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

Protein networking: nicotinic acetylcholine receptors and their protein-protein-associations

Nicotinic acetylcholine receptors (nAChR) are complex transmembrane proteins involved in neurotransmission in the nervous system and at the neuromuscular junction. nAChR disorders may lead to severe, potentially fatal pathophysiological states. To date, the receptor has been the focus of basic and applied research to provide novel therapeutic interventions. Since most studies have investigated only the nAChR itself, it is necessary to consider the receptor as part of its protein network to understand or elucidate-specific pathways. On its way through the secretory pathway, the receptor interacts with several chaperones and proteins. This review takes a closer look at these molecular interactions and focuses especially on endoplasmic reticulum biogenesis, secretory pathway sorting, Golgi maturation, plasma membrane presentation, retrograde internalization, and recycling. Additional knowledge regarding the nAChR protein network may lead to a more detailed comprehension of the fundamental pathomechanisms of diseases or may lead to the discovery of novel therapeutic drug targets.

Deguelin Restores Paclitaxel Sensitivity in Paclitaxel-Resistant Ovarian Cancer Cells via Inhibition of the EGFR Signaling Pathway

Background

Ovarian cancer is one of women's malignancies with the highest mortality among gynecological cancers. Paclitaxel is used in first-line ovarian cancer chemotherapy. Research on paclitaxel-resistant ovarian cancer holds significant clinical importance.

Methods

Cell viability and flow cytometric assays were conducted at different time and concentration points of deguelin and paclitaxel treatment. Immunoblotting was performed to assess the activation status of key signaling molecules important for cell survival and proliferation following treatment with deguelin and paclitaxel. The fluo-3 acetoxymethyl assay for P-glycoprotein transport activity assay and cell viability assay in the presence of N-acetyl-L-cysteine were also conducted.

Results

Cell viability and flow cytometric assays demonstrated that deguelin resensitized paclitaxel in a dose- and time-dependent manner. Cotreatment with deguelin and paclitaxel inhibited EGFR and its downstream signaling molecules, including AKT, ERK, STAT3, and p38 MAPK, in SKOV3-TR cells. Interestingly, cotreatment with deguelin and paclitaxel suppressed the expression level of EGFR via the lysosomal degradation pathway. Cotreatment did not affect the expression and function of P-glycoprotein. N-acetyl-L-cysteine failed to restore cell cytotoxicity when used in combination with deguelin and paclitaxel in SKOV3-TR cells. The expression of BCL-2, MCL-1, and the phosphorylation of the S155 residue of BAD were downregulated. Moreover, inhibition of paclitaxel resistance by deguelin was also observed in HeyA8-MDR cells.

Conclusion

Our research showed that deguelin effectively suppresses paclitaxel resistance in SKOV3-TR ovarian cancer cells by downregulating the EGFR and its downstream signaling pathway and modulating the BCL-2 family proteins. Furthermore, deguelin exhibits inhibitory effects on paclitaxel resistance in HeyA8-MDR ovarian cancer cells, suggesting a potential mechanism for paclitaxel resensitization that may not be cell-specific. These findings suggest that deguelin holds promise as an anticancer therapeutic agent for overcoming chemoresistance in ovarian cancer.

Endosomal Arl4A attenuates EGFR degradation by binding to the ESCRT-II component VPS36

Ligand-induced epidermal growth factor receptor (EGFR) endocytosis followed by endosomal EGFR signaling and lysosomal degradation plays important roles in controlling multiple biological processes. ADP-ribosylation factor (Arf)-like protein 4 A (Arl4A) functions at the plasma membrane to mediate cytoskeletal remodeling and cell migration, whereas its localization at endosomal compartments remains functionally unknown. Here, we report that Arl4A attenuates EGFR degradation by binding to the endosomal sorting complex required for transport (ESCRT)-II component VPS36. Arl4A plays a role in prolonging the duration of EGFR ubiquitinylation and deterring endocytosed EGFR transport from endosomes to lysosomes under EGF stimulation. Mechanistically, the Arl4A-VPS36 direct interaction stabilizes VPS36 and ESCRT-III association, affecting subsequent recruitment of deubiquitinating-enzyme USP8 by CHMP2A. Impaired Arl4A-VPS36 interaction enhances EGFR degradation and clearance of EGFR ubiquitinylation. Together, we discover that Arl4A negatively regulates EGFR degradation by binding to VPS36 and attenuating ESCRT-mediated late endosomal EGFR sorting.

Loss of CPAP causes sustained EGFR signaling and epithelial-mesenchymal transition in oral cancer

Higher epidermal growth factor receptor (EGFR) signaling can contribute to tumor metastasis and resistance to therapies in oral squamous cell carcinoma (OSCC). EGFR signaling can promote epithelial-mesenchymal transition (EMT) in OSCC. EMT is a process by which epithelial cells acquire invasive properties and it can contribute to tumor metastasis. Not only do the abnormal functions of microtubule and microtubule-organizing centers (MTOC) such as centrosomes lead to cancers, but also the malignant tissues are characterized by aberrant centriolar features and amplified centrosomes. Microtubule inhibition therapies increase the sensitivity to EGFR targeting drugs in various cancers. In this study, we show that the loss of expression of a microtubule/tubulin binding protein, centrosomal protein 4.1-associated protein (CPAP), which is critical for centriole biogenesis and normal functioning of the centrosome, caused an increase in the EGFR levels and its signaling and, enhanced the EMT features and invasiveness of OSCC cells. Further, depletion of CPAP enhanced the tumorigenicity of these cells in a xeno-transplant model. Importantly, CPAP loss-associated EMT features and invasiveness of multiple OSCC cells were attenuated upon depletion of EGFR in them. On the other hand, we found that CPAP protein levels were higher in EGF treated OSCC cells as well as in oral cancer tissues, suggesting that the frequently reported aberrant centriolar features of tumors are potentially a consequence, but not the cause, of tumor progression. Overall, our novel observations show that, in addition to its known indispensable role in centrosome biogenesis, CPAP also plays a vital role in suppressing tumorigenesis in OSCC by facilitating EGFR homeostasis.

Cooperation and Interplay between EGFR Signalling and Extracellular Vesicle Biogenesis in Cancer

Epidermal growth factor receptor (EGFR) takes centre stage in carcinogenesis throughout its entire cellular trafficking odyssey. When loaded in extracellular vesicles (EVs), EGFR is one of the key proteins involved in the transfer of information between parental cancer and bystander cells in the tumour microenvironment. To hijack EVs, EGFR needs to play multiple signalling roles in the life cycle of EVs. The receptor is involved in the biogenesis of specific EV subpopulations, it signals as an active cargo, and it can influence the uptake of EVs by recipient cells. EGFR regulates its own inclusion in EVs through feedback loops during disease progression and in response to challenges such as hypoxia, epithelial-to-mesenchymal transition and drugs. Here, we highlight how the spatiotemporal rules that regulate EGFR intracellular function intersect with and influence different EV biogenesis pathways and discuss key regulatory features and interactions of this interplay. We also elaborate on outstanding questions relating to EGFR-driven EV biogenesis and available methods to explore them. This mechanistic understanding will be key to unravelling the functional consequences of direct anti-EGFR targeted and indirect EGFR-impacting cancer therapies on the secretion of pro-tumoural EVs and on their effects on drug resistance and microenvironment subversion.

Deficiency of GABARAP but not its Paralogs Causes Enhanced EGF-induced EGFR Degradation

The γ-aminobutyric acid type A receptor-associated protein (GABARAP) and its close paralogs GABARAPL1 and GABARAPL2 constitute a subfamily of the autophagy-related 8 (Atg8) protein family. Being associated with a variety of dynamic membranous structures of autophagic and non-autophagic origin, Atg8 proteins functionalize membranes by either serving as docking sites for other proteins or by acting as membrane tethers or adhesion factors. In this study, we describe that deficiency for GABARAP alone, but not for its close paralogs, is sufficient for accelerated EGF receptor (EGFR) degradation in response to EGF, which is accompanied by the downregulation of EGFR-mediated MAPK signaling, altered target gene expression, EGF uptake, and EGF vesicle composition over time. We further show that GABARAP and EGFR converge in the same distinct compartments at endogenous GABARAP expression levels in response to EGF stimulation. Furthermore, GABARAP associates with EGFR in living cells and binds to synthetic peptides that are derived from the EGFR cytoplasmic tail in vitro. Thus, our data strongly indicate a unique and novel role for GABARAP during EGFR trafficking.

Targeting microtubules sensitizes drug resistant lung cancer cells to lysosomal pathway inhibitors

Oncogene-addicted cancers are predominantly driven by specific oncogenic pathways and display initial exquisite sensitivity to designer therapies, but eventually become refractory to treatments. Clear understanding of lung tumorigenic mechanisms is essential for improved therapies.

Methods: Lysosomes were analyzed in EGFR-WT and mutant cells and corresponding patient samples using immunofluorescence or immunohistochemistry and immunoblotting. Microtubule organization and dynamics were studied using immunofluorescence analyses. Also, we have validated our findings in a transgenic mouse model that contain EGFR-TKI resistant mutations.

Results: We herein describe a novel mechanism that a mutated kinase disrupts the microtubule organization and results in a defective endosomal/lysosomal pathway. This prevents the efficient degradation of phosphorylated proteins that become trapped within the endosomes and continue to signal, therefore amplifying downstream proliferative and survival pathways. Phenotypically, a distinctive subcellular appearance of LAMP1 secondary to microtubule dysfunction in cells expressing EGFR kinase mutants is seen, and this may have potential diagnostic applications for the detection of such mutants. We demonstrate that lysosomal-inhibitors re-sensitize resistant cells to EGFR tyrosine-kinase inhibitors (TKIs). Identifying the endosome-lysosome pathway and microtubule dysfunction as a mechanism of resistance allows to pharmacologically intervene on this pathway.

Conclusions: We find that the combination of microtubule stabilizing agent and lysosome inhibitor could reduce the tumor progression in EGFR TKI resistant mouse models of lung cancer.

Adenovirus early region 3 RIDα protein limits NFκB signaling through stress-activated EGF receptors

The host limits adenovirus infections by mobilizing immune systems directed against infected cells that also represent major barriers to clinical use of adenoviral vectors. Adenovirus early transcription units encode a number of products capable of thwarting antiviral immune responses by co-opting host cell pathways. Although the EGF receptor (EGFR) was a known target for the early region 3 (E3) RIDα protein encoded by nonpathogenic group C adenoviruses, the functional role of this host-pathogen interaction was unknown. Here we report that incoming viral particles triggered a robust, stress-induced pathway of EGFR trafficking and signaling prior to viral gene expression in epithelial target cells. EGFRs activated by stress of adenoviral infection regulated signaling by the NFκB family of transcription factors, which is known to have a critical role in the host innate immune response to infectious adenoviruses and adenovirus vectors. We found that the NFκB p65 subunit was phosphorylated at Thr254, shown previously by other investigators to be associated with enhanced nuclear stability and gene transcription, by a mechanism that was attributable to ligand-independent EGFR tyrosine kinase activity. Our results indicated that the adenoviral RIDα protein terminated this pathway by co-opting the host adaptor protein Alix required for sorting stress-exposed EGFRs in multivesicular endosomes, and promoting endosome-lysosome fusion independent of the small GTPase Rab7, in infected cells. Furthermore RIDα expression was sufficient to down-regulate the same EGFR/NFκB signaling axis in a previously characterized stress-activated EGFR trafficking pathway induced by treatment with the pro-inflammatory cytokine TNF-α. We also found that cell stress activated additional EGFR signaling cascades through the Gab1 adaptor protein that may have unappreciated roles in the adenoviral life cycle. Similar to other E3 proteins, RIDα is not conserved in adenovirus serotypes associated with potentially severe disease, suggesting stress-activated EGFR signaling may contribute to adenovirus virulence.

Proteomics reveals novel protein associations with early endosomes in an epidermal growth factor-dependent manner

The epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase that is an integral component of proliferative signaling. EGFRs on the cell surface become activated upon EGF binding and have an increased rate of endocytosis. Once in the cytoplasm, the EGF·EGFR complex is trafficked to the lysosome for degradation, and signaling is terminated. During trafficking, the EGFR kinase domain remains active, and the internalized EGFR can continue signaling to downstream effectors. Although effector activity varies based on the EGFR's endocytic location, it is not clear how this occurs. In an effort to identify proteins that uniquely associate with the internalized, liganded EGFR in the early endosome, we developed an early endosome isolation strategy to analyze their protein composition. Post-nuclear supernatant from HeLa cells stimulated with and without EGF were separated on an isotonic 17% Percoll gradient. The gradient was fractionated, and early endosomal fractions were pooled and immunoisolated with an EEA1 mAb. The isolated endosomes were validated by immunoblot using antibodies against organelle-specific marker proteins and transmission EM. These early endosomes were also subjected to LC-MS/MS for proteomic analysis. Five proteins were detected in endosomes in a ligand-dependent manner: EGFR, RUFY1, STOML2, PTPN23, and CCDC51. Knockdown of RUFY1 or PTPN23 by RNAi indicated that both proteins play a role in EGFR trafficking. These experiments indicate that endocytic trafficking of activated EGFR changes the protein composition, membrane trafficking, and signaling potential of the early endosome.

Spatio-temporal regulation of EGFR signaling by the Eps15 homology domain-containing protein 3 (EHD3)

The epidermal growth factor (EGF) receptor EGFR is a major receptor tyrosine kinase whose role in gliomagenesis is well established. We have recently identified EHD3 [Eps15 homology (EH) domain-containing protein 3], an endocytic trafficking regulatory protein, as a putative brain tumor suppressor. Here, we investigate the underlying mechanisms, by establishing a novel mechanistic and functional connection between EHD3 and the EGFR signaling pathway. We show that, in response to stimulation with the EGF ligand, EHD3 accelerates the rate of EGFR degradation by dramatically increasing its ubiquitination. As part of this process, EHD3 also regulates EGFR endosomal trafficking by diverting it away from the recycling route into the degradative pathway. Moreover, we found that upon EGF activation, rather than affecting the total MAPK and AKT downstream signaling, EHD3 decreases endosome-based signaling of these two pathways, thus suggesting the contribution of EHD3 in the spatial regulation of EGFR signaling. This function explains the higher sensitivity of EHD3-expressing cells to the growth-inhibitory effects of EGF. In summary, this is the first report supporting a mechanism of EHD3-mediated tumor suppression that involves the attenuation of endosomal signaling of the EGFR oncogene.

Metabolic flux-driven sialylation alters internalization, recycling, and drug sensitivity of the epidermal growth factor receptor (EGFR) in SW1990 pancreatic cancer cells

In prior work we reported that advanced stage, drug-resistant pancreatic cancer cells (the SW1990 line) can be sensitized to the EGFR-targeting tyrosine kinase inhibitors (TKIs) erlotinib and gefitinib by treatment with 1,3,4-O-Bu₃ManNAc (Bioorg. Med. Chem. Lett. (2015) 25(6):1223-7). Here we provide mechanistic insights into how this compound inhibits EGFR activity and provides synergy with TKI drugs. First, we showed that the sialylation of the EGFR receptor was at most only modestly enhanced (by ∼20 to 30%) compared to overall ∼2-fold increase in cell surface levels of this sugar. Second, flux-driven sialylation did not alter EGFR dimerization as has been reported for cancer cell lines that experience increased sialylation due to spontaneous mutations. Instead, we present evidence that 1,3,4-O-Bu₃ManNAc treatment weakens the galectin lattice, increases the internalization of EGFR, and shifts endosomal trafficking towards non-clathrin mediated (NCM) endocytosis. Finally, by evaluating downstream targets of EGFR signaling, we linked synergy between 1,3,4-O-Bu₃ManNAc and existing TKI drugs to a shift from clathrin-coated endocytosis (which allows EGFR signaling to continue after internalization) towards NCM endocytosis, which targets internalized moieties for degradation and thereby rapidly diminishes signaling.

Induction of Tyrosine Phosphorylation of UV-Activated EGFR by the Beta-Human Papillomavirus Type 8 E6 Leads to Papillomatosis

Epidemiological evidence is accumulating that beta-human papillomaviruses (HPV) synergize with UV-light in the development of precancerous actinic keratosis, and cutaneous squamous cell carcinomas (cSCC), one of the most common cancers in the Caucasian population. We previously demonstrated the tumorigenic activity of beta-HPV type 8 (HPV8) in the skin of transgenic mice and its cooperation with UV-light. Analysis of underlying mechanisms now showed that in keratinocytes expressing the HPV8E6 protein a transient increase of tyrosine phosphorylated epidermal growth factor receptor (EGFR) in response to UV-irradiation occurred, while EGFR tyrosine phosphorylation, i.e., receptor tyrosine kinase (RTK)-activity was hardly affected in empty vector control cells. FACS and immunofluorescences revealed that the EGFR was internalized into early endosomes in response to UV-exposure in both, HPV8E6 positive and in control cells, yet with a higher rate in the presence of HPV8E6. Moreover, only in HPV8E6 expressing keratinocytes the EGFR was further sorted into CD63+ intraluminal vesicles, indicative for trafficking to late endosomes. The latter requires the ubiquitination of the EGFR, and in correlation, we could show that only in HPV8E6 positive keratinocytes the EGFR was ubiquitinated upon UV-exposure. HPV8E6 and tyrosine phosphorylated EGFR directly interacted which was enhanced by UV-irradiation. The treatment of K14-HPV8E6 transgenic mice with Canertinib, an inhibitor of the RTK-activity of the EGFR, suppressed skin papilloma growth in response to UV-irradiation. This confirms the crucial role of the RTK-activity of the EGFR in HPV8E6 and UV-mediated papillomatosis in transgenic mice. Taken together, our results demonstrate that HPV8E6 alters the signaling of the UV-activated EGFR and this is a critical step in papilloma formation in response to UV-light in transgenic mice. Our results provide a molecular basis how a beta-HPV type may support early steps of skin tumor formation in cooperation with UV-light.

The spatiotemporal regulation of RAS signalling

Nearly 30% of human tumours harbour mutations in RAS family members. Post-translational modifications and the localisation of RAS within subcellular compartments affect RAS interactions with regulator, effector and scaffolding proteins. New insights into the control of spatiotemporal RAS signalling reveal that activation kinetics and subcellular compartmentalisation are tightly coupled to the generation of specific biological outcomes. Computational modelling can help utilising these insights for the identification of new targets and design of new therapeutic approaches.

FIH-1 disrupts an LRRK1/EGFR complex to positively regulate keratinocyte migration

Factor inhibiting hypoxia-inducible factor 1 (FIH-1; official symbol HIF1AN) is a hydroxylase that negatively regulates hypoxia-inducible factor 1α but also targets other ankyrin repeat domain-containing proteins such as Notch receptor to limit epithelial differentiation. We show that FIH-1 null mutant mice exhibit delayed wound healing. Importantly, in vitro scratch wound assays demonstrate that the positive role of FIH-1 in migration is independent of Notch signaling, suggesting that this hydroxylase targets another ankyrin repeat domain-containing protein to positively regulate motogenic signaling pathways. Accordingly, FIH-1 increases epidermal growth factor receptor (EGFR) signaling, which in turn enhances keratinocyte migration via mitogen-activated protein kinase pathway, leading to extracellular signal-regulated kinase 1/2 activation. Our studies identify leucine-rich repeat kinase 1 (LRRK1), a key regulator of the EGFR endosomal trafficking and signaling, as an FIH-1 binding partner. Such an interaction prevents the formation of an EGFR/LRRK1 complex, necessary for proper EGFR turnover. The identification of LRRK1 as a novel target for FIH-1 provides new insight into how FIH-1 functions as a positive regulator of epithelial migration.

Lipid rafts, KCa/ClCa/Ca2+ channel complexes and EGFR signaling: Novel targets to reduce tumor development by lipids?

Membrane lipid rafts are distinct plasma membrane nanodomains that are enriched with cholesterol, sphingolipids and gangliosides, with occasional presence of saturated fatty acids and phospholipids containing saturated acyl chains. It is well known that they organize receptors (such as Epithelial Growth Factor Receptor), ion channels and their downstream acting molecules to regulate intracellular signaling pathways. Among them are Ca2+ signaling pathways, which are modified in tumor cells and inhibited upon membrane raft disruption. In addition to protein components, lipids from rafts also contribute to the organization and function of Ca2+ signaling microdomains. This article aims to focus on the lipid raft KCa/ClCa/Ca2+ channel complexes that regulate Ca2+ and EGFR signaling in cancer cells, and discusses the potential modification of these complexes by lipids as a novel therapeutic approach in tumor development. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Live-cell fluorescence imaging reveals high stoichiometry of Grb2 binding to the EGF receptor sustained during endocytosis

Activation of epidermal growth factor (EGF) receptor (EGFR) leads to its interaction with Grb2, a dual-function adapter mediating both signaling through Ras and receptor endocytosis. We used time-lapse three-dimensional imaging by spinning disk confocal microscopy to analyze trafficking of EGFR and Grb2 in living HeLa cells stimulated with low, physiological concentrations of EGFR ligands. Endogenous Grb2 was replaced in these cells by Grb2 fused to yellow fluorescent protein (YFP). After transient residence in the plasma membrane, Rhodamine-conjugated EGF (EGF-Rh) and Grb2-YFP were rapidly internalized and accumulated in endosomes. Quantitative image analysis revealed that on average two Grb2-YFP molecules were colocalized with one EGF-Rh in cells stimulated with 2 ng/ml EGF-Rh, and the excess of Grb2-YFP over EGF-Rh was even higher when a receptor-saturating concentration of EGF-Rh was used. Therefore, we hypothesize that a single EGFR molecule can be simultaneously associated with functionally distinct Grb2 interaction partners during and after endocytosis. Continuous presence of Grb2-YFP in endosomes was also observed when EGFR was activated by transforming growth factor-α and amphiregulin, suggesting that endosomal EGFRs remain ligand occupied and signaling competent, despite the fact that these growth factors are thought to dissociate from the receptor at acidic pH. The prolonged localization and activity of EGFR-Grb2 complexes in endosomes correlated with the sustained activation of extracellular stimulus-regulated kinase 1/2, suggesting that endosomal EGFRs contribute significantly to this signaling pathway. We propose that endosomal EGFRs function to extend signaling in time and space to compensate for rapid downregulation of surface EGFRs in cells with low receptor expression levels.

An Internal Standard-Assisted Synthesis and Degradation Proteomic Approach Reveals the Potential Linkage between VPS4B Depletion and Activation of Fatty Acid β-Oxidation in Breast Cancer Cells

The endosomal/lysosomal system, in particular the endosomal sorting complexes required for transport (ESCRTs), plays an essential role in regulating the trafficking and destination of endocytosed receptors and their associated signaling molecules. Recently, we have shown that dysfunction and down-regulation of vacuolar protein sorting 4B (VPS4B), an ESCRT-III associated protein, under hypoxic conditions can lead to the abnormal accumulation of epidermal growth factor receptor (EGFR) and aberrant EGFR signaling in breast cancer. However, the pathophysiological consequences of VPS4B dysfunction remain largely elusive. In this study, we used an internal standard-assisted synthesis and degradation mass spectrometry (iSDMS) method, which permits the direct measurement of protein synthesis, degradation and protein dynamic expression, to address the effects of VPS4B dysfunction in altering EGF-mediated protein expression. Our initial results indicate that VPS4B down-regulation decreases the expression of many proteins involved in glycolytic pathways, while increased the expression of proteins with roles in mitochondrial fatty acid β-oxidation were up-regulated in VPS4B-depleted cells. This observation is also consistent with our previous finding that hypoxia can induce VPS4B down-regulated, suggesting that the adoption of fatty acid β-oxidation could potentially serve as an alternative energy source and survival mechanism for breast cancer cells in response to hypoxia-mediated VPS4B dysfunction.

Monocyte to macrophage differentiation-associated (MMD) positively regulates ERK and Akt activation and TNF-α and NO production in macrophages

Macrophage activation is modulated by both environmental cues and endogenous programs. In the present study, we investigated the role of a PAQR family protein, monocyte to macrophage differentiation-associated (MMD), in macrophage activation and unveiled its underlying molecular mechanism. Our results showed that while MMD expression could be detected in all tissues examined, its expression level is significantly up-regulated upon monocyte differentiation. Within cells, EGFP-MMD fusion protein could be co-localized to endoplasmic reticulum, mitochondria, Golgi apparatus, but not lysosomes and cytoplasm. MMD expression is up-regulated in macrophages after LPS stimulation, and this might be modulated by RBP-J, the critical transcription factor of Notch signaling. Overexpression of MMD in macrophages increased the production of TNF-α and NO upon LPS stimulation. We found that MMD overexpression enhanced ERK1/2 and Akt phosphorylation in macrophages after LPS stimulation. Blocking Erk or Akt by pharmacological agent reduced TNF-α or NO production in MMD-overexpressing macrophages, respectively. These results suggested that MMD modulates TNF-α and NO production in macrophages, and this process might involves Erk or Akt.

I787 provides signals for c-Kit receptor internalization and functionality that control mast cell survival and development

Mast cell (MC) differentiation, survival, and activation are controlled by the membrane tyrosine kinase c-Kit upon interaction with stem cell factor (SCF). Here we describe a single point mutation induced by N-ethyl-N-nitrosurea (ENU) mutagenesis in C57BL/6J mice-an A to T transversion at position 2388 (exon 17) of the c-Kit gene, resulting in the isoleucine 787 substitution by phenylalanine (787F), and analyze the consequences of this mutation for ligand binding, signaling, and MC development. The Kit(787F/787F) mice carrying the single amino acid exchange of c-Kit lacks both mucosal and connective tissue-type MCs. In bone marrow-derived mast cells (BMMCs), the 787F mutation does not affect SCF binding and c-Kit receptor shedding, but strongly impairs SCF-induced cytokine production, degranulation enhancement, and apoptosis rescue. Interestingly, c-Kit downstream signaling in 787F BMMCs is normally initiated (Erk1/2 and p38 activation as well as c-Kit autophosphorylation) but fails to be sustained thereafter. In addition, 787F c-Kit does not efficiently mediate Cbl activation, leading to the absence of subsequent receptor ubiquitination and impaired c-Kit internalization. Thus, I787 provides nonredundant signals for c-Kit internalization and functionality.

Lovastatin inhibits EGFR dimerization and AKT activation in squamous cell carcinoma cells: potential regulation by targeting rho proteins

We recently showed the ability of lovastatin to inhibit the function of the epidermal growth factor receptor (EGFR) and its downstream signaling of the phosphatidylinositol-3 kinase/AKT pathway. Combining lovastatin with gefitinib, a potent EGFR inhibitor, induced synergistic cytotoxicity in various tumor-derived cell lines. In this study, lovastatin treatment was found to inhibit ligand-induced EGFR dimerization in squamous cell carcinoma cells and its activation of AKT and its downstream targets 4E-binding protein 1 and S6 kinase 1. This inhibition was associated with global protein translational inhibition shown by a decrease in RNA associated polysome fractions. The effects of lovastatin on EGFR function were reversed by the addition of geranylgeranyl pyrophosphate, which functions as a protein membrane anchor. Lovastatin treatment induced actin cytoskeletal disorganization and the expression of geranylgeranylated rho family proteins that regulate the actin cytoskeleton, including rhoA. Lovastatin-induced rhoA was inactive as EGF stimulation failed to activate rhoA and inhibition of the rho-associated kinase, a target and mediator of rhoA function, with Y-27632 also showed inhibitory effects on EGFR dimerization. The ability of lovastatin to inhibit EGFR dimerization is a novel exploitable mechanism regulating this therapeutically relevant target. To explore the potential clinical significance of this combination, we evaluated the effect of statin on the overall survival (OS) and disease-specific survival (DSS) of patients with advanced non-small-cell lung cancer enrolled in the NCIC Clinical Trials Group phase III clinical trials BR21 (EGFR tyrosine kinase inhibitor erlotinib versus placebo) and BR18 (carboplatin and paclitaxel with or without the metalloproteinase inhibitor BMS275291). In BR18, use of statin did not affect OS or DSS. In BR21, patients showed a trend for improvement in OS (HR: 0.69, P=0.098) and DSS (HR: 0.62, P=0.048), but there was no statin x treatment interaction effect (P=0.34 and P=0.51 for OS and DSS, respectively).

Intracellular signaling: peripatetic Ras

Ras proteins traffic between the plasma membrane and endomembranes and signal from the cytosolic face of a variety of organelles. Palmitoylated N-Ras and H-Ras signal from early endosomes. A recent study reports that K-Ras resides on and signals from various types of endosomes, including late endosomes/lysosomes and multivesicular bodies.

Ras/MAPK signaling from endomembranes

Signal transduction along the Ras/MAPK pathway has been generally thought to take place at the plasma membrane. It is now evident that the plasma membrane is not the only platform capable of Ras/MAPK signal induction. Fusion of Ras with green fluorescent protein and the development of genetically encoded fluorescent probes for Ras activation have revealed signaling events on a variety of intracellular membranes including endosomes, the Golgi apparatus and the endoplasmic reticulum. Thus, the Ras/MAPK pathway is spatially compartmentalized within cells and this may afford greater complexity of signal output.

Compartmentalized signalling: Ras proteins and signalling nanoclusters

Differential subcellular compartmentalization of the three main Ras isoforms (H-Ras, N-Ras and K-Ras) is believed to underlie their biological differences. Modulatable interactions between cellular membranes and Ras C-terminal hypervariable region motifs determine differences in trafficking and the relative proportions of each isoform in cell-surface signalling nanoclusters and intracellular endoplasmic reticulum/Golgi, endosomal and mitochondrial compartments. Ras regulators, effectors and scaffolds are also differentially distributed, potentially enabling preferential coupling to specific signalling pathways in each subcellular location. Here we summarize the mechanisms underlying compartment-specific Ras signalling and the outputs generated.

Ras acylation, compartmentalization and signaling nanoclusters (Review)

Ras proteins have become paradigms for isoform- and compartment-specific signaling. Recent work has shown that Ras isoforms are differentially distributed within cell surface signaling nanoclusters and on endomembranous compartments. The critical feature regulating Ras protein localization and isoform-specific functions is the C-terminal hypervariable region (HVR). In this review we discuss the differential post-translational modifications and reversible targeting functions of Ras isoform HVR motifs. We describe how compartmentalized Ras signaling has specific functional consequences and how cell surface signaling nanoclusters generate precise signaling outputs.

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.

Signaling from endosomes: location makes a difference

In all transmembrane receptor systems the kinetics of receptor trafficking upon ligand stimulation is maintained in a balance between degradative and recycling pathways in order to keep homeostasis and to strictly control receptor-mediated signaling. Endocytosis is commonly considered as an efficient mechanism of uptake and transport of membrane-associated signaling molecules leading to attenuation of ligand-induced responses. Accumulating evidence, however, shows that signaling from internalized receptors not only continues in endosomal compartments, but that there are also distinct signaling events that require endocytosis. Endocytic organelles form a dynamic network of subcellular compartments, which actively control the timing, amplitude, and specificity of signaling. In this review we provide examples in which signal transduction either requires an active endocytic machinery, or directly originates from various types of endosomes. Based on recent discoveries, we emphasize the close interdependence between signaling and endocytosis, and the physiological relevance of endocytic transport in health and disease.

The central proline rich region of POB1/REPS2 plays a regulatory role in epidermal growth factor receptor endocytosis by binding to 14-3-3 and SH3 domain-containing proteins

BACKGROUND

The human POB1/REPS2 (Partner of RalBP1) protein is highly conserved in mammals where it has been suggested to function as a molecular scaffold recruiting proteins involved in vesicular traffic and linking them to the actin cytoskeleton remodeling machinery. More recently POB1/REPS2 was found highly expressed in androgen-dependent prostate cancer cell lines, while one of its isoforms (isoform 2) is down regulated during prostate cancer progression.

RESULTS

In this report we characterize the central proline rich domain of POB1/REPS2 and we describe for the first time its functional role in receptor endocytosis. We show that the ectopic expression of this domain has a dominant negative effect on the endocytosis of activated epidermal growth factor receptor (EGFR) while leaving transferrin receptor endocytosis unaffected. By a combination of different approaches (phage display, bioinformatics predictions, peptide arrays, mutagenic analysis, in vivo co-immunoprecipitation), we have identified two closely spaced binding motifs for 14-3-3 and for the SH3 of the proteins Amphiphysin II and Grb2. Differently from wild type, proline rich domains that are altered in these motifs do not inhibit EGFR endocytosis, suggesting that these binding motifs play a functional role in this process.

CONCLUSION

Our findings are relevant to the characterization of the molecular mechanism underlying the involvement of POB1/REPS2, SH3 and 14-3-3 proteins in receptor endocytosis, suggesting that 14-3-3 could work by bridging the EGF receptor and the scaffold protein POB1/REPS2.

Molecular Mechanisms that Regulate Epidermal Growth Factor Receptor Inactivation

The Epidermal Growth Factor Receptor (EGFR) is the prototypical receptor tyrosine kinase (RTK). These cell surface receptors are integral membrane proteins that bind ligands on their extracellular domain and relay that information to within the cell. The activated EGFR regulates diverse cell fates such as growth, proliferation, differentiation, migration, and apoptosis. These signaling properties are important for the appropriate development and maintenance of an organism. However, when inappropriately controlled, due to EGFR overexpression or hyperactivation, these signaling events are characteristic of many cancers. It remains unclear whether the uncontrolled EGFR activity leads to cell transformation or is a consequence of cell transformation. Regardless of the cause, increased EGFR activity serves both as a biomarker in the diagnosis of some cancers and is a molecular target for anti-cancer therapies. The promising results with current anti-EGFR therapies suggest that the receptor is a viable molecular target for a limited number of applications. However, to become an effective therapeutic target for other cancers that have elevated levels of EGFR activity, current approaches for inhibiting EGFR signaling will need to be refined. Here we describe the molecular mechanisms that regulate EGFR inactivation and discuss their potential as therapeutic targets for inhibiting EGFR signaling.

Rap1-PDZ-GEF1 interacts with a neurotrophin receptor at late endosomes, leading to sustained activation of Rap1 and ERK and neurite outgrowth

Neurotrophins, such as NGF and BDNF, induce sustained activation of Rap1 small G protein and ERK, which are essential for neurite outgrowth. We show involvement of a GDP/GTP exchange factor (GEF) for Rap1, PDZ-GEF1, in these processes. PDZ-GEF1 is activated by GTP-Rap1 via a positive feedback mechanism. Upon NGF binding, the TrkA neurotrophin receptor is internalized from the cell surface, passes through early endosomes, and arrives in late endosomes. A tetrameric complex forms between PDZ-GEF1, synaptic scaffolding molecule and ankyrin repeat-rich membrane spanning protein which interacts directly with the TrkA receptor. At late endosomes, the complex induces sustained activation of Rap1 and ERK, resulting in neurite outgrowth. In cultured rat hippocampal neurons, PDZ-GEF1 is recruited to late endosomes in a BDNF-dependent manner involved in BDNF-induced neurite outgrowth. Thus, the interaction of PDZ-GEF1 with an internalized neurotrophin receptor transported to late endosomes induces sustained activation of both Rap1 and ERK and neurite outgrowth.

Endosomal transport of neurotrophins: roles in signaling and neurodegenerative diseases

The internalization and retrograde axonal transport of neurotrophin receptors is important for their retrograde signal transduction supporting neuronal differentiation, plasticity, and survival. To influence transcription, neurotrophin signals initiated at synapses have to be conveyed retrogradely to the cell body. Signaling endosomes containing neurotrophin receptor signaling complexes mediate retrograde neurotrophin signaling from synapses to the nucleus. Interestingly, many neurodegenerative diseases, including Alzheimer's disease, Niemann Pick disease Type C, and Charcot-Marie-Tooth neuropathies, show alterations of vesicular transport, suggesting that traffic jams within neuronal processes may cause neurodegeneration. Although most of these diseases are complex and may be modulated by diverse pathways contributing to neuronal death, altered neurotrophin transport is emerging as a strong candidate influence on neurodegeneration. In this article, we review the mechanisms of internalization and endocytic trafficking of neurotrophin receptors, and discuss the potential roles of perturbations in neurotrophin trafficking in a number of neurodegenerative diseases.

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.

Compartmentalized Ras/MAPK signaling

Signal transduction down the Ras/MAPK pathway, including that critical to T cell activation, proliferation, and differentiation, has been generally considered to occur at the plasma membrane. It is now clear that the plasma membrane does not represent the only platform for Ras/MAPK signaling. Moreover, the plasma membrane itself is no longer considered a uniform structure but rather a patchwork of microdomains that can compartmentalize signaling. Signaling on internal membranes was first recognized on endosomes. Genetically encoded fluorescent probes for signaling events such as GTP/GDP exchange on Ras have revealed signaling on a variety of intracellular membranes, including the Golgi apparatus. In fibroblasts, Ras is activated on the plasma membrane and Golgi with distinct kinetics. The pathway by which Golgi-associated Ras becomes activated involves PLCgamma and RasGRP1 and may also require retrograde trafficking of Ras from the plasma membrane to the Golgi as a consequence of depalmitoylation. Thus, the Ras/MAPK pathway represents a clear example of compartmentalized signaling.

Control of MAPK signalling: from complexity to what really matters

Oncogenesis results from changes in kinetics or in abundance of proteins in signal transduction networks. Recently, it was shown that control of signalling cannot reside in a single gene product, and might well be dispersed over many components. Which of the reactions in these complex networks are most important, and how can the existing molecular information be used to understand why particular genes are oncogenes whereas others are not? We implement a new method to help address such questions. We apply control analysis to a detailed kinetic model of the epidermal growth factor-induced mitogen-activated protein kinase network. We determine the control of each reaction with respect to three biologically relevant characteristics of the output of this network: the amplitude, duration and integrated output of the transient phosphorylation of extracellular signal-regulated kinase (ERK). We confirm that control is distributed, but far from randomly: a small proportion of reactions substantially control signalling. In particular, the activity of Raf is in control of all characteristics of the transient profile of ERK phosphorylation, which may clarify why Raf is an oncogene. Most reactions that really matter for one signalling characteristic are also important for the other characteristics. Our analysis also predicts the effects of mutations and changes in gene expression.

Differential endocytic sorting of p75NTR and TrkA in response to NGF: a role for late endosomes in TrkA trafficking

NGF binds to two receptors, p75NTR and TrkA. The endosomal trafficking of receptors is of emerging importance for the understanding of their signaling. We compared the endocytic trafficking of the two NGF receptors in PC12 cells. Both p75NTR and TrkA were internalized in response to NGF and colocalized with early endosomes. However, surprisingly, the subsequent endosomal trafficking paths of both NGF receptors diverged: whereas p75NTR recycled back to the surface, TrkA moved to late endosomes and underwent lysosomal degradation. By performing subcellular fractionations of NGF stimulated PC12 cells, tyrosine-phosphorylated TrkA was recovered in fractions corresponding to late endosomes. This implicates these organelles as novel endosomal NGF signaling platforms. Furthermore, the trafficking of NGF receptors could be manipulated by pharmacological means. Disrupting p75NTR recycling diminished TrkA activation in response to low concentrations of NGF, demonstrating a functional role for the recycling of p75NTR.

Endosome function and dysfunction in Alzheimer's disease and other neurodegenerative diseases

Endocytosis is universally important in cell function. In the brain, the roles of endosomes are relatively more complex due to the unique polar morphology of neurons and specialized needs for inter-cellular communication. New evidence shows that endosome function is altered in a surprising range of neurodegenerative disorders, including in several inherited neurologic disorders where the causative mutations occur in genes that regulate endosome function. In Alzheimer's disease (AD), endosome abnormalities are among the earliest neuropathologic features to develop and have now been closely linked to genetic risk factors for AD, including APP triplication in Trisomy 21 (Down syndrome, DS) and ApoE4 genotype in sporadic AD. Recent findings on endosome regulation and developmental and late-onset neurodegenerative disease disorders are beginning to reveal how endocytic pathway impairment may lead to neuronal dysfunction and cell death in these disorders and may also promote amyloidogenesis in AD.

Exploiting type 3 complement receptor for TNF-alpha suppression, immune evasion, and progressive pulmonary fungal infection

TNF-alpha is crucial in defense against intracellular microbes. Host immune cells use type 3 complement receptors (CR3) to regulate excess TNF-alpha production during physiological clearance of apoptotic cells. BAD1, a virulence factor of Blastomyces dermatitidis, is displayed on yeast and released during infection. BAD1 binds yeast to macrophages (Mphi) via CR3 and CD14 and suppresses TNF-alpha, which is required for resistance. We investigated whether blastomyces adhesin 1 (BAD1) exploits host receptors for immune deviation and pathogen survival. Soluble BAD1 rapidly entered Mphi, accumulated intracellularly by 10 min after introduction to cells, and trafficked to early and late endosomes. Inhibition of receptor recycling by monodansyl cadaverine blocked association of BAD1 with Mphi and reversed TNF-alpha suppression in vitro. Inhibition of BAD1 uptake with cytochalasin D and FcR-redirected delivery of soluble BAD1 as Ag-Ab complexes or of wild-type yeast opsonized with IgG similarly reversed TNF-alpha suppression. Hence, receptor-mediated entry of BAD1 is requisite in TNF-alpha suppression, and the route of entry is critical. Binding of soluble BAD1 to Mphi of CR3(-/-) and CD14(-/-) mice was reduced to 50 and 33%, respectively, of that in wild-type mice. Mphi of CR3(-/-) and CD14(-/-) mice resisted soluble BAD1 TNF-alpha suppression in vitro, but, in contrast to CR3(-/-) cells, CD14(-/-) cells were still subject to suppression mediated by surface BAD1 on wild-type yeast. CR3(-/-) mice resisted both infection and TNF-alpha suppression in vivo, in contrast to wild-type and CD14(-/-) mice. BAD1 of B. dermatitidis thus co-opts normal host cell physiology by exploiting CR3 to subdue TNF-alpha production and foster pathogen survival.

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.

TULA: an SH3- and UBA-containing protein that binds to c-Cbl and ubiquitin

Downregulation of protein tyrosine kinases is a major function of the multidomain protein c-Cbl. This effect of c-Cbl is critical for both negative regulation of normal physiological stimuli and suppression of cellular transformation. In spite of the apparent importance of these effects of c-Cbl, their own regulation is poorly understood. To search for possible novel regulators of c-Cbl, we purified a number of c-Cbl-associated proteins by affinity chromatography and identified them by mass spectrometry. Among them, we identified the UBA- and SH3-containing protein T-cell Ubiquitin LigAnd (TULA), which can also bind to ubiquitin. Functional studies in a model system based on co-expression of TULA, c-Cbl, and EGF receptor in 293T cells demonstrate that TULA is capable of inhibiting c-Cbl-mediated downregulation of EGF receptor. Furthermore, modulation of TULA concentration in Jurkat T-lymphoblastoid cells demonstrates that TULA upregulates the activity of both Zap kinase and NF-AT transcription factor. Therefore, our study indicates that TULA counters the inhibitory effect of c-Cbl on protein tyrosine kinases and, thus, may be involved in the regulation of biological effects of c-Cbl. Finally, our results suggest that TULA-mediated inhibition of the effects of c-Cbl on protein tyrosine kinases is caused by TULA-induced ubiquitylation and degradation of c-Cbl.

Novel biphasic traffic of endocytosed EGF to recycling and degradative compartments in lacrimal gland acinar cells

The purpose of this study was to delineate the traffic patterns of EGF and EGF receptors (EGFR) in primary cultured acinar epithelial cells from rabbit lacrimal glands. Uptake of [(125)I]-EGF exhibited saturable and non-saturable, temperature-dependent components, suggesting both receptor-mediated and fluid phase endocytosis. Accumulation of [(125)I] was time-dependent over a 120-min period, but the content of intact [(125)I]-EGF decreased after reaching a maximum at 20 min. Analytical fractionation by sorbitol density gradient centrifugation and phase partitioning indicated that within 20 min at 37 degrees C [(125)I] reached an early endosome, basal-lateral recycling endosome, pre-lysosome, and lysosome. Small components of the label also appeared to reach the Golgi complex and trans-Golgi network. Intact [(125)I]-EGF initially accumulated in the recycling endosome; the content in the recycling endosome subsequently decreased, and by 120 min increased amounts of [(125)I]-labeled degradation products appeared in the pre-lysosomes and lysosomes. Confocal microscopy imaging of FITC-EGF and LysoTrackerRed revealed FITC enriched in a dispersed system of non-acidic compartments at 20 min and in acidic compartments at 120 min. Both confocal immunofluorescence microscopy and analytical fractionation indicated that the intracellular EGFR pool was much larger than the plasma membrane-expressed pool at all times. Cells loaded with [(125)I]-EGF released a mixture of intact EGF and [(125)I]-labeled degradation products. The observations indicate that in lacrimal acinar cells, EGFR and EGF-EGFR complexes continually traffic between the plasma membranes and a system of endomembrane compartments; EGF-stimulation generates time-dependent signals that initially decrease, then increase, EGF-EGFR traffic to degradative compartments.

The SRC homology 2 domain of Rin1 mediates its binding to the epidermal growth factor receptor and regulates receptor endocytosis

Activated epidermal growth factor receptors (EGFRs) recruit intracellular proteins that mediate receptor signaling and endocytic trafficking. Rin1, a multifunctional protein, has been shown to regulate EGFR internalization (1). Here we show that EGF stimulation induces a specific, rapid, and transient membrane recruitment of Rin1 and that recruitment is dependent on the Src homology 2 (SH2) domain of Rin1. Immunoprecipitation of EGFR is accompanied by co-immunoprecipitation of Rin1 in a time- and ligand-dependent manner. Association of Rin1 and specifically the SH2 domain of Rin1 with the EGFR was dependent on tyrosine phosphorylation of the intracellular domain of the EGFR. The recruitment of Rin1, observed by light microscopy, indicated that although initially cytosolic, Rin1 was recruited to both plasma membrane and endosomes following EGF addition. Moreover, the expression of the SH2 domain of Rin1 substantially impaired the internalization of EGF without affecting internalization of transferrin. Finally, we found that Rin1 co-immunoprecipitated with a number of tyrosine kinase receptors but not with cargo endocytic receptors. These results indicate that Rin1 provides a link via its SH2 domain between activated tyrosine kinase receptors and the endocytic pathway through the recruitment and activation of Rab5a.

Effects of EGF on expression of phosphorylated p44/42 MAPK in rat small intestine after ischemia-reperfusion injury

AIM

To investigate the effects of EGF on the characteristics of phosphrylated p44/42 MAPK expression and its biological significance in EGF-induced gut repair after ischemia-reperfusion (I/R) injury.

METHODS

A total of 80 Wistar rats were randomly divided into four groups, namely EGF treated group (E), normal saline control (R), ischemia group (I) and sham operated control (C). In group E and R, the rats were treated with intravenous EGF 100 μg/kg/rat or normal saline respectively after 45 minutes of superior mesenteric artery occlusion. Blood samples were collected at 2, 6, 12 and 24 hours after reperfusion and plasma D-lactate were determined. Tissue samples from intestine were also taken for histological analysis and immunohistochemical analysis of phospho-p44/42 MAPK.

RESULTS

The changes of histological structure and D-lactate indicated that the intestinal barrier was damaged after intestinal I/R injury, while EGF treatment significantly improved the outcome. In group C and I positive signals of phospho-p44/42 MAPK were mainly located in the cytoplasm of the intestinal villi and crypts, while in group I positive cells increased significantly (P<0.05). In group R, positive signals were found in almost all the cells and the percentage of positive nuclei increased with the time of reperfusion, reaching its peak after 12h of reperfusion. In group E, the percentages were higher than those in group R and the peak of nuclear expression was earlier.

CONCLUSION

EGF administration improves the outcome of I/R induced intestinal damage. After I/R the expression and nuclear translocation of phspho-p44/42 MAPK increases with the time of reperfusion, suggesting its role in intestinal reconstitution. EGF treatment induces its early expression and translocation into the nucleus, suggesting the significance of p44/42 MAPK signaling pathway in EGF-induced gut repair.

Cholesterol depletion from the plasma membrane triggers ligand-independent activation of the epidermal growth factor receptor

We recently demonstrated that depletion of plasma membrane cholesterol with methyl-beta-cyclodextrin (MbetaCD) caused activation of MAPK (Chen, X., and Resh, M. D. (2001) J. Biol. Chem. 276, 34617-34623). MAPK activation was phosphatidylinositol 3-kinase (PI3K)-dependent and involved increased tyrosine phosphorylation of the p85 subunit of PI3K. We next determined whether MbetaCD treatment induced tyrosine phosphorylation of other cellular proteins. Here we report that cholesterol depletion of serum-starved COS-1 cells with MbetaCD or filipin caused an increase in Tyr(P) levels of a 180-kDa protein that was identified as the epidermal growth factor receptor (EGFR). Cross-linking experiments showed that MbetaCD induced dimerization of EGFR, indicative of receptor activation. Reagents that block release of membrane-bound EGFR ligands did not affect MbetaCD-induced tyrosine phosphorylation of EGFR, indicating that MbetaCD activation of EGFR is ligand-independent. Moreover, MbetaCD treatment resulted in increased tyrosine phosphorylation of EGFR downstream targets and Ras activation. Incubation of cells with the specific EGFR inhibitor AG4178 blocked MbetaCD-induced phosphorylation of EGFR, SHC, phospholipase C-gamma, and Gab-1 as well as MAPK activation. We conclude that cholesterol depletion from the plasma membrane by MbetaCD causes ligand-independent activation of EGFR, resulting in MAPK activation by PI3K and Ras-dependent mechanisms. Moreover, these studies reveal a novel mode of action of MbetaCD, in addition to its ability to disrupt membrane rafts.

CIN85 participates in Cbl-b-mediated down-regulation of receptor tyrosine kinases

The Cbl family of ubiquitin ligases in mammals contains three members, Cbl, Cbl-b, and Cbl-3, that are involved in down-regulation of receptor tyrosine kinases (RTKs) by mediating receptor ubiquitination and degradation. More recently, a novel pathway has been identified whereby Cbl promotes internalization of EGF receptor via a CIN85/endophilin pathway that is functionally separable from the ubiquitin ligase activity of Cbl (1). Here we show that Cbl-b, but not Cbl-3, utilize the same mechanism to down-regulate multiple RTKs. CIN85 was shown to bind to the minimal binding domain identified in the carboxyl terminus of Cbl-b. Ligand-induced phosphorylation of Cbl-b further increased their interactions and led to a rapid and sustained recruitment of CIN85 in the complex with EGF or PDGF receptors. Inhibition of binding between CIN85 and Cbl-b was sufficient to impair Cbl-b-mediated internalization of EGF receptors, while being dispensable for Cbl-b-directed polyubiquitination of EGF receptors. Moreover, CIN85 and Cbl/Cbl-b were constitutively associated with activated PDGF, EGF, or c-Kit receptors in several tumor cell lines. Our data reveal a common pathway utilized by Cbl and Cbl-b that may have an important and redundant function in negative regulation of ligand-activated as well as oncogenically activated RTKs in vivo.

Signal transduction and endocytosis: close encounters of many kinds

Binding of hormones, growth factors and other cell modulators to cell-surface receptors triggers a complex array of signal-transduction events. The activation of many receptors also accelerates their endocytosis. Endocytic transport is important in regulating signal transduction and in mediating the formation of specialized signalling complexes. Conversely, signal-transduction events modulate specific components of the endocytic machinery. Recent studies of protein tyrosine kinases and G-protein-coupled receptors have shed new light on the mechanisms and functional consequences of this bidirectional interplay between signalling and membrane-transport networks.

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.