growth factor receptor (EGFR), HER2/neu, and proliferating cell nuclear antigen (PCNA). EGFR was expressed in 97.3% of tumors. Strong EGFR immunoreactivity correlated with a high proliferative rate (p=0.0 14). No obvious relationship existed between HER2/neu immunoreactivity and proliferative rate. The strong correlation between strong EGFR immunoreactivity and tumor proliferation suggests that the EGFR may represent a therapeutic target in anal cancers.

Exploring naturally occurring ivy nanoparticles as an alternative biomaterial

Arabinoglactan protein (AGP)-rich nanoparticles obtained from the sticky exudates of Hedera helix (English ivy), have shown promising potential to be used in nanomedicine owing to their excellent aqueous solubility, low intrinsic viscosity, biocompatibility, and biodegradability. In this study, the feasibilities of utilizing ivy nanoparticles (INPs) as nano-carriers for delivering chemotherapeutic drugs in cancer therapy and as nano-fillers to develop novel scaffolds for tissue engineering in regenerative medicine are evaluated. Via electrostatic and hydrophobic interactions, pH-responsive nanoconjugates are formed between the INPs and the doxorubicin (DOX) with an entrapment ratio of 77.9±3.9%. While the INPs show minimal cytotoxicity, the formed INP-DOX conjugates exhibit substantially stronger cytotoxic activity than free DOX against multiple cancer cell lines, suggesting a synergistic effect is established upon conjugation. The anti-cancer effects of the INP-DOX conjugates are further evaluated via in vivo xenograft assays by subcutaneously implanting DOX resistant cell line, SW620/Ad-300, into nude mice. The tumor volumes in mice treated with the INP-DOX conjugates are significantly less than those of the mice treated with free DOX. In addition, the INPs are further exploited as nano-fillers to develop fibrous scaffolds with collagen, via mimicking the porous matrix where the INPs are embedded under natural condition. Enhanced adhesion of smooth muscle cells (SMCs) and accelerated proliferation of mouse aortic SMCs are observed in this newly constructed scaffold. Overall, the results obtained from the present study suggest great potential of the INPs to be used as biocompatible nanomaterials in nanomedicine. The AGP-rich INP renders a glycoprotein architecture that is amenable for modification according to the functional designs, capable of being developed as versatile nanomaterials for extensive biomedical applications.

STATEMENT OF SIGNIFICANCE

Naturally occurring organic nanomaterials have drawn increasing interest for their potential biomedical applications in recent years. In this study, a new type of naturally occurring nanoparticles obtained from the sticky exudates on the adventitious roots of English ivy (H. helix), was explored for its potential biomedical application. In particular, the feasibilities of utilizing ivy nanoparticles (INPs) as nano-carriers for delivering chemotherapeutic drugs in cancer therapy and as nano-fillers to develop novel scaffolds for tissue engineering in regenerative medicine were evaluated both in vitro and in vivo. Overall, the results obtained from the present study suggest the great potential of the INPs to be used as biocompatible nanomaterials in nanomedicine. This study may open a totally new frontier for exploring the biomedical application of naturally occurring nanomaterials.

Epidermal growth factor receptor subunit locations determined in hydrated cells with environmental scanning electron microscopy

Imaging single epidermal growth factor receptors (EGFR) in intact cells is presently limited by the available microscopy methods. Environmental scanning electron microscopy (ESEM) of whole cells in hydrated state in combination with specific labeling with gold nanoparticles was used to localize activated EGFRs in the plasma membranes of COS7 and A549 cells. The use of a scanning transmission electron microscopy (STEM) detector yielded a spatial resolution of 3 nm, sufficient to identify the locations of individual EGFR dimer subunits. The sizes and distribution of dimers and higher order clusters of EGFRs were determined. The distance between labels bound to dimers amounted to 19 nm, consistent with a molecular model. A fraction of the EGFRs was found in higher order clusters with sizes ranging from 32–56 nm. ESEM can be used for quantitative whole cell screening studies of membrane receptors, and for the study of nanoparticle-cell interactions in general.

Plasma membrane--cortical cytoskeleton interactions: a cell biology approach with biophysical considerations

From a biophysical standpoint, the interface between the cell membrane and the cytoskeleton is an intriguing site where a "two-dimensional fluid" interacts with an exceedingly complex three-dimensional protein meshwork. The membrane is a key regulator of the cytoskeleton, which not only provides docking sites for cytoskeletal elements through transmembrane proteins, lipid binding-based, and electrostatic interactions, but also serves as the source of the signaling events and molecules that control cytoskeletal organization and remolding. Conversely, the cytoskeleton is a key determinant of the biophysical and biochemical properties of the membrane, including its shape, tension, movement, composition, as well as the mobility, partitioning, and recycling of its constituents. From a cell biological standpoint, the membrane-cytoskeleton interplay underlies--as a central executor and/or regulator--a multitude of complex processes including chemical and mechanical signal transduction, motility/migration, endo-/exo-/phagocytosis, and other forms of membrane traffic, cell-cell, and cell-matrix adhesion. The aim of this article is to provide an overview of the tight structural and functional coupling between the membrane and the cytoskeleton. As biophysical approaches, both theoretical and experimental, proved to be instrumental for our understanding of the membrane/cytoskeleton interplay, this review will "oscillate" between the cell biological phenomena and the corresponding biophysical principles and considerations. After describing the types of connections between the membrane and the cytoskeleton, we will focus on a few key physical parameters and processes (force generation, curvature, tension, and surface charge) and will discuss how these contribute to a variety of fundamental cell biological functions.

Characterization of the diffusion of epidermal growth factor receptor clusters by single particle tracking

A number of studies have shown that receptors of the epidermal growth factor receptor family (ErbBs) exist as higher-order oligomers (clusters) in cell membranes in addition to their monomeric and dimeric forms. Characterizing the lateral diffusion of such clusters may provide insights into their dynamics and help elucidate their functional relevance. To that end, we used single particle tracking to study the diffusion of clusters of the epidermal growth factor (EGF) receptor (EGFR; ErbB1) containing bound fluorescently-labeled ligand, EGF. EGFR clusters had a median diffusivity of 6.8×10(-11)cm(2)/s and were found to exhibit different modes of transport (immobile, simple, confined, and directed) similar to that previously reported for single EGFR molecules. Disruption of actin filaments increased the median diffusivity of EGFR clusters to 10.3×10(-11)cm(2)/s, while preserving the different modes of diffusion. Interestingly, disruption of microtubules rendered EGFR clusters nearly immobile. Our data suggests that microtubules may play an important role in the diffusion of EGFR clusters either directly or perhaps indirectly via other mechanisms. To our knowledge, this is the first report probing the effect of the cytoskeleton on the diffusion of EGFR clusters in the membranes of live cells.

EZYprep LC-coupled MALDI-TOF/TOF MS: an improved matrix spray application for phosphopeptide characterisation

The quality of MALDI-TOF mass spectrometric analysis is highly dependent on the matrix and its deposition strategy. Although different matrix-deposition methods have specific advantages, one major problem in the field of proteomics, particularly with respect to quantitation, is reproducibility between users or laboratories. Compounding this is the varying crystal homogeneity of matrices depending on the deposition strategy used. Here, we describe a novel optimised matrix-deposition strategy for LC-MALDI-TOF/TOF MS using an automated instrument that produces a nebulised matrix "mist" under controlled atmospheric conditions. Comparisons of this with previously reported strategies showed the method to be advantageous for the atypical matrix, 2,5-DHB, and improved phosphopeptide ionisation when compared with deposition strategies for CHCA. This optimised DHB matrix-deposition strategy with LC-MALDI-TOF/TOF MS, termed EZYprep LC, was subsequently optimised for phosphoproteome analysis and compared to LC-ESI-IT-MS and a previously reported approach for phosphotyrosine identification and characterisation. These methods were used to map phosphorylation on epidermal growth factor-stimulated epidermal growth factor receptor to gauge the sensitivity of the proposed method. EZYprep DHB LC-MALDI-TOF/TOF MS was able to identify more phosphopeptides and characterise more phosphorylation sites than the other two proteomic strategies, thus proving to be a sensitive approach for phosphoproteome analysis.

TNK2 preserves epidermal growth factor receptor expression on the cell surface and enhances migration and invasion of human breast cancer cells

Introduction

Amplification of the TNK2 gene in primary tumours correlates with poor prognosis. In accordance, TNK2 overexpression was shown to promote invasion of cancer cells – but the mechanism by which TNK2 mediates these effects is unresolved. TNK2 was suggested to regulate Cdc42-driven migration by activation of breast cancer antioestrogen resistance 1 (BCAR1); however, distinct from this effect is evidence for a role of TNK2 in the regulation of epidermal growth factor receptor (EGFR) endocytosis and degradation. In the present study we sought to investigate whether negative targeting of TNK2 by siRNA could be used to inhibit cancer cell invasion, to establish the contribution of its effect on the EGFR and to consequently attempt to resolve the issue of TNK2's mechanism of action.

Methods

We used siRNA to knockdown expression of TNK2 and its proposed effector BCAR1 in order to analyse the effect of this knockdown on cancer cell behaviour in vitro. We examined morphological changes using phase-contrast microscopy and immunohistochemistry. Functional parameters examined included apoptosis, proliferation, migration and invasion. We also performed flow cytometry analysis to examine EGFR cell surface expression and carried out western blot to examine the total EGFR levels.

Results

We observed that targeting of TNK2 by siRNA in breast cancer cells resulted in distinct morphological changes characterised by a stellate appearance and an absence of protrusions at membrane edges. These changes were not recapitulated upon siRNA targeting of BCAR1. We thus hypothesised that a component of the effects induced by TNK2 may be independent of BCAR1. Consistent with the idea of an alternative mechanism for TNK2, we observed that TNK2 associates with activated EGFR in breast cancer cells in a TNK2-kinase-independent manner. Furthermore, we demonstrated that TNK2 functions to maintain EGFRs on the cell surface. We could demonstrate that the main functional effect of activating these surface EGFRs in breast cancer cells is stimulation of migration. In accordance, TNK2 silencing by siRNA led to a significant reduction in cell surface EGFR and to a concomitant decrease in the migratory and invasive capacity of breast cancer cells.

Conclusion

Our data suggest that TNK2 can enhance migration and invasion of breast cancer cells via preservation of EGFR expression, notwithstanding its previously reported signalling via BCAR1, explaining its oncogenic behaviour in vitro and correlation with metastatic human breast cancer in vivo.

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.

Cellular dynamics of EGF receptor-targeted synthetic viruses

The epidermal growth factor receptor (EGFR) is overexpressed on a high percentage of human carcinomas. EGFR is an attractive therapeutic target for tissue-specific targeting by non-viral vectors in cancer gene therapy. In this study we analyzed and compared the effects of EGFR-targeted and untargeted polyplexes in respect to internalization into EGFR overexpressing HuH7 cells. Uptake kinetics and internalization dynamics were evaluated by flow cytometry and single-particle tracking. Our results clearly show that EGFR targeting leads to faster and more efficient internalization compared with untargeted particles. After 5 minutes 50% of the EGFR-targeted polyplexes were internalized, whereas untargeted polyplexes reached only approximately 20% internalization even after 20 minutes. In addition, single-particle tracking revealed a three-phase dynamics of the internalization process, and this was generally observed for polyplexes independent of targeting. Phase I was characterized by slow, actin cytoskeleton-mediated movement of the particles with drift, and included the internalization process. During phase II particles displayed increased velocities with normal and confined diffusion in the cytoplasm. Phase III was characterized by fast active transport along microtubules. Targeting of polyplexes for receptor-mediated endocytosis by the EGFR resulted in shortening of phase I and strongly accelerated internalization.

Binding of the Alzheimer amyloid β-peptide to neuronal cell membranes by fluorescence correlation spectroscopy

The deposition of the Alzheimer amyloid beta-peptide (Abeta) fibrils in brain is a key step in Alzheimer's disease. The aggregated Abeta is found to be toxic to neurons since cells die when the aggregated Abeta is added to the cell culture medium. However, target of action of Abeta to cells is unknown. We have applied the fluorescence correlation spectroscopy (FCS) technique to study the existence of a receptor or target molecule for the Alzheimer amyloid beta-peptide (Abeta) in cultured human cerebral cortical neurons. FCS measurement of the fluorophore rhodamine-labeled Abeta (Rh-Abeta) shows diffusion times: 0.1 ms, 1.1 ms and 5.9 ms. Thus, 0.1 ms corresponds to the unbound Rh-Abeta, and 1.1 ms and 5.9 ms correspond to slowly diffusing complexes of Rh-Abeta bound to a kind of receptor or target molecule for Abeta. Addition of excess non-labeled Abeta is accompanied by a competitive displacement, showing that the Abeta binding is specific. Full saturation of the Abeta binding is obtained at nanomolar concentrations, indicating that the Abeta binding is of high affinity. The notion that using FCS we have found a kind of receptor or target molecule for Abeta makes an important point that Abeta kills cells possibly by affecting cell membranes via a receptor or target molecule. This study is of highly significance since it suggests that Abeta possibly affects neuronal cell membranes of Alzheimer patients via a receptor or target molecule.

Role of extracellular matrix-cell interaction and epidermal growth factor (EGF) on EGF-receptors and actin cytoskeleton arrangement in infantile pituitary cells

Epidermal growth factor (EGF) induces changes in cell morphology, actin cytoskeleton, and adhesion processes in cultured infantile pituitary cells. The extracellular matrix, through integrin engagement, collaborates with growth factors in cell signaling. We have examined the participation of collagen I/III and collagen plus fibronectin in the EGF response of infantile pituitary cells with respect to their cell morphology and actin cytoskeleton. As a comparison, we have used poly-lysine as a substrate. Infantile cells elicit the EGF response when they are associated with extracellular matrix proteins, but no response can be obtained with poly-lysine as the substrate. Cells acquire a flattened shape and organize their actin filaments and vinculin as in focal adhesions. Because the EGF receptor (EGFR) is linked to the actin cytoskeleton in other cells structuring a microdomain in cell signaling, we have investigated this association and substrate adhesion participation in infantile pituitary cells. The proportion of EGFR associated with the actin cytoskeleton is approximately 31%; no difference has been observed between the substrates used. Cells in suspension show actin-associated EGFR, suggesting an association independent of cell adhesion. However, no colocalization of EGFRs with actin fibers has been observed, suggesting an indirect association. Compared with beta(1)-integrin, which is linked to actin fibers through structural proteins, EGFR binds more strongly with the actin cytoskeleton. This study thus shows cell adhesion dependence on the EGF effect in the actin cytoskeleton arrangement; this is probably favored by the actin fiber/EGFR association that facilitates the cell signaling pathways for actin cytoskeleton organization in infantile pituitary cells.

Spatial modeling of dimerization reaction dynamics in the plasma membrane: Monte Carlo vs. continuum differential equations

Bimolecular reactions in the plasma membrane, such as receptor dimerization, are a key signaling step for many signaling systems. For receptors to dimerize, they must first diffuse until a collision happens, upon which a dimerization reaction may occur. Therefore, study of the dynamics of cell signaling on the membrane may require the use of a spatial modeling framework. Despite the availability of spatial simulation methods, e.g., stochastic spatial Monte Carlo (MC) simulation and partial differential equation (PDE) based approaches, many biological models invoke well-mixed assumptions without completely evaluating the importance of spatial organization. Whether one is to utilize a spatial or non-spatial simulation framework is therefore an important decision. In order to evaluate the importance of spatial effects a priori, i.e., without performing simulations, we have assessed the applicability of a dimensionless number, known as second Damköhler number (Da), defined here as the ratio of time scales of collision and reaction, for 2-dimensional bimolecular reactions. Our study shows that dimerization reactions in the plasma membrane with Da approximately >0.1 (tested in the receptor density range of 10(2)-10(5)/microm(2)) require spatial modeling. We also evaluated the effective reaction rate constants of MC and simple deterministic PDEs. Our simulations show that the effective reaction rate constant decreases with time due to time dependent changes in the spatial distribution of receptors. As a result, the effective reaction rate constant of simple PDEs can differ from that of MC by up to two orders of magnitude. Furthermore, we show that the fluctuations in the number of copies of signaling proteins (noise) may also depend on the diffusion properties of the system. Finally, we used the spatial MC model to explore the effect of plasma membrane heterogeneities, such as receptor localization and reduced diffusivity, on the dimerization rate. Interestingly, our simulations show that localization of epidermal growth factor receptor (EGFR) can cause the diffusion limited dimerization rate to be up to two orders of magnitude higher at higher average receptor densities reported for cancer cells, as compared to a normal cell.

Computational modeling reveals molecular details of epidermal growth factor binding

BACKGROUND

The ErbB family of receptors are dysregulated in a number of cancers, and the signaling pathway of this receptor family is a critical target for several anti-cancer drugs. Therefore a detailed understanding of the mechanisms of receptor activation is critical. However, despite a plethora of biochemical studies and recent single particle tracking experiments, the early molecular mechanisms involving epidermal growth factor (EGF) binding and EGF receptor (EGFR) dimerization are not as well understood. Herein, we describe a spatially distributed Monte Carlo based simulation framework to enable the simulation of in vivo receptor diffusion and dimerization.

RESULTS

Our simulation results are in agreement with the data from single particle tracking and biochemical experiments on EGFR. Furthermore, the simulations reveal that the sequence of receptor-receptor and ligand-receptor reaction events depends on the ligand concentration, receptor density and receptor mobility.

CONCLUSION

Our computer simulations reveal the mechanism of EGF binding on EGFR. Overall, we show that spatial simulation of receptor dynamics can be used to gain a mechanistic understanding of receptor activation which may in turn enable improved cancer treatments in the future.

Regulated EGF receptor binding to F-actin modulates receptor phosphorylation

The epidermal growth factor receptor (EGFR) is known to bind to the F-actin cytoskeleton in intact cells, and this interaction has been suggested to sequester the EGFR signal transduction system to specific loci within the cell. In this study, the interaction of the EGFR with actin is examined in a reconstituted cell free system. Soluble protein components of the cytosol from A431 cells are shown to dramatically enhance binding of the EGFR to F-actin in a saturable, concentration-dependent fashion. Most of the intracellular C-terminal portion of the EGFR is found to be required for this interaction. Binding of the EGFR to F-actin strongly deactivates the receptor by diminishing EGFR autophosphorylation activity and enhancing tyrosine phosphatase activity toward the EGFR. These results suggest that a ternary or larger protein signaling complex forms on an F-actin cellular scaffold, providing a spatially restricted signal modulation site that can be regulated by cytoskeletal remodeling.

Differential cytoskeletal association of ErbB1 and ErbB2 during keratinocyte differentiation

ErbB1 and ErbB2 display differential subcellular localization in human skin and cultured keratinocytes. To determine whether ErbB1 and ErbB2 also differ in cytoskeletal binding properties, normal human keratinocytes grown under conditions favoring a basal or differentiated phenotype were repeatedly extracted in a non-ionic detergent buffer. In basaloid keratinocytes, cytoskeletal association of ErbB1 and ErbB2 was limited. ErbB1 ( approximately 5%) was tightly associated with the cytoskeleton, compared to <1% of ErbB2 (p=0.004). After EGF stimulation, activated ErbB1 and ERK associated with the cytoskeleton to a greater extent than did total ErbB1 and total ERK. Association of ErbB2 increased markedly in differentiated keratinocytes, whereas association of ErbB1 was similar in basaloid and differentiated cells. Cytoskeletal association of ErbB2 correlated with plasma membrane localization. These results suggest that ErbB1 and ErbB2 employ different mechanisms of plasma membrane targeting during keratinocyte differentiation, and that cytoskeletal association may facilitate the coupling of activated ErbB1 and ERK.

Effect of ErbB2 coexpression on the kinetic interactions of epidermal growth factor with its receptor in intact cells

We have extended the use of stopped-flow mixing and fluorescence anisotropy detection to investigate in real-time the effects of ErbB2 coexpression on the kinetic interactions of epidermal growth factor (EGF) with the EGF receptor. Using stable 32D-derived cell lines expressing both the EGF receptor and ErbB2, and fluorescein-labeled H22Y murine EGF (F-EGF), a series of association and dissociation experiments were performed in which the kinetic interaction of F-EGF with cells was monitored by observing time-dependent changes in fluorescence anisotropy following rapid mixing. Data were collected at various concentrations of F-EGF and multiple cell densities, using cells that express similar levels of the EGF receptor but different levels of ErbB2, and then analyzed by fitting to a two independent receptor-class model using global analysis techniques. The recovered kinetic parameters indicated that the coexpression of ErbB2 had relatively modest effects on recovered rate constants and calculated K(d) values, but a significant effect on the fraction of receptors associated with the high-affinity receptor class. This effect on the fraction of high-affinity receptors depended on the relative expression of ErbB2, as higher ErbB2 expression levels correlated with a larger fraction of high-affinity receptors. Further, the increase in the fraction of high-affinity receptors due to the presence of ErbB2 occurred without any change in the total number of EGF binding sites per cell. Thus, we have identified modulation of the relative populations of high- and low-affinity classes of EGF receptors as a consequence of coexpression of ErbB2 with the EGF receptor.

Heterogeneity of signal transduction at the subcellular level: microsphere-based focal EGF receptor activation and stimulation of Shc translocation

Epidermal growth factor receptor (EGFR, erbB1) activation and translocation of the Shc adaptor protein to activated receptors were analyzed at the subcellular level by dual-label immunofluorescence and confocal laser scanning microscopy in conjunction with a new microsphere-based protocol. In the Quantitative Microsphere Recruitment Assay (QMRA) introduced here, epidermal growth factor-coated 1 μm diameter microspheres were distributed over the surface of adherent tissue culture cells expressing the receptor. High-resolution confocal microscopy of a fusion construct of the receptor and the green fluorescent protein expressed in Chinese hamster ovary cells demonstrated that engulfment and internalization of the microspheres occurred rapidly within minutes, and in a receptor activation-dependent manner. In human epidermoid carcinoma A431 cells, receptor activation and Shc translocation persisted over the 20-minute time course of the experiments. However, at the subcellular level the positive correlation of receptor activation and Shc translocation observed at 5-8 minutes dissipated, indicating a time-dependent decoupling of the two events and variation in the kinetics of signal transduction for different subcellular locations.

Heterodimerization of the epidermal-growth-factor (EGF) receptor and ErbB2 and the affinity of EGF binding are regulated by different mechanisms

When clathrin-dependent endocytosis is inhibited in HeLa cells by overexpression of a K44A (Lys(44)-->Ala) mutant of the GTPase dynamin, high-affinity binding of epidermal growth factor (EGF) to the EGF receptor (EGFR) is disrupted [Ringerike, Stang, Johannessen, Sandnes, Levy and Madshus (1998) J. Biol. Chem. 273, 16639-16642]. We now report that the effect of [K44A]dynamin on EGF binding was counteracted by incubation with the non-specific kinase inhibitor staurosporine (SSP), implying that a protein kinase is responsible for disrupted high-affinity binding of EGF upon overexpression of [K44A]dynamin. The effect of [K44A]dynamin on EGF binding was not due to altered phosphorylation of the EGFR, suggesting that the activated kinase is responsible for phosphorylation of a substrate other than EGFR. The number of EGFR molecules was increased in cells overexpressing [K44A]dynamin, while the number of proto-oncoprotein ErbB2 molecules was unaltered. EGF-induced receptor dimerization was not influenced by overexpression of [K44A]dynamin. ErbB2-EGFR heterodimer formation was found to be ligand-independent, and the number of heterodimers was not altered by overexpression of [K44A]dynamin. Neither SSP nor the phorbol ester PMA, which disrupts high-affinity EGF-EGFR interaction, had any effect on the EGFR homo- or hetero-dimerization. Furthermore, the EGF-induced tyrosine phosphorylation of ErbB2 was not affected by overexpression of [K44A]dynamin, implying that EGFR-ErbB2 dimers were fully functional. Our results strongly suggest that high-affinity binding of EGF and EGFR-ErbB2 heterodimerization are regulated by different mechanisms.

Overexpression of N-acetylglucosaminyltransferase III enhances the epidermal growth factor-induced phosphorylation of ERK in HeLaS3 cells by up-regulation of the internalization rate of the receptors

N-Acetylglucosaminyltransferase III (GnT-III) is a key enzyme that inhibits the extension of N-glycans by introducing a bisecting N-acetylglucosamine residue. In this study we investigated the effect of GnT-III on epidermal growth factor (EGF) signaling in HeLaS3 cells. Although the binding of EGF to the epidermal growth factor receptor (EGFR) was decreased in GnT-III transfectants to a level of about 60% of control cells, the EGF-induced activation of extracellular signal-regulated kinase (ERK) in GnT-III transfectants was enhanced to approximately 1.4-fold that of the control cells. A binding analysis revealed that only low affinity binding of EGF was decreased in the GnT-III transfectants, whereas high affinity binding, which is considered to be responsible for the downstream signaling, was not altered. EGF-induced autophosphorylation and dimerization of the EGFR in the GnT-III transfectants were the same levels as found in the controls. The internalization rate of EGFR was, however, enhanced in the GnT-III transfectants as judged by the uptake of (125)I-EGF and Oregon Green-labeled EGF. When the EGFR internalization was delayed by dansylcadaverine, the up-regulation of ERK phosphorylation in GnT-III transfectants was completely suppressed to the same level as control cells. These results suggest that GnT-III overexpression in HeLaS3 cells resulted in an enhancement of EGF-induced ERK phosphorylation at least in part by the up-regulation of the endocytosis of EGFR.

Ligand-Receptor Interactions in the Membrane of Cultured Cells Monitored by Fluorescence Correlation Spectroscopy

We investigated the specific binding of epidermal growth factor (EGF) to its membrane-bound receptors in cultured cells. The specificity of the binding was attested by the consistent displacement of bound rhodamine-labeled EGF (Rh-EGF) following addition of 1000-fold molar excess of unlabeled EGF. The binding specificity of EGF was further confirmed when vascular EGF was unable to displace Rh-EGF binding, demonstrating no cross-reaction. Evidence for the specific interactions was verified by an equilibrium saturation binding experiment. EGF binding to the cell membranes is saturated at nanomolar concentration. The Scatchard plots show a binding process with K(ass) of 1.5 x 10(9) M(-1). The dissociation kinetics follow a single exponential function characteristic for a relatively slow dissociation process with k(diss) = 2.9 x 10(-4) s(-1). The appearance of two binding complexes through the distribution of diffusion times may suggest that these are representatives of two different forms or subtypes of EGF receptors. This study is of pharmaceutical significance as it provides evidence that fluorescence correlation spectroscopy can be used as a rapid technique for studying ligand-receptor interactions in cell cultures. This is a step forward toward large-scale drug screening in cell cultures.

Attachment of A-431 cells on immobilized antibodies to the EGF receptor promotes cell spreading and reorganization of the microfilament system

EGF-like sequences, inherent in a number of extracellular matrix proteins, participate in cell adhesion. It is possible that interactions of these sequences with EGF receptors (EGFR) affect actin filament organization. It was shown previously [Khrebtukova et al., 1991: Exp. Cell Res. 194:48-55] that antibodies specific to EGFR induce capping of these receptors and redistribution of cytoskeletal proteins in A-431 cells. Here we report that A-431 cells attach and spread on solid substrata coated with antibodies to EGFR, even in the absence of serum. Thus, EGFR can act as an adhesion protein and promote microfilament reorganization. Binding of the cells to the EGFR-antibody resulted in the formation of a unique cell shape characterized by numerous, actin-based filopodia radiating from the cell body, but without membrane ruffles. There was also a conspicuous circular belt of actin-containing fibers inside the cell margin, and many irregular actin aggregates in the perinuclear area. The morphologies and actin distributions in A-431 cells spread on fibronectin or laminin 2/4 were very different. On fibronectin, cells had polygonal shapes with numerous stress-fibers and thick actin-containing fibers along the cell edges. On laminin-covered substrata, the cells became fusiform and acquired broad leading lamellae with ruffles. In these cells, there were also a few bundles of filaments running the whole length of the cell body, and shorter bundles extending through the leading lamellae towards the membrane ruffles in the cell edge. These effects and those seen with immobilized EGF suggest that different ligand/receptor complexes induce specific reorganizations of the microfilament system.

Thermodynamic mixing of molecular states of the epidermal growth factor receptor modulates macroscopic ligand binding affinity

The epidermal growth factor receptor (EGFr), when expressed on the cell surface, has long been known to display two distinct affinities for epidermal growth factor (EGF) binding. In addition, the treatment of cells expressing the EGFr with phorbol esters has been shown to cause a loss of the high-affinity binding capacity of the receptor. In the present study, point mutations that alter acidic or phosphorylation sites have been made in an intracellular domain near Tyr-992 (residues 988-992) of the EGFr. Equilibrium (125)I-EGF binding studies demonstrate that the conversion of Tyr-992 into glutamate induces a 4-fold decrease in the EGFr apparent low-affinity dissociation constant, whereas the mutation of two acidic residues, Asp-988 and Glu-991, or the conversion of Tyr-992 into phenylalanine does not alter EGFr affinity. Phorbol ester treatment of EGFr-expressing Chinese hamster ovary cells results in a loss of high-affinity binding and an increase in the apparent low-affinity dissociation constant of the receptor, similar to the effect of a truncation mutant in which the C-terminal 190 residues are deleted. These results are examined in the context of a new model for regulation of the affinity of the EGFr for EGF in which a cytosolic particle stabilizes the high-affinity conformation of the EGFr and a rapid equilibrium exists between EGFr high-affinity and low-affinity conformations. This model demonstrates that the macroscopic affinities of the EGFr can differ from the affinities of individual EGFr molecules and provides a theoretical framework whereby the measured affinities of the EGFr are modulated by intracellular interactions.

Retinoic acid affects the EGF-R signaling pathway during differentiation induction of human endometrial adenocarcinoma cells

We have shown that moderately differentiated endometrial adenocarcinoma (RL95-2) cells differentiate in response to retinoic acid treatment, illustrated by their reorganization of actin filaments and cell enlargement (Carter et al., Anticancer Res. 16, 17-24, 1996). Tyrphostin, an inhibitor of epidermal growth factor receptor (EGF-R)-associated protein tyrosine kinases, caused a dramatic reorganization of actin filaments in RL95-2 cells, similar to retinoic-acid-treated cells (Carter and Bellido, J. Cell. Physiol. 178, 320-332, 1999). We evaluated the possibility that the differentiating effects of retinoids are due to retinoic-acid-induced decreases in phosphorylation of EGF-R and changes in downstream effector proteins. Retinoic acid caused a decrease in tyrosine phosphorylation of EGF-R. Retinoic acid treatment induced a dramatic actin filament reorganization and cell enlargement. Treatment with EGF reversed this effect, because cells treated with retinoic acid followed by EGF only possessed disrupted actin aggregates and appeared small, thus resembling medium controls. Retinoic acid induced a relocalization and decrease in the amount of Shc protein, another actin-binding protein which is an adaptor protein for EGF-R signaling. In addition, retinoic acid induced a relocalization of gelsolin from the plasma membrane to the cytoplasm. Retinoic acid decreased cell detachment in detachment assays; one-half as many retinoic-acid-treated cells detached as in controls. These results are consistent with the idea that retinoic acid induces differentiation of RL95-2 cells by interfering with the EGF-R signaling pathway.

Single-molecule imaging of EGFR signalling on the surface of living cells

The early events in signal transduction from the epidermal growth factor (EGF) receptor (EGFR) are dimerization and autophosphorylation of the receptor, induced by binding of EGF. Here we observe these events in living cells by visualizing single molecules of fluorescent-dye-labelled EGF in the plasma membrane of A431 carcinoma cells. Single-molecule tracking reveals that the predominant mechanism of dimerization involves the formation of a cell-surface complex of one EGF molecule and an EGFR dimer, followed by the direct arrest of a second EGF molecule, indicating that the EGFR dimers were probably preformed before the binding of the second EGF molecule. Single-molecule fluorescence-resonance energy transfer shows that EGF-EGFR complexes indeed form dimers at the molecular level. Use of a monoclonal antibody specific to the phosphorylated (activated) EGFR reveals that the EGFR becomes phosphorylated after dimerization.

Association of the Ras to mitogen-activated protein kinase signal transduction pathway with microfilaments. Evidence for a p185(neu)-containing cell surface signal transduction particle linking the mitogenic pathway to a membrane-microfilament association site

Microvilli of the aggressive 13762 ascites mammary adenocarcinoma contain a large, microfilament-associated signal transduction particle whose scaffolding is a stable glycoprotein complex (Li, Y., Hua, F., Carraway, K. L., and Carraway, C. A. C. (1999) J. Biol. Chem. 274, 25651-25658) associated with the growth factor receptor p185(neu). The receptor is constitutively tyrosine-phosphorylated in the cells and microvilli, predicting that it should recruit mitogenic pathway components to this membrane-microfilament interaction site. Immunoprecipitation of cell lysates with anti-phosphotyrosine and immunoblotting showed phosphorylated forms of the mitogenic pathway proteins Shc and MAPK in addition to p185(neu), suggesting that the Ras to MAPK mitogenic pathway is activated. Immunoblotting of p185(neu)-containing microvillar fractions revealed the presence in each of stably associated Shc, Grb-2, Sos, Ras, Raf, mitogen-activated protein kinase kinase, and mitogen-activated protein kinase/extracellular signal-regulated kinase, as well as the transcription factor-phosphorylating kinase Rsk. All of these pathway components co-immunoprecipitated with p185(neu) from cleared lysates of microvilli solubilized under microfilament-depolymerizing conditions. The recruitment of constitutively phosphorylated p185(neu) and the activated mitogenic pathway proteins to this membrane-microfilament interaction site provides a physical model for integrating the assembly of the mitogenic pathway with the transmission of growth factor signal to the cytoskeleton. This linkage is probably a requisite step in the global cytoskeleton remodeling accompanying mitogenesis.

Growth factor-induced signal transduction in adherent mammalian cells is sensitive to gravity

Epidermal growth factor (EGF) activates a well-characterized signal transduction cascade in a wide variety of cells. This activation leads to increased cell proliferation in most cell types. Among the early effects evoked by EGF are receptor clustering, cell rounding, and early gene expression. The influence of gravity on EGF-induced EGF receptor clustering and gene expression as well as on actin polymerization and cell rounding have been investigated in adherent A431 epithelial cells with the use of sounding rockets to create microgravity conditions. EGF-induced c-fos and c-jun expression decreased in microgravity. This was caused by alteration of the EGF receptor and protein kinase C-mediated signal transduction pathways. In contrast, neither the binding of EGF to the receptor nor the receptor clustering were changed under microgravity conditions. Because cell morphology was also modulated under microgravity conditions, and the growth factor-induced signal transduction cascades have been demonstrated to be linked to the actin microfilament system, it is tempting to suggest that the actin microfilament system constitutes the gravity-sensitive cell component.

Surface redistribution of interferon gamma-receptor and its colocalization with the actin cytoskeleton

BACKGROUND

Specific antibodies for human IFN gamma-R1 were used to examine its mobilization in Colo 205 cells.

METHODS

We report here that antibody-IFN gamma-R1 complex induced capping and actin colocalization. Pretreatment with cytochalasin D abolished this capping. To define the role of the IFN gamma-R1 in the possible interaction with actin, transfected murine fibroblasts cell line with human cDNA IFN gamma-R1 were used.

RESULTS

Only those cells expressing the full receptor and cultured in suspension polarized the receptor and this colocalized with actin filaments. Nevertheless, cells truncated in their intracellular domain displayed no capping and actin remained unaltered either in suspension or in monolayer culture conditions. A mutant bearing an IFN gamma-R1 with substitutions in positions 270-271 of the intracellular domain redistributed both IFN gamma-R1 and actin as micropatches instead of capping. Mutation in 256-303 residues resulted in IFN gamma-R1 microaggregates but actin remained unchanged.

CONCLUSIONS

These experimental models allowed us to highlight an apparent receptor-microfilament association through the intracellular domain of IFN gamma-R1, and to specifically locate it within the intracellular region 256-303 that has been identified as relevant for ligand-receptor internalization and biological function.

Epidermal growth factor receptor activation is localized within low-buoyant density, non-caveolar membrane domains

Increasing evidence for the organization of cell-surface proteins and lipids into different detergent-insoluble rafts led us to investigate epidermal growth factor (EGF) receptor activation in the plasma membranes of A431 carcinoma cells, using a combination of cell fractionation and immunoprecipitation techniques. Density-gradient centrifugation of sodium carbonate cell extracts revealed that the vast majority of both stimulated and unstimulated EGF receptors were concentrated in a caveolin-rich light membrane (CLM) fraction, with the biochemical characteristics of detergent-insoluble glycolipid-rich domains (DIGs). However, ultrastructural analysis of the CLM fraction revealed that it contained a heterogeneous collection of vesicles, some with sizes greater than that expected for individual caveolae. Experiments with detergent-solubilized cells and isolated CLMs indicated that, in contrast with caveolin, EGF receptors were unlikely to be localized to DIG domains. Furthermore, immunoisolation of caveolin from CLMs revealed that EGF receptor activation occurs in a compartment distinct from caveolae. Similarly, using an anti-(EGF receptor) antibody, the bulk of the cellular caveolin was not co-immunoprecipitated from CLMs, thereby confirming that these two proteins reside in separate membrane domains. The deduction that caveolar signalling and EGF receptor activation occur in separable rafts argues for a multiplicity of signal transduction compartments within the plasma membrane. In addition, by demonstrating that EGF receptor activation is compartmentalized within low-density, non-caveolar regions of the plasma membrane, it is also shown that the co-localization of proteins in a CLM fraction is insufficient to prove caveolar localization.

High-affinity binding of epidermal growth factor (EGF) to EGF receptor is disrupted by overexpression of mutant dynamin (K44A)

Activation of the epidermal growth factor receptor (EGFR) kinase was analyzed in cells conditionally defective for clathrin-dependent endocytosis by overexpression of mutant dynamin (K44A). EGF-induced autophosphorylation of the EGFR on ice was strongly reduced in cells overexpressing mutant dynamin, and consistently, binding analyses showed that high-affinity EGFRs were lost. In the absence of mutant dynamin the cells displayed both high- and low-affinity EGFR. At 4 degreesC EGF-EGFR localized mainly outside coated pits regardless of expression of mutant dynamin. However, also low-affinity EGFR efficiently moved to coated pits upon incubating cells at 37 degreesC. Thus, expression of mutant dynamin disrupts high-affinity binding of EGF, but not ligand-induced recruitment of EGFR to clathrin-coated pits.

EGF stimulates an increase in actin nucleation and filament number at the leading edge of the lamellipod in mammary adenocarcinoma cells

Stimulation of metastatic MTLn3 cells with EGF causes the rapid extension of lamellipods, which contain a zone of F-actin at the leading edge. In order to establish the mechanism for accumulation of F-actin at the leading edge and its relationship to lamellipod extension in response to EGF, we have studied the kinetics and location of EGF-induced actin nucleation activity in MTLn3 cells and characterized the actin dynamics at the leading edge by measuring the changes at the pointed and barbed ends of actin filaments upon EGF stimulation of MTLn3 cells. The major result of this study is that stimulation of MTLn3 cells with EGF causes a transient increase in actin nucleation activity resulting from the appearance of free barbed ends very close to the leading edge of extending lamellipods. In addition, cytochalasin D causes a significant decrease in the total F-actin content in EGF-stimulated cells, indicating that both actin polymerization and depolymerization are stimulated by EGF. Pointed end incorporation of rhodamine-labeled actin by the EGF stimulated cells is 2.12+/−0.47 times higher than that of control cells. Since EGF stimulation causes an increase in both barbed and pointed end incorporation of rhodamine-labeled actin in the same location, the EGF-stimulated nucleation sites are more likely due either to severing of pre-existing filaments or de novo nucleation of filaments at the leading edge thereby creating new barbed and pointed ends. The timing and location of EGF-induced actin nucleation activity in MTLn3 cells can account for the observed accumulation of F-actin at the leading edge and demonstrate that this F-actin rich zone is the primary actin polymerization zone after stimulation.

Characterization of a mutant profilin with reduced actin-binding capacity: effects in vitro and in vivo

We are investigating structure-function relationships in profilin and actin by site-specific mutagenesis using a yeast, Saccharomyces cerevisiae, expression system to produce wild-type and mutant proteins. This paper shows that deleting proline 96 and threonine 97, which are located close to the major actin binding site on profilin, did not significantly alter the interaction between profilin and phosphatidylinositol 4,5-bisphosphate, nor did it affect the profilin:poly(L-proline) interaction. The mutant protein, however, had a lower capacity to bind to actin in vitro than wild-type profilin, though it showed a slightly increased profilin-enhanced nucleotide exchange on the actin. When microinjected into Swiss 3T3 mouse fibroblasts or porcine aortic endothelial cells, the mutant profilin did not change the organization of the microfilament system like the wild-type profilin did. This provides further evidence that profilin controls microfilament organization in the cell by interacting directly with actin.

Identification of an intracellular domain of the EGF receptor required for high-affinity binding of EGF

Although all EGF receptors in EGF receptor-expressing cells are molecularly identical, they can be subdivided in two different classes that have either a high or a low affinity for EGF. Specifically the high-affinity class is associated with filamentous actin. To determine whether the interaction of the EGF receptor with actin induces its high-affinity state, we studied EGF-binding properties of an EGF receptor mutant that lacks the actin-binding site. Interestingly, we found that cells expressing this mutant receptor still display both high- and low-affinity classes of EGF receptors, indicating that the actin-binding domain does not determine the high-affinity binding state. By further mutational analysis we identified a receptor domain, within the tyrosine kinase domain, that regulates the affinity for EGF.

Cytoskeletal and morphological changes associated with the specific suppression of the epidermal growth factor receptor tyrosine kinase activity in A431 human epidermoid carcinoma

The epidermal growth factor (EGF) receptor is well known as a mediator of mitogenic signaling and its tyrosine kinase activity has been suggested as a viable target in cancer chemotherapy. To explore the consequences of abolishing the kinase activity of this receptor, we have utilized a potent and specific inhibitor of the enzyme, PD 153035, to sustain a long-term suppression of its activity. This compound inhibits EGF receptor autophosphorylation in cells with an IC50 in the low nanomolar range and does not block PDGF or FGF receptor kinase until concentrations are greater than 10 microM. [1] Human epidermoid carcinoma A431 cells were grown in the presence of PD 153035 and were passed weekly until cells grew in the presence of 1 microM inhibitor. These cells, referred to as A431R, showed a remarkable change in morphology, becoming flattened and spread out. A comparison of the sensitivity of EGF receptor autophosphorylation to PD 153035 between A431 and A431R showed a similar dose response, indicating that the cells had not developed any defect in the kinase which might make it resistant to the inhibitor. Likewise, EGF receptor autophosphorylation in response to exogenously added EGF, as well as receptor internalization, was similar between the two cell lines. Furthermore, analysis of A431R cells by flow cytometry showed no significant change in DNA content or percentage of cells in any one phase of the cell cycle compared to the parent line. 125I-labeled EGF/receptor binding studies showed that receptor number in the A431R cells was equivalent to that of the parent line; however, the Scatchard plot was linear, in contrast to the typical biphasic plot obtained with the parent cells, implying a loss of high-affinity receptors. Cytoskeletal preparations from both cell lines indicated that the A431R had fourfold less EGF receptor associated with the cytoskeleton than A431. This was accompanied by a remarkable increase in polymerized actin stress fibers throughout the A431R cells, which most likely accounts for their flattened morphology. The A431R cells also exhibited a twofold increase in the expression of focal adhesion kinase, which is consistent with a greater contact area for their cell surface and increase in focal adhesions. Finally, although the A431R cells have a doubling time of 24 h, similar to that of the parent line, these cells stop growing as the monolayer approaches confluence, reminiscent of the contact inhibition seen in nontransformed cells. These data indicate that long-term suppression of the EGF receptor tyrosine kinase activity in A431 human epidermoid carcinoma results in certain cellular properties which are more consistent with a differentiated and nontransformed phenotype.

Distribution of guanylate cyclase within photoreceptor outer segments

Guanylate cyclases play an essential role in the recovery of vertebrate photoreceptor cells after light activation. Here, we have investigated how one such guanylate cyclase, RetGC-1, is distributed within light- and dark-adapted rod photoreceptor cells. Guanylate cyclase activity partitioned with the photoreceptor outer segment (OS) cytoskeleton in a light-sensitive manner. RetGC-1 was found to bind actin filaments in actin blot overlays, suggesting a mechanism for its association with the OS cytoskeleton. In retinal sections, this enzyme was immunodetected only in the OSs, where it appeared to be distributed throughout the disk membranes.

Oligomerization of epidermal growth factor receptors on A431 cells studied by time-resolved fluorescence imaging microscopy. A stereochemical model for tyrosine kinase receptor activation

The aggregation states of the epidermal growth factor receptor (EGFR) on single A431 human epidermoid carcinoma cells were assessed with two new techniques for determining fluorescence resonance energy transfer: donor photobleaching fluorescence resonance energy transfer (pbFRET) microscopy and fluorescence lifetime imaging microscopy (FLIM). Fluorescein-(donor) and rhodamine-(acceptor) labeled EGF were bound to the cells and the extent of oligomerization was monitored by the spatially resolved FRET efficiency as a function of the donor/acceptor ratio and treatment conditions. An average FRET efficiency of 5% was determined after a low temperature (4 degrees C) incubation with the fluorescent EGF analogs for 40 min. A subsequent elevation of the temperature for 5 min caused a substantial increase of the average FRET efficiency to 14% at 20 degrees C and 31% at 37 degrees C. In the context of a two-state (monomer/dimer) model for the EGFR, these FRET efficiencies were consistent with minimal average receptor dimerizations of 13, 36, and 69% at 4, 20, and 37 degrees C, respectively. A431 cells were pretreated with the monoclonal antibody mAb 2E9 that specifically blocks EGF binding to the predominant population of low affinity EGFR (15). The average FRET efficiency increased dramatically to 28% at 4 degrees C, indicative of a minimal receptor dimerization of 65% for the subpopulation of high affinity receptors. These results are in accordance with prior studies indicating that binding of EGF leads to a fast and temperature-dependent microclustering of EGFR, but suggest in addition that the high affinity functional subclass of receptors on quiescent A431 cells are present in a predimerized or oligomerized state. We propose that the transmission of the external ligand-binding signal to the cytoplasmic domain is effected by a concerted relative rotational rearrangement of the monomeric units comprising the dimeric receptor, thereby potentiating a mutual activation of the tyrosine kinase domains.

Barriers for lateral diffusion of transferrin receptor in the plasma membrane as characterized by receptor dragging by laser tweezers: fence versus tether

Our previous results indicated that the plasma membrane of cultured normal rat kidney fibroblastic cell is compartmentalized for diffusion of receptor molecules, and that long-range diffusion is the result of successive intercompartmental jumps (Sako, Y. and Kusumi, A. 1994. J. Cell Biol. 125:1251-1264). In the present study, we characterized the properties of intercompartmental boundaries by tagging transferrin receptor (TR) with either 210-nm-phi latex or 40-nm-phi colloidal gold particles, and by dragging the particle-TR complexes laterally along the plasma membrane using laser tweezers. Approximately 90% of the TR-particle complexes showed confined-type diffusion with a microscopic diffusion coefficient (Dmicro) of approximately 10(-9) cm2/s and could be dragged past the intercompartmental boundaries in their path by laser tweezers at a trapping force of 0.25 pN for gold-tagged TR and 0.8 pN for latex-tagged TR. At lower dragging forces between 0.05 and 0.1 pN, particle-TR complexes tended to escape from the laser trap at the boundaries, and such escape occurred in both the forward and backward directions of dragging. The average distance dragged was half of the confined distance of TR, which further indicates that particle-TR complexes escape at the compartment boundaries. Since variation in the particle size (40 and 210 nm, the particles are on the extracellular surface of the plasma membrane) hardly affects the diffusion rate and behavior of the particle-TR complexes at the compartment boundaries, and since treatment with cytochalasin D or vinblastin affects the movements of TR (Sako and Kusumi as cited above), argument has been advanced that the boundaries are present in the cytoplasmic domain. Rebound of the particle-TR complexes when they escape from the laser tweezers at the compartment boundaries suggests that the boundaries are elastic structures. These results are consistent with the proposal that the compartment boundaries consist of membrane skeleton or a membrane-associated part of the cytoskeleton (membrane skeleton fence model). Approximately 10% of TR exhibited slower diffusion (Dmicro approximately 10(-10)-10(-11) cm2/s) and binding to elastic structures.

Epidermal growth factor induces rapid and transient association of phospholipase C-gamma 1 with EGF-receptor and filamentous actin at membrane ruffles of A431 cells

Addition of epidermal growth factor to A431 cells results in dramatic changes in cell morphology. Initially the cells form membrane ruffles accompanied by increased actin polymerization, followed by cell rounding. Activation of the tyrosine kinase of the receptor by binding epidermal growth factor leads also to phosphorylation and activation of phospholipase C-gamma 1, a key enzyme in the phosphoinositide pathway. In this study we have investigated the localization of phospholipase C-gamma 1 during cell activation by epidermal growth factor. It is shown that addition of the growth factor to A431 cells leads to a translocation of phospholipase C-gamma 1 from the cytosol to the membrane fraction. Interestingly, this relocation is exclusively directed to the membrane ruffles. Most of the phospholipase C-gamma 1 associates to the membrane and a small fraction to the underlying skeleton. Immunocytochemical studies demonstrated that phospholipase C-gamma 1 co-localizes with the epidermal growth factor receptor and also filamentous actin at the membrane ruffles. Moreover, using anti-phosphotyrosine antibodies we found that the membrane ruffles are significantly enriched in phosphotyrosyl proteins. Between 5 and 10 minutes after stimulation the membrane ruffles disappear and also the co-localization of phospholipase C-gamma 1 with the epidermal growth factor receptor and filamentous actin. These results support the notion that activation of A431 cells by epidermal growth factor leads to the formation of a signalling complex of its receptor, phospholipase C-gamma 1 and filamentous actin which is primarily localized at membrane ruffles.

Signaling, mitogenesis and the cytoskeleton: where the action is

Stimulation of mitogenesis by the epidermal growth factor (EGF) operates through a pathway involving the receptor, the small G-protein Ras and protein kinases of the MAP kinase cascade. It is proposed that two of the critical steps of that pathway utilize localization of components to the plasma membrane where Ras is located: recruitment of the nucleotide exchange protein Sos to the phosphorylated EGF receptor via a complex with the SH2/SH3-containing protein Grb2 and recruitment of the protein kinase Raf to activated Ras. Moreover, it is then proposed that Raf associates with the cytoskeleton at the membrane as it is being activated. Other signaling elements, including class I receptor kinases, nonreceptor tyrosine kinases and tyrosine phosphatases, are known to function at specific cellular sites. These observations have led us to propose that localization of signaling components, and particularly sites at membrane-microfilament interfaces, play critical roles in cellular regulation.

EGF causes hyperproliferation and apoptosis in T51B cells: involvement of high and low affinity EGFR binding sites

We have recently described an in vitro model for liver homeostasis which involves growing T51B rat liver epithelial cells with 1 nM epidermal growth factor (EGF). There is an initial period of hyperplasia, lasting about 3 days, which is followed by an increase in apoptosis. The cell density returns to around the confluent control level 5 days after EGF addition. The dose response of T51B cells to EGF shows three distinct growth patterns. We have carried out EGF binding studies that suggest that the occupancy of the low affinity binding site of the EGF receptor, is responsible for the hyperproliferation seen when the cells are grown with high doses of EGF. These studies also suggest that the apoptosis could be triggered by down-regulation of the receptor, in a manner analogous to the removal of a trophic hormone in other systems.

Inositol 1,4,5-trisphosphate receptor-like protein in plasmalemmal caveolae is linked to actin filaments

We reported that a plasmalemmal inositol 1,4,5-trisphosphate receptor-like protein (PM InsP3R-L) is localized in caveolae of various non-neuronal cells in vivo (Fujimoto et al. (1992) J. Cell Biol. 119, 1507–1513). In the present study, we investigated the distribution of PM InsP3R-L in cultured cells. In mouse epidermal keratinocytes (Pam 212) cultured in standard Ca2+ (1.8 mM), PM InsP3R-L was distributed densely in the vicinity of cell-to-cell contacts. In contrast, when Pam cells were cultured in low Ca2+ (0.06 mM) without making cell-to-cell contacts, PM InsP3R-L was observed randomly; by restoring the Ca2+ concentration, the circumferential actin filaments became obvious and the density of PM InsP3R-L increased in the contact region. Treatment of Pam cells with cytochalasin D caused aggregation of caveolae where PM InsP3R-L as well as F-actin and fodrin were localized. In bovine aortic endothelial cells, PM InsP3R-L was aligned along actin filaments crossing the cytoplasm in various directions. PM InsP3R-L of Pam cells was hardly extracted by treatment with 0.5% Triton X-100 or 60 mM octyl-glucoside in a cytoskeleton-stabilizing buffer for 15 minutes at 4 degrees C. The results show that the distribution of caveolae bearing PM InsP3R-L changes when the actin cytoskeleton is modified. They also indicate that the association of PM InsP3R-L with actin filaments may mediate the redistribution of caveolae. Since caveolae are thought to be related to signal transduction, their location defined by the actin cytoskeleton may affect the site where cellular reaction is to occur in response to various stimuli.

Cloning and expression of a cytoskeleton-associated diacylglycerol kinase that is dominantly expressed in cerebellum

A third species of diacylglycerol kinase (EC 2.7.1.107) cDNA was cloned from a rat brain cDNA library. The isolated cDNA encoded a 788-amino acid, 88-kDa polypeptide. This isozyme shared 58% identity with the previously isolated rat 80-kDa and 90-kDa diacylglycerol kinases. EF hand motifs, cysteine-rich zinc finger-like sequences, and putative ATP-binding site were all conserved among these isozymes. The 88-kDa diacylglycerol kinase was expressed specifically in brain and localized predominantly in cerebellar Purkinje cells. This isozyme was associated equally with particulate and supernatant fractions in cDNA-transfected COS-7 cells and dominantly with the particulate fraction in the brain. After Triton X-100 extraction, this isozyme remained in the detergent-insoluble cytoskeletal fraction of the brain and transfected COS-7 cells.