Immunocytochemical localization of Shc and activated EGF receptor in early endosomes after EGF stimulation of HeLa cells

Piezo1 activates noncanonical EGFR endocytosis and signaling

EGFR-ERK signaling controls cell cycle progression during development, homeostasis, and disease. While EGF ligand and mechanical inputs can activate EGFR-ERK signaling, the molecules linking mechanical force to this axis have remained mysterious. We previously found that stretch promotes mitosis via the stretch-activated ion channel Piezo1 and ERK signaling. Here, we show that Piezo1 provides the missing link between mechanical signals and EGFR-ERK activation. While both EGF- and Piezo1-dependent activation trigger clathrin-mediated EGFR endocytosis and ERK activation, EGF relies on canonical tyrosine autophosphorylation, whereas Piezo1 involves Src-p38 kinase-dependent serine phosphorylation. In addition, unlike EGF, ex vivo lung slices treated with Piezo1 agonist promoted cell cycle re-entry via nuclear ERK, AP-1 (FOS and JUN), and YAP accumulation, typical of regenerative and malignant signaling. Our results suggest that mechanical activation via Piezo1, Src, and p38 may be more relevant to controlling repair, regeneration, and cancer growth than tyrosine kinase signaling via canonical EGF signaling, suggesting an alternative therapeutic approach.

EGFR-RAS-MAPK signaling is confined to the plasma membrane and associated endorecycling protrusions

The subcellular localization of RAS GTPases defines the operational compartment of the EGFR-ERK1/2 signaling pathway within cells. Hence, we used live-cell imaging to demonstrate that endogenous KRAS and NRAS tagged with mNeonGreen are predominantly localized to the plasma membrane. NRAS was also present in the Golgi apparatus and a tubular, plasma-membrane derived endorecycling compartment, enriched in recycling endosome markers (TERC). In EGF-stimulated cells, there was essentially no colocalization of either mNeonGreen-KRAS or mNeonGreen-NRAS with endosomal EGFR, which, by contrast, remained associated with endogenous Grb2-mNeonGreen, a receptor adaptor upstream of RAS. ERK1/2 activity was diminished by blocking cell surface EGFR with cetuximab, even after most ligand-bound, Grb2-associated EGFRs were internalized. Endogenous mCherry-tagged RAF1, an effector of RAS, was recruited to the plasma membrane, with subsequent accumulation in mNG-NRAS–containing TERCs. We propose that a small pool of surface EGFRs sustain signaling within the RAS-ERK1/2 pathway and that RAS activation persists in TERCs, whereas endosomal EGFR does not significantly contribute to ERK1/2 activity.

The role of the ShcD and RET interaction in neuroblastoma survival and migration

Preliminary screening data showed that the ShcD adaptor protein associates with the proto-oncogene RET receptor tyrosine kinase. In the present study, we aimed to investigate the molecular interaction between ShcD and RET in human neuroblastoma cells and study the functional impact of this interaction. We were able to show that ShcD immunoprecipitated with RET from SK-N-AS neuroblastoma cell lysates upon GDNF treatment. This result was validated by ShcD-RET co-localization, which was visualized using a fluorescence microscope. ShcD-RET coexpression promoted ShcD and RET endosomal localization, resulting in unexpected inhibition of the downstream ERK and AKT pathways. Interestingly, ShcD-RET association reduced the viability and migration of SK-N-AS cells. Although ShcD was previously shown to trigger melanoma cell migration and tumorigenesis, our data showed an opposite role for ShcD in neuroblastoma SK-N-AS cells via its association with RET in GDNF-treated cells. In conclusion, ShcD acts as a switch molecule that promotes contrasting biological responses depending on the stimulus ad cell type.

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The melanoma associated Shc adaptor, ShcD, is found to interact with Ret oncogene receptor in SK-N-AS neuroblastoma cells.

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ShcD and Ret coexpression favoures their endosomal localization.

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ShcD-Ret association has suppressed ERK and AKT signalling.

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The functional consequence of ShcD and Ret interaction was shown to negatively affect cell survival and cellular migration in.

A Small Organic Molecule Blocks EGFR Transport into the Nucleus by the Nonclassical Pathway Resulting in Repression of Cancer Invasion

In addition to the traditional epidermal growth factor receptor (EGFR) signaling pathways, nuclear EGFR has been shown to control multiple cellular functions, including cell proliferation and invasion. It has been reported that EGFR is transported into the nucleus after forming a complex with KPNA/KPNB1 or KPNB1. Herein, it is shown that EGFR can interact with both KP and KPNA, but EGF-activated EGFR mostly binds with KPNB1 through the pull-down assay. Also, a small organic molecule (1), an effective binder of KPNB1, inhibits the interaction between EGFR and KPNB1 in the nonclassical transport pathway, but not KPNA. Furthermore, treatment of cancer cells with 1 noticeably blocks the nuclear entry of EGFR, which results in significant suppression of invasion by lung cancer H1299 cells. These findings show that 1 is an effective inhibitor of EGFR/KPNB1 interactions in vitro, it may be used in cellular studies as a tool to determine the role of nuclear EGFR, and it is a drug candidate.

Insights into the Shc Family of Adaptor Proteins

The Shc family of adaptor proteins is a group of proteins that lacks intrinsic enzymatic activity. Instead, Shc proteins possess various domains that allow them to recruit different signalling molecules. Shc proteins help to transduce an extracellular signal into an intracellular signal, which is then translated into a biological response. The Shc family of adaptor proteins share the same structural topography, CH2-PTB-CH1-SH2, which is more than an isoform of Shc family proteins; this structure, which includes multiple domains, allows for the posttranslational modification of Shc proteins and increases the functional diversity of Shc proteins. The deregulation of Shc proteins has been linked to different disease conditions, including cancer and Alzheimer’s, which indicates their key roles in cellular functions. Accordingly, a question might arise as to whether Shc proteins could be targeted therapeutically to correct their disturbance. To answer this question, thorough knowledge must be acquired; herein, we aim to shed light on the Shc family of adaptor proteins to understand their intracellular role in normal and disease states, which later might be applied to connote mechanisms to reverse the disease state.

Ovarian Cancer Differential Interactome and Network Entropy Analysis Reveal New Candidate Biomarkers

Ovarian cancer is one of the most common cancers and has a high mortality rate due to insidious symptoms and lack of robust diagnostics. A hitherto understudied concept in cancer pathogenesis may offer new avenues for innovation in ovarian cancer biomarker development. Cancer cells are characterized by an increase in network entropy, and several studies have exploited this concept to identify disease-associated gene and protein modules. We report in this study the changes in protein-protein interactions (PPIs) in ovarian cancer within a differential network (interactome) analysis framework utilizing the entropy concept and gene expression data. A compendium of six transcriptome datasets that included 140 samples from laser microdissected epithelial cells of ovarian cancer patients and 51 samples from healthy population was obtained from Gene Expression Omnibus, and the high confidence human protein interactome (31,465 interactions among 10,681 proteins) was used. The uncertainties of the up- or downregulation of PPIs in ovarian cancer were estimated through an entropy formulation utilizing combined expression levels of genes, and the interacting protein pairs with minimum uncertainty were identified. We identified 105 proteins with differential PPI patterns scattered in 11 modules, each indicating significantly affected biological pathways in ovarian cancer such as DNA repair, cell proliferation-related mechanisms, nucleoplasmic translocation of estrogen receptor, extracellular matrix degradation, and inflammation response. In conclusion, we suggest several PPIs as biomarker candidates for ovarian cancer and discuss their future biological implications as potential molecular targets for pharmaceutical development as well. In addition, network entropy analysis is a concept that deserves greater research attention for diagnostic innovation in oncology and tumor pathogenesis.

Teaching an old dogma new tricks: twenty years of Shc adaptor signalling

Shc (Src homology and collagen homology) proteins are considered prototypical signalling adaptors in mammalian cells. Consisting of four unique members, ShcA, B, C and D, and multiple splice isoforms, the family is represented in nearly every cell type in the body, where it engages in an array of fundamental processes to transduce environmental stimuli. Two decades of investigation have begun to illuminate the mechanisms of the flagship ShcA protein, whereas much remains to be learned about the newest discovery, ShcD. It is clear, however, that the distinctive modular architecture of Shc proteins, their promiscuous phosphotyrosine-based interactions with a multitude of membrane receptors, involvement in central cascades including MAPK (mitogen-activated protein kinase) and Akt, and unconventional contributions to oxidative stress and apoptosis all require intricate regulation, and underlie diverse physiological function. From early cardiovascular development and neuronal differentiation to lifespan determination and tumorigenesis, Shc adaptors have proven to be more ubiquitous, versatile and dynamic than their structures alone suggest.

Neuropilin-1-dependent regulation of EGF-receptor signaling

Neuropilin-1 (NRP1) is a coreceptor for multiple extracellular ligands. NRP1 is widely expressed in cancer cells and in advanced human tumors; however, its functional relevance and signaling mechanisms are unclear. Here, we show that NRP1 expression controls viability and proliferation of different cancer cells, independent of its short intracellular tail. We found that the extracellular domain of NRP1 interacts with the EGF receptor (EGFR) and promotes its signaling cascade elicited upon EGF or TGF-α stimulation. Upon NRP1 silencing, the ability of ligand-bound EGFR to cluster on the cell surface, internalize, and activate the downstream AKT pathway is severely impaired. EGFR is frequently activated in human tumors due to overexpression, mutation, or sustained autocrine/paracrine stimulation. Here we show that NRP1-blocking antibodies and NRP1 silencing can counteract ligand-induced EGFR activation in cancer cells. Thus our findings unveil a novel molecular mechanism by which NRP1 can control EGFR signaling and tumor growth.

EGF-induced ERK-activation downstream of FAK requires rac1-NADPH oxidase

Reactive oxygen species (ROS) function as signaling molecules mainly by reversible oxidation of redox-sensitive target proteins. ROS can be produced in response to integrin ligation and growth factor stimulation through Rac1 and its effector protein NADPH oxidase. One of the central roles of Rac1-NADPH oxidase is actin cytoskeletal rearrangement, which is essential for cell spreading and migration. Another important regulator of cell spread is focal adhesion kinase (FAK), a coordinator of integrin and growth factor signaling. Here, we propose a novel role for NADPH oxidase as a modulator of the FAK autophosphorylation site. We found that Rac1-NADPH oxidase enhanced the phosphorylation of FAK at Y397. This site regulates FAK's ability to act as a scaffold for EGF-mediated signaling, including activation of ERK. Accordingly, we found that EGF-induced activation of FAK at Y925, the following activation of ERK, and phosphorylation of FAK at the ERK-regulated S910-site depended upon NADPH oxidase. Furthermore, the inhibition of NADPH oxidase caused excessive focal adhesions, which is in accordance with ERK and FAK being modulators of focal adhesion dissociation. Our data suggest that Rac1 through NADPH oxidase is part of the signaling pathway constituted by FAK, Rac1, and ERK that regulates focal adhesion disassembly during cell spreading.

Engineered bivalent ligands to bias ErbB receptor-mediated signaling and phenotypes

The ErbB receptor family is dysregulated in many cancers, and its therapeutic manipulation by targeted antibodies and kinase inhibitors has resulted in effective chemotherapies. However, many malignancies remain refractory to current interventions. We describe a new approach that directs ErbB receptor interactions, resulting in biased signaling and phenotypes. Due to known receptor-ligand affinities and the necessity of ErbB receptors to dimerize to signal, bivalent ligands, formed by the synthetic linkage of two neuregulin-1β (NRG) moieties, two epidermal growth factor (EGF) moieties, or an EGF and a NRG moiety, can potentially drive homotypic receptor interactions and diminish formation of HER2-containing heterodimers, which are implicated in many malignancies and are a prevalent outcome of stimulation by native, monovalent EGF, or NRG. We demonstrate the therapeutic potential of this approach by showing that bivalent NRG (NN) can bias signaling in HER3-expressing cancer cells, resulting in some cases in decreased migration, inhibited proliferation, and increased apoptosis, whereas native NRG stimulation increased the malignant potential of the same cells. Hence, this new approach may have therapeutic relevance in ovarian, breast, lung, and other cancers in which HER3 has been implicated.

RNA helicase A is a DNA-binding partner for EGFR-mediated transcriptional activation in the nucleus

EGF induces the translocation of EGF receptor (EGFR) from the cell surface to the nucleus where EGFR activates gene transcription through its binding to an AT-rich sequence (ATRS) of the target gene promoter. However, how EGFR, without a DNA-binding domain, can bind to the gene promoter is unclear. In the present study, we show that RNA helicase A (RHA) is an important mediator for EGFR-induced gene transactivation. EGF stimulates the interaction of EGFR with RHA in the nucleus of cancer cells. The EGFR/RHA complex then associates with the target gene promoter through binding of RHA to the ATRS of the target gene promoter to activate its transcription. Knockdown of RHA expression in cancer cells abrogates the binding of EGFR to the target gene promoter, thereby reducing EGF/EGFR-induced gene expression. In addition, interruption of EGFR-RHA interaction decreases the EGFR-induced promoter activity. Consistently, we observed a positive correlation of the nuclear expression of EGFR, RHA, and cyclin D1 in human breast cancer samples. These results indicate that RHA is a DNA-binding partner for EGFR-mediated transcriptional activation in the nucleus.

Cell proliferation and epidermal growth factor signaling in non-small cell lung adenocarcinoma cell lines are dependent on Rin1

Rin1 is a Rab5 guanine nucleotide exchange factor that plays an important role in Ras-activated endocytosis and growth factor receptor trafficking in fibroblasts. In this study, we show that Rin1 is expressed at high levels in a large number of non-small cell lung adenocarcinoma cell lines, including Hop62, H650, HCC4006, HCC827, EKVX, HCC2935, and A549. Rin1 depletion from A549 cells resulted in a decrease in cell proliferation that was correlated to a decrease in epidermal growth factor receptor (EGFR) signaling. Expression of wild type Rin1 but not the Rab5 guanine nucleotide exchange factor-deficient Rin1 (Rin1Delta) complemented the Rin1 depletion effects, and overexpression of Rin1Delta had a dominant negative effect on cell proliferation. Rin1 depletion stabilized the cell surface levels of EGFR, suggesting that internalization was necessary for robust signaling in A549 cells. In support of this conclusion, introduction of either dominant negative Rab5 or dominant negative dynamin decreased A549 proliferation and EGFR signaling. These data demonstrate that proper internalization and endocytic trafficking are critical for EGFR-mediated signaling in A549 cells and suggest that up-regulation of Rin1 in A549 cell lines may contribute to their proliferative nature.

Cell proliferation and epidermal growth factor signaling in non-small cell lung adenocarcinoma cell lines are dependent on Rin1

Rin1 is a Rab5 guanine nucleotide exchange factor that plays an important role in Ras-activated endocytosis and growth factor receptor trafficking in fibroblasts. In this study, we show that Rin1 is expressed at high levels in a large number of non-small cell lung adenocarcinoma cell lines, including Hop62, H650, HCC4006, HCC827, EKVX, HCC2935, and A549. Rin1 depletion from A549 cells resulted in a decrease in cell proliferation that was correlated to a decrease in epidermal growth factor receptor (EGFR) signaling. Expression of wild type Rin1 but not the Rab5 guanine nucleotide exchange factor-deficient Rin1 (Rin1Delta) complemented the Rin1 depletion effects, and overexpression of Rin1Delta had a dominant negative effect on cell proliferation. Rin1 depletion stabilized the cell surface levels of EGFR, suggesting that internalization was necessary for robust signaling in A549 cells. In support of this conclusion, introduction of either dominant negative Rab5 or dominant negative dynamin decreased A549 proliferation and EGFR signaling. These data demonstrate that proper internalization and endocytic trafficking are critical for EGFR-mediated signaling in A549 cells and suggest that up-regulation of Rin1 in A549 cell lines may contribute to their proliferative nature.

Differential effects of EGFR ligands on endocytic sorting of the receptor

Endocytic downregulation is a pivotal mechanism turning off signalling from the EGF receptor (EGFR). It is well established that whereas EGF binding leads to lysosomal degradation of EGFR, transforming growth factor (TGF)-α causes receptor recycling. TGF-α therefore leads to continuous signalling and is a more potent mitogen than EGF. In addition to EGF and TGF-α, five EGFR ligands have been identified. Although many of these ligands are upregulated in cancers, very little is known about their effect on EGFR trafficking.

We have compared the effect of six different ligands on endocytic trafficking of EGFR. We find that, whereas they all stimulate receptor internalization, they have very diverse effects on endocytic sorting. Heparin-binding EGF-like growth factor and Betacellulin target all EGFRs for lysosomal degradation. In contrast, TGF-α and epiregulin lead to complete receptor recycling. EGF leads to lysosomal degradation of the majority but not all EGFRs. Amphiregulin does not target EGFR for lysosomal degradation but causes fast as well as slow EGFR recycling. The Cbl ubiquitin ligases, especially c-Cbl, are responsible for EGFR ubiquitination after stimulation with all ligands, and persistent EGFR phosphorylation and ubiquitination largely correlate with receptor degradation.

Altered EGFR localization and degradation in human breast cancer cells with an amphiregulin/EGFR autocrine loop

The epidermal growth factor receptor (EGFR) and its ligand amphiregulin (AR) have been shown to be co-over expressed in breast cancer. We have previously shown that an AR/EGFR autocrine loop is required for SUM149 human breast cancer cell proliferation, motility and invasion. We also demonstrated that AR can induce these altered phenotypes when expressed in the normal mammary epithelial cell line MCF10A, or by exposure of these cells to AR in the medium. In the present studies, we demonstrate that SUM149 cells and immortalized human mammary epithelial MCF10A cells that over express AR (MCF10A AR) or are cultured in the presence of exogenous AR, express higher levels of EGFR protein than MCF10A cells cultured in EGF. Pulse-chase analysis showed that EGFR protein remained stable in the presence of AR, yet was degraded in the presence of EGF. Consistent with this observation, tyrosine 1045 on the EGFR, the c-cbl binding site, exhibited less phosphorylation following stimulation with AR than following stimulation with EGF. Ubiquitination of the receptor was also dramatically less following stimulation with AR than following stimulation with EGF. Flow cytometry analysis showed that EGFR remained on the cell surface following stimulation with AR but was rapidly internalized following stimulation with EGF. Immunofluorescence and confocal microscopy confirmed the flow cytometry results. EGFR in MCF10A cells cultured in the presence of EGF exhibited a predominantly intracellular, punctate localization. In stark contrast, SUM149 cells and MCF10A cells growing in the presence of AR expressed EGFR predominantly on the membrane and at cell-cell junctions. We propose that AR alters EGFR internalization and degradation in a way that favors accumulation of EGFR at the cell surface and ultimately leads to changes in EGFR signaling.

ERK-dependent threonine phosphorylation of EGF receptor modulates receptor downregulation and signaling

Epidermal growth factor (EGF) signaling is critical in normal and aberrant cellular behavior. Extracellular signal-regulated kinase (ERK) mediates important downstream aspects of EGF signaling. Additionally, EGFR undergoes MEK1-dependent ERK consensus site phosphorylation in response to EGF or cytokines such as growth hormone (GH) and prolactin (PRL). GH- or PRL-induced EGFR phosphorylation alters subsequent EGF-induced EGFR downregulation and signal characteristics in an ERK-dependent fashion. We now use reconstitution to study mutation of the sole EGFR ERK phosphorylation consensus residue, (669)T. CHO-GHR cells, which lack EGFR and express GHR, were stably transfected to express human wild-type or T669A ((669)T changed to alanine) EGFRs at similar abundance. Treatment of cells with GH or EGF caused phosphorylation of WT, but not T669A EGFR, in an ERK activity-dependent fashion that was detected with an antibody that recognizes phosphorylation of ERK consensus sites, indicating that (669)T is required for this phosphorylation. Notably, EGF-induced downregulation of EGFR abundance was much more rapid in cells expressing EGFR T669A vs. WT EGFR. Further, pretreatment with the MEK1/ERK inhibitor PD98059 enhanced EGF-induced EGFR loss in cells expressing WT EGFR, but not EGFR T669A, suggesting that the ERK-dependent effects on EGFR downregulation required phosphorylation of (669)T. In signaling experiments, EGFR T669A displayed enhanced acute (15 min) EGFR tyrosine phosphorylation (reflecting EGFR kinase activity) compared to WT EGFR. Further, acute EGF-induced ubiquitination of WT EGFR was markedly enhanced by PD98059 pretreatment and was increased in EGFR T669A-expressing cells independent of PD98059. These signaling data suggest that ERK-mediated (669)T phosphorylation negatively modulates EGF-induced EGFR kinase activity. We furthered these investigations using a human fibrosarcoma cell line that endogenously expresses EGFR and ErbB-2 and also harbors an activating Ras mutation. In these cells, EGFR was constitutively detected with the ERK consensus site phosphorylation-specific antibody and EGF-induced EGFR downregulation was modest, but was substantially enhanced by pretreatment with MEK1/ERK inhibitor. Collectively, these data indicate that ERK activity, by phosphorylation of a threonine residue in the EGFR juxtamembrane cytoplasmic domain, modulates EGFR trafficking and signaling.

EGF and amphiregulin differentially regulate Cbl recruitment to endosomes and EGF receptor fate

EGF-R [EGF (epidermal growth factor) receptor] ligands can promote or inhibit cell growth. The biological outcome of receptor activation is dictated, at least in part, by ligand-specified patterns of endocytic trafficking. EGF-R trafficking downstream of the ligands EGF and TGF-alpha (transforming growth factor-alpha) has been investigated extensively. However, less is known about EGF-R fates induced by the ligands BTC (betacellulin) and AR (amphiregulin). We undertook comparative analyses to identify ligand-specific molecular events that regulate EGF-R trafficking and degradation. EGF (17 nM) and BTC (8.5 nM) induced significant EGF-R degradation, with or without ectopic expression of the ubiquitin ligase Cbl. Human recombinant AR (17 nM) failed to affect receptor degradation in either case. Notably, levels of ligand-induced EGF-R ubiquitination did not correlate strictly with receptor degradation. Dose-response experiments revealed that AR at a saturating concentration was a partial agonist at the EGF-R, with approx. 40% efficacy (relative to EGF) at inducing receptor tyrosine phosphorylation, ubiquitination and association with Cbl. EGF-R down-regulation and degradation also were compromised upon cell stimulation with AR (136 nM). These outcomes correlated with decreased degradation of the Cbl substrate and internalization inhibitor hSprouty2. Downstream of the hSprouty2 checkpoint in AR-stimulated cells, Cbl-free EGF-R was incorporated into endosomes from which Cbl-EGF-R complexes were excluded. Our results suggest that the AR-specific EGF-R fate results from decreased hSprouty2 degradation and reduced Cbl recruitment to underphosphorylated EGF-R, two effects that impair EGF-R trafficking to lysosomes.

c-Src trafficking and co-localization with the EGF receptor promotes EGF ligand-independent EGF receptor activation and signaling

c-Src is a non-receptor tyrosine kinase that associates with both the plasma membrane and endosomal compartments. In many human cancers, especially breast cancer, c-Src and the EGF receptor (EGFR) are overexpressed. Dual overexpression of c-Src and EGFR correlates with a Src-dependent increase in activation of EGFR, and synergism between these two tyrosine kinases increases the mitogenic activity of EGFR. Despite extensive studies of the functional interaction between c-Src and EGFR, little is known about the interactions in the trafficking pathways for the two proteins and how that influences signaling. Given the synergism between c-Src and EGFR, and the finding that EGFR is internalized and can signal from endosomes, we hypothesized that c-Src and EGFR traffic together through the endocytic pathway. Here we use a regulatable c-SrcGFP fusion protein that is a bona fide marker for c-Src to show that c-Src undergoes constitutive macropinocytosis from the plasma membrane into endocytic compartments. The movement of c-Src was dependent on its tyrosine kinase activity. Stimulation of cells with EGF revealed that c-Src traffics into the cell with activated EGFR and that c-Src expression and kinase activity prolongs EGFR activation. Surprisingly, even in the absence of EGF addition, c-Src expression induced activation of EGFR and of EGFR-mediated downstream signaling targets ERK and Shc. These data suggest that the synergy between c-Src and EGFR also occurs as these two kinases traffic together, and that their co-localization promotes EGFR-mediated signaling.

Mechanistic aspects of crosstalk between GH and PRL and ErbB receptor family signaling

Growth hormone (GH) and prolactin (PRL) are anterior pituitary hormones that have multiple roles in growth and metabolism. Both hormones are important in mammary development and breast cancer. The epidermal growth factor (EGF) family of peptides and the receptors that they activate (the ErbB family) are also major players in mammary biology and pathophysiology. Recent studies in signal transduction have highlighted the interplay between signaling pathways referred to as crosstalk. In this review, cell biological and signaling studies related to crosstalk between GH and PRL and the ErbB family are discussed. In particular, the role of GH- and PRL-induced phosphorylation of ErbB receptors in regulating EGF responsiveness is highlighted with attention to potential pathophysiological relevance.

Epidermal growth factor receptor fate is controlled by Hrs tyrosine phosphorylation sites that regulate Hrs degradation

Hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs) is an endosomal protein essential for the efficient sorting of activated growth factor receptors into the lysosomal degradation pathway. Hrs undergoes ligand-induced tyrosine phosphorylation on residues Y329 and Y334 downstream of epidermal growth factor receptor (EGFR) activation. It has been difficult to investigate the functional roles of phosphoHrs, as only a small proportion of the cellular Hrs pool is detectably phosphorylated. Using an HEK 293 model system, we found that ectopic expression of the protein Cbl enhances Hrs ubiquitination and increases Hrs phosphorylation following cell stimulation with EGF. We exploited Cbl's expansion of the phosphoHrs pool to determine whether Hrs tyrosine phosphorylation controls EGFR fate. In structure-function studies of Cbl and EGFR mutants, the level of Hrs phosphorylation and rapidity of apparent Hrs dephosphorylation correlated directly with EGFR degradation. Differential expression of wild-type versus Y329,334F mutant Hrs in Hrs-depleted cells revealed that one or both tyrosines regulate ligand-dependent Hrs degradation, as well as EGFR degradation. By modulating Hrs ubiquitination, phosphorylation, and protein levels, Cbl may control the composition of the endosomal sorting machinery and its ability to target EGFR for lysosomal degradation.

Subcellular localization of epidermal growth factor receptor in human submandibular gland

The subcellular distribution of the epidermal growth factor receptor (EGFr) was demonstrated in the normal human submandibular gland by means of immunogold cytochemistry. EGFr labelling appeared in both acinar and ductal cells, where strong immunoreactivity was associated with a tubulovesicular system near the basolateral surfaces. In addition, groups of reactive vesicles were highlighted among secretory granules of both serous and mucous cells and at the apex of ductal cells. Basolateral vesicles were interpreted as being a result of EGFr internalization after activation by an exogenous ligand, although the functional meaning of those located apically remains unclear.

Src Homology 2 Domain-based High Throughput Assays for Profiling Downstream Molecules in Receptor Tyrosine Kinase Pathways

Src homology 2 (SH2) domains are evolutionary conserved small protein modules that bind specifically to tyrosine-phosphorylated peptides. More than 100 SH2 domains have been identified in proteins encoded by the human genome. The binding specificity of these domains plays a critical role in signaling within the cell, mediating the relocalization and interaction of proteins in response to changes in tyrosine phosphorylation states. Here we developed an SH2 domain profiling method based on a multiplexed fluorescent microsphere assay in which various SH2 domains are used to probe the global state of tyrosine phosphorylation within a cell and to screen synthetic peptides that specifically bind to each SH2 domain. The multiplexed, fluorescent microsphere-based assay is a recently developed technology that can potentially detect a wide variety of interactions between biological molecules. We constructed 25-plex SH2 domain-GST fusion protein-conjugated fluorescent microsphere sets to investigate phosphorylation-mediated cell signaling through the specific binding of SH2 domains to activated target proteins. The response of HeLa, COS-1, A431, and 293 cells and four breast cancer cell lines to epidermal growth factor and insulin were quantitatively profiled using this novel microsphere-based, multiplexed, high throughput assay system.

Altered regulation of EGF receptor signaling following a partial hepatectomy

We have studied epidermal growth factor receptor (EGFR) phosphorylation and localization in the pre-replicative phase of liver regeneration induced by a 70% partial hepatectomy (PH), and how a PH affects EGFR activation and trafficking. When Western blotting was performed on livers after PH with antibodies raised against activated forms of EGFR autophosphorylation sites, no marked increase in EGFR tyrosine phosphorylation was observed. However, events associated with attenuation of EGFR signals were observed. Two hours after PH, we found increased EGFR ubiquitination and internalization, followed by receptor downregulation. Furthermore, EGFR phosphorylation following an injection of EGF was reduced after PH. This reduction correlated with an increased activation of PKC and a distinct augmentation in the phosphorylation of the PKC-regulated T654-site of EGFR. When primary cultured hepatocytes were treated with tetradecanoylphorbol acetate (TPA) to induce T654-phosphorylation of EGFR, we found colocalization of a fraction of EGFR with EEA1, downregulation of EGF-mediated EGFR autophosphorylation, altered ligand-induced intracellular sorting of EGFR, and increased mitogenic signaling through the EGFR-Ras-Raf-ERK pathway. Further, we found that both TPA and a PH enhanced EGF-induced proliferation of hepatocytes. In conclusion, our results suggest that hepatocyte priming involves modulation of EGFR that enhances its ability to mediate growth factor responses without an increase in its receptor tyrosine kinase-activity. This may be a pre-replicative competence event that increases growth factor effects during G1 progression.

EGF signalling in prostate cancer cell lines is inhibited by a high expression level of the endocytosis protein REPS2

In advanced prostate cancer, cellular changes occur leading to a transition from androgen-dependent to androgen-independent growth. During this transition, proliferation of androgen-dependent prostate cancer cells becomes more and more dependent on growth factors, like the epidermal growth factor (EGF). Endocytosis of growth factor receptors, one of the mechanisms that controls growth factor signalling, was observed to be markedly changed in advanced metastatic prostate cancer. Internalisation and signalling of EGF receptors was examined in different prostate cancer cell lines, in relation to the expression level of the endocytosis-related REPS2 gene. It was observed that a high level of REPS2 correlates with reduced EGF-internalisation. To investigate this more thoroughly, prostate cancer cells with inducible REPS2 expression were generated. Using these cells, it was found that REPS2-induction indeed results in reduction of EGF-internalisation. Furthermore, when EGF receptor signalling was evaluated, by examination of mRNA expression for several EGF-responsive genes (EGF receptor, EGR-1, Fos and Jun), it was observed that induced expression of REPS2 exerts an inhibiting effect on this signalling. From these experiments, it is concluded that increased REPS2 expression negatively affects EGF receptor internalisation and subsequent signalling. Therefore, decreased REPS2 expression during prostate cancer progression, observed in earlier work, may result in enhanced EGF receptor expression and signalling, which could add to the androgen-independent state of advanced prostate cancer.

Identification of 14-3-3zeta as an EGF receptor interacting protein

The 14-3-3 proteins are known to interact with a number of proteins involved in the regulation of cell signaling. Here, we describe an association of 14-3-3zeta with the epidermal growth factor receptor (EGFR) that is rapidly induced by EGF. The 1028-EGFR truncated mutant which lacks the cytoplasmic tail from amino acids 1029-1186 identified the binding site for 14-3-3 to be between amino acid 1028 and the receptor carboxyl terminus. Mutational deletion of serine residues 1046, 1047, 1057 and 1142 did not inhibit EGF-induced 14-3-3 association with the receptor. Immunofluorescence microscopy indicated an EGF-induced co-localization of EGFR and HA-14-3-3zeta along the plasma membrane. Our finding adds to the growing complexity of EGF receptor signaling and indicates a role for 14-3-3 proteins in EGF receptor signaling or regulation.

UV-radiation-induced internalization of the epidermal growth factor receptor requires distinct serine and tyrosine residues in the cytoplasmic carboxy-terminal domain

The mechanism of UV-radiation-induced EGF receptor (EGFR) internalization remains to be established. In the present study, we found UV-radiation-mediated internalization of the EGFR to be dependent on the cytoplasmic carboxy-terminal region. UV radiation was unable to induce internalization of EGFR carboxy-terminal truncation mutants where all or four of the five major autophosphorylation sites were missing (963- and 1028-EGFR, respectively). Mutational removal of serine residues 1046, 1047, 1057 and 1142 within the carboxy-terminal receptor region was also sufficient to abolish UV-radiation-induced internalization of the EGFR. Furthermore, the UV-radiation-induced internalization was abrogated for an EGFR mutated in tyrosine 1045 (Y1045F), the major c-Cbl binding site. However, UV radiation did not induce phosphorylation at tyrosine 1045, in contrast to the prominent phosphorylation induced by EGF. Our results suggest a mechanism for UV-radiation-induced internalization of EGFR involving a conformational change that is dependent on structural elements formed by specific serine and tyrosine residues in the carboxy-terminal domain.

Serine mutations that abrogate ligand-induced ubiquitination and internalization of the EGF receptor do not affect c-Cbl association with the receptor

In the present study, we examined EGF-induced internalization, degradation and trafficking of the epidermal growth factor receptor (EGFR) mutated at serines 1046, 1047, 1057 and 1142 located in its cytoplasmic carboxy-terminal region. We found the serine-mutated EGFR to be inhibited in EGF-induced internalization and degradation in NIH3T3 cells. We therefore tested the hypothesis that these mutations affect ligand-induced c-Cbl association with the receptor, leading to inhibited receptor ubiquitination. EGF was unable to induce ubiquitination of the serine-mutated EGFR, yet EGF-induced phosphorylation of the c-Cbl-binding site at tyrosine 1045, and c-Cbl-EGFR association, was unaffected. To compare the relevance of these serine residues with tyrosine 1045 in their regulation of c-Cbl binding and receptor ubiquitination, we analysed an EGFR mutated at tyrosine 1045 (Y1045F). EGF-induced c-Cbl-EGFR binding was partially inhibited, and receptor ubiquitination was abrogated in cells expressing Y1045F-EGFR. In contrast, ligand-induced internalization and degradation of the Y1045F mutant was similar to that of wild-type EGFR. Together, our data indicate that the serine residues and tyrosine 1045 are essential for EGF-induced receptor ubiquitination, but only the serine residues are critical for EGFR internalization and degradation.

Increase in epidermal growth factor receptor protein induced in osteoblastic cells after exposure to flow of culture media

To investigate how bone cells respond to mechanical stimuli, we subjected osteoblastic cells to fluid flow. We and others already reported that in a culture system of osteoblast-like cells, ERK1/2, Shc, and other proteins were tyrosine-phosphorylated by medium flow and the early response gene, egr-1 or c-fos mRNA, increased. These are the same as events found after stimulation by various growth factors. Moreover, because there were also reports suggesting that growth factor signaling is involved in the responses to mechanical stimuli, we examined the change in epidermal growth factor (EGF) receptor in the cells exposed to medium flow. The results demonstrated that EGF receptor protein increased after exposure to medium flow. This increase did not occur without serum in media, and the addition of EGF restored it. Furthermore, leupeptin blocked this increase. These results suggest that degradation of EGF-occupied EGF receptor by leupeptin-sensitive protease(s) in endosomes decreased with exposure to medium flow. This was presumed to participate, at least in part, in signaling of fluid flow.

Rac activation upon cell-cell contact formation is dependent on signaling from the epidermal growth factor receptor

Cadherins are transmembrane receptors that mediate cell-cell adhesion. They play an essential role in embryonic development and maintenance of tissue architecture. The Rho family small GTPases regulate actin cytoskeletal dynamics in different cell types. The function of two family members, Rho and Rac, is required for the stability of cadherins at cell-cell contacts. Consistent with the published data we have found that Rac is activated upon induction of intercellular adhesion in epithelial cells. This activation is dependent on functional cadherins (Nakagawa, M., Fukata, M., Yamaga, M., Itoh, N., and Kaibuchi, K. (2001) J. Cell Sci. 114, 1829-1838; Noren, N. K., Niessen, C. M., Gumbiner, B. M., and Burridge, K. (2001) J. Biol. Chem. 276, 3305-3308). Here we show for the first time that clustering of cadherins using antibody-coated beads is sufficient to promote Rac activation. In the presence of Latrunculin B, Rac can be partially activated by antibody-clustered cadherins. These results suggest that actin polymerization is not required for initial Rac activation. Contrary to what has been described before, phosphatidylinositol 3-kinases are not involved in Rac activation following cell-cell adhesion in keratinocytes. Interestingly, inhibition of epidermal growth factor receptor signaling efficiently blocks the increased Rac-GTP levels observed after contact formation. We conclude that cadherin-dependent adhesion can activate Rac via epidermal growth factor receptor signaling.

Huntingtin-associated protein 1 interacts with hepatocyte growth factor-regulated tyrosine kinase substrate and functions in endosomal trafficking

Huntingtin-associated protein 1 (HAP1) is a novel protein of unknown function with a higher binding affinity for the mutant form of Huntington's disease protein huntingtin. Here we report that HAP1 interacts with hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs), a mammalian homologue of yeast vacuolar protein sorting protein Vps27p involved in the endosome-to-lysosome trafficking. This novel interaction was identified in a yeast two-hybrid screen using full-length Hrs as bait, and confirmed by in vitro binding assays and co-immunoprecipitation experiments. Deletion analysis reveals that the association of HAP1 with Hrs is mediated via a coiled-coil interaction between the central coiled-coil domains of both proteins. Immunofluorescence and subcellular fractionation studies show that HAP1 co-localizes with Hrs on early endosomes. Like Hrs, overexpression of HAP1 causes the formation of enlarged early endosomes, and inhibits the degradation of internalized epidermal growth factor receptors. Whereas overexpression of HAP1 does not affect either constitutive or ligand-induced receptor-mediated endocytosis, it potently blocks the trafficking of endocytosed epidermal growth factor receptors from early endosomes to late endosomes. These findings implicate, for the first time, the involvement of HAP1 in the regulation of vesicular trafficking from early endosomes to the late endocytic compartments.

Stress kinase p38 mediates EGFR transactivation by hyperosmolar concentrations of sorbitol

Activation of the epidermal growth factor receptor (EGFR) has been shown to occur by ligand-dependent and ligand-independent mechanisms. Different molecular mechanisms have been found to be responsible for ligand-independent receptor transactivation. Here, we show that hyperosmolar concentrations of sorbitol activate the EGFR in human keratinocytes. Experiments using specific inhibitors of EGFR phosphorylation show that the increased amount of activated receptors is the result of a decreased rate of dephosphorylation. Furthermore, sorbitol treatment results in a strong activation of stress kinase p38. Treatment of the cells with SB203580, a known inhibitor of p38 alpha and beta kinases, results in impairment of receptor activation, indicating that the stress kinase is involved in receptor activation modulation. This is further reinforced by experiments showing that addition of Toxin B, known to be an inhibitor of the small Rho GTPases rac1, cdc42, and Rho A/B, to the cells results in a strong induction of EGFR activation. Our results point, therefore, to a mechanism by which osmotic shock activates EGFR through the small Rho GTPases-p38 stress kinase pathway.

Calmodulin regulates intracellular trafficking of epidermal growth factor receptor and the MAPK signaling pathway

The epidermal growth factor receptor (EGFR) is a member of the tyrosine kinase receptor family involved in signal transduction and the regulation of cellular proliferation and differentiation. It is also a calmodulin-binding protein. To examine the role of calmodulin in the regulation of EGFR, the effect of calmodulin antagonist, W-13, on the intracellular trafficking of EGFR and the MAPK signaling pathway was analyzed. W-13 did not alter the internalization of EGFR but inhibited its recycling and degradation, thus causing the accumulation of EGF and EGFR in enlarged early endosomal structures. In addition, we demonstrated that W-13 stimulated the tyrosine phosphorylation of EGFR and consequent recruitment of Shc adaptor protein with EGFR, presumably through inhibition of the calmodulin-dependent protein kinase II (CaM kinase II). W-13-mediated EGFR phosphorylation was blocked by metalloprotease inhibitor, BB94, indicating a possible involvement of shedding in this process. However, MAPK activity was decreased by W-13; dissection of this signaling pathway showed that W-13 specifically interferes with Raf-1 activity. These data are consistent with the regulation of EGFR by calmodulin at several steps of the receptor signaling and trafficking pathways.

Coordinated traffic of Grb2 and Ras during epidermal growth factor receptor endocytosis visualized in living cells

Activation of the epidermal growth factor receptor (EGFR) triggers multiple signaling pathways and rapid endocytosis of the epidermal growth factor (EGF)-receptor complexes. To directly visualize the compartmentalization of molecules involved in the major signaling cascade, activation of Ras GTPase, we constructed fusions of Grb2, Shc, H-Ras, and K-Ras with enhanced cyan fluorescent protein (CFP) or yellow fluorescent protein (YFP), and used live-cell fluorescence imaging microscopy combined with the fluorescence resonance energy transfer (FRET) technique. Stimulation of cells by EGF resulted in the accumulation of large pools of Grb2-CFP and YFP-Shc in endosomes, where these two adaptor proteins formed a complex with EGFR. H-Ras and K-Ras fusion proteins were found at the plasma membrane, particularly in ruffles and lamellipodia, and also in endosomes independently of GTP/GDP loading and EGF stimulation. The relative amount of endosomal H-Ras was higher than that of K-Ras, whereas K-Ras predominated at the plasma membrane. On application of EGF, Grb2, and Ras converge in the same endosomes through the fusion of endosomes containing either Grb2 or Ras or through the joint internalization of two proteins from the plasma membrane. To examine the localization of the GTP-bound form of Ras, we used a FRET assay that exploits the specific interaction of GTP-bound CFP-Ras with the YFP-fused Ras binding domain of c-Raf. FRET microscopy revealed that GTP-bound Ras is located at the plasma membrane, mainly in ruffles and at the cell edges, as well as in endosomes containing EGFR. These data point to the potential for endosomes to serve as sites of generation for persistent signaling through Ras.

Role of calmodulin in the modulation of the MAPK signalling pathway and the transactivation of epidermal growth factor receptor mediated by PKC

We have recently shown that calmodulin (CaM) regulates the trafficking of epidermal growth factor receptor (EGFR) as well as the mitogen-activated protein kinase (MAPK) signalling pathway. However, the overall regulation of the MAPK pathway is achieved through a complex interplay of other several upstream effectors including G-proteins, EGF, EGFR, protein kinase C (PKC), phosphatidylinositol-3-kinase and CaM. In order to understand the role of CaM in the PKC-mediated transactivation of EGFR we have analysed the effect of a CaM antagonist, N-(4-aminobutyl)-5-chloro-2-naphthalenesulfonamide, on the 12-O-tetradecanoylphorbol-13-acetate-mediated activation of EGFR and the subsequent MAPK activation. The results show that CaM interferes with MAPK activation and the transactivation of EGFR mediated by PKC.

Fluorescent histochemical techniques for analysis of intracellular signaling

Intracellular signaling relies on the orchestrated cooperation of signaling proteins and modules, their intracellular localization, and membrane trafficking. Recently, a repertoire of fluorescence-based techniques, which significantly increases our potential for detailed studies of the involved mechanisms, has been introduced. Microscopic techniques with increased resolution have been combined with improved techniques for detection of signaling proteins. Transfections of fluorescently tagged proteins have allowed in vivo microscopy of their trafficking and interactions with other proteins and intracellular structures. We present an overview of general signaling principles and a description of techniques based on fluorescent microscopy suited for studies of signaling mechanisms.

UV induces tyrosine kinase-independent internalisation and endosome arrest of the EGF receptor

We have compared the activation and trafficking of epidermal growth factor receptor (EGFR) induced by UV light and EGF. Tyrosine phosphorylation of EGFR was not detected in UV-exposed cells by immunoblotting of whole cell lysates or EGFR immunoprecipitates with antibodies specific for each of the five activated autophosphorylation sites of EGFR. In addition, EGFR of UV-irradiated cells did not demonstrate increased 32P-incorporation. However, UV-exposed cells demonstrated a gel mobility shift of EGFR, which was not abolished by alkaline phosphatase treatment. UV-exposure did not induce dimerisation of EGFR. Furthermore, UV induced internalisation of EGFR without polyubiquitination or degradation. UV-exposed EGFR was transferred to early endosomes and arrested in transferrin-accessible endosomes close to the cell surface. Whereas inhibition of the EGFR tyrosine kinase effectively inhibited tyrosine phosphorylation and internalisation of EGF-activated EGFR, internalisation of UV-exposed EGFR was unaffected. UV induced neither relocalisation of Shc and Grb2 nor activation of Raf, but activation of MEK and MAPK was observed.

Our work indicates that UV induces internalisation of EGFR independent of its phosphorylation or receptor tyrosine kinase activation, and altered EGFR trafficking compared with ligand-activated receptor. In addition, MAPK activation by UV does not appear to be mediated by EGFR activation.

Pharmacodynamic studies of the epidermal growth factor receptor inhibitor ZD1839 in skin from cancer patients: histopathologic and molecular consequences of receptor inhibition

PURPOSE

The epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor ZD1839 (Iressa; AstraZeneca Pharmaceuticals, Alderley Park, United Kingdom) is under development as an anticancer agent. We studied the pharmacodynamic effects of ZD1839 on EGFR in the skin, an EGFR-dependent tissue, in cancer patients participating in ZD1839 phase I clinical trials.

PATIENTS AND METHODS

We studied 104 pre- and/or on-ZD1839 therapy ( approximately at day 28 of therapy) skin biopsies from 65 patients receiving escalating doses of daily oral ZD1839. We measured ZD1839 effects on EGFR activation by immunohistochemistry using an antibody specific for the activated (phosphorylated) EGFR. Effects on receptor signaling (activated mitogen-activated protein kinase [MAPK]), proliferation, p27(KIP1), and maturation were also assessed.

RESULTS

Histopathologically, the stratum corneum of the epidermis was thinner during therapy (P <.001). In hair follicles, prominent keratin plugs and microorganisms were found in dilated infundibula. ZD1839 suppressed EGFR phosphorylation in all EGFR-expressing cells (P <.001). In addition, ZD1839 inhibited MAPK activation (P <.001) and reduced keratinocyte proliferation index (P <.001). Concomitantly, ZD1839 increased the expression of p27(KIP1) (P <.001) and maturation markers (P <.001) and increased apoptosis (P <.001). These effects were observed at all dose levels, before reaching dose-limiting toxicities.

CONCLUSION

ZD1839 inhibits EGFR activation and affects downstream receptor-dependent processes in vivo. These effects were profound at doses well below the one producing unacceptable toxicity, a finding that strongly supports pharmacodynamic assessments to select optimal doses instead of a maximum-tolerated dose for definitive efficacy and safety trials.

Epidermal growth factor receptor mutation type III transfected into a small cell lung cancer cell line is predominantly localized at the cell surface and enhances the malignant phenotype

In the present study we transfected the epidermal growth factor receptor (EGFR)-negative small cell lung cancer cell line, GLC3, with the type III EGFR mutation (EGFRvIII). The EGFRvIII protein could be detected by Western blot analysis as a 145-kDa protein, which by immunohistochemistry appeared to be localized at the cell surface. Ultrastructurally EGFRvIII was expressed mainly at the cell surface with clusters at cell-cell contacts. In the in vitro invasion assay, GLC3-EGFRvIII cells had a approximately 5-fold increased invasion compared with uninduced GLC3-EGFRvIII, GLC3-Tet-On and the parental cell line. GLC3-Tet-On appeared uniform in size with adherence junctions at cell-cell contacts. In uninduced GLC3-EGFRvIII cells adherence junctions were also present but less distinct. In doxycycline-pretreated GLC3-EGFRvIII cells, adherence junctions were absent. We conclude that the expression of EGFRvIII results in a more malignant phenotype. This effect appears to involve the disruption of adherence junctions.

Regulation of epidermal growth factor receptor signaling by endocytosis and intracellular trafficking

Ligand activation of the epidermal growth factor receptor (EGFR) leads to its rapid internalization and eventual delivery to lysosomes. This process is thought to be a mechanism to attenuate signaling, but signals could potentially be generated after endocytosis. To directly evaluate EGFR signaling during receptor trafficking, we developed a technique to rapidly and selectively isolate internalized EGFR and associated molecules with the use of reversibly biotinylated anti-EGFR antibodies. In addition, we developed antibodies specific to tyrosine-phosphorylated EGFR. With the use of a combination of fluorescence imaging and affinity precipitation approaches, we evaluated the state of EGFR activation and substrate association during trafficking in epithelial cells. We found that after internalization, EGFR remained active in the early endosomes. However, receptors were inactivated before degradation, apparently due to ligand removal from endosomes. Adapter molecules, such as Shc, were associated with EGFR both at the cell surface and within endosomes. Some molecules, such as Grb2, were primarily found associated with surface EGFR, whereas others, such as Eps8, were found only with intracellular receptors. During the inactivation phase, c-Cbl became EGFR associated, consistent with its postulated role in receptor attenuation. We conclude that the association of the EGFR with different proteins is compartment specific. In addition, ligand loss is the proximal cause of EGFR inactivation. Thus, regulated trafficking could potentially influence the pattern as well as the duration of signal transduction.

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.

Re-localization of activated EGF receptor and its signal transducers to multivesicular compartments downstream of early endosomes in response to EGF

The rapid internalization of receptor tyrosine kinases after ligand binding has been assumed to be a negative modulation of signal transduction. However, accumulating data indicate that signal transduction from internalized cell surface receptors also occurs from endosomes. We show that a substantial fraction of tyrosine-phosphorylated epidermal growth factor receptor (EGFR) and Shc, Grb2 and Cbl after internalization relocates from early endosomes to compartments which are negative for the early endosomes, recycling vesicle markers EEA1 and transferrin in EGF-stimulated cells. These compartments contained the multivesicular body and late endosome marker CD63, and the late endosome and lysosome marker LAMP-1, and showed a multivesicular morphology. Subcellular fractionation revealed that activated EGFR, adaptor proteins and activated ERK 1 and 2 were located in EEA1-negative and LAMP-1-positive fractions. Co-immunoprecipitations showed EGFR in complex with both Shc, Grb2 and Cbl. Treatment with the weak base chloroquine or inhibitors of lysosomal enzymes after EGF stimulation induced an accumulation of tyrosine-phosphorylated EGFR and Shc in EEA1-negative and CD63-positive vesicles after a 120-min chase period. This was accompanied by a sustained activation of ERK 1 and 2. These results suggest that EGFR signaling is not spatially restricted to the plasma membrane, primary vesicles and early endosomes, but is continuing from late endocytic trafficking organelles maturing from early endosomes.

Hrs interacts with sorting nexin 1 and regulates degradation of epidermal growth factor receptor

Hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs) is a mammalian homologue of yeast vacuolar protein sorting (Vps) protein Vps27p; however, the role of Hrs in lysosomal trafficking is unclear. Here, we report that Hrs interacts with sorting nexin 1 (SNX1), a recently identified mammalian homologue of yeast Vps5p that recognizes the lysosomal targeting code of epidermal growth factor receptor (EGFR) and participates in lysosomal trafficking of the receptor. Biochemical analyses demonstrate that Hrs and SNX1 are ubiquitous proteins that exist in both cytosolic and membrane-associated pools, and that the association of Hrs and SNX occurs on cellular membranes but not in the cytosol. Furthermore, endogenous SNX1 and Hrs form a approximately 550-kDa complex that excludes EGFR. Immunofluorescence and subcellular fractionation studies show that Hrs and SNX1 colocalize on early endosomes. By using deletion analysis, we have mapped the binding domains of Hrs and SNX1 that mediate their association. Overexpression of Hrs or its SNX1-binding domain inhibits ligand-induced degradation of EGFR, but does not affect either constitutive or ligand-induced receptor-mediated endocytosis. These results suggest that Hrs may regulate lysosomal trafficking through its interaction with SNX1.