Suppression of EGFR endocytosis by dynamin depletion reveals that EGFR signaling occurs primarily at the plasma membrane

The AP-2 complex interacts with γ-TuRC and regulates the proliferative capacity of neural progenitors

Centrosomes are organelles that nucleate microtubules via the activity of gamma-tubulin ring complexes (γ-TuRC). In the developing brain, centrosome integrity is central to the progression of the neural progenitor cell cycle, and its loss leads to microcephaly. We show that NPCs maintain centrosome integrity via the endocytic adaptor protein complex-2 (AP-2). NPCs lacking AP-2 exhibit defects in centrosome formation and mitotic progression, accompanied by DNA damage and accumulation of p53. This function of AP-2 in regulating the proliferative capacity of NPCs is independent of its role in clathrin-mediated endocytosis and is coupled to its association with the GCP2, GCP3, and GCP4 components of γ-TuRC. We find that AP-2 maintains γ-TuRC organization and regulates centrosome function at the level of MT nucleation. Taken together, our data reveal a novel, noncanonical function of AP-2 in regulating the proliferative capacity of NPCs and open new avenues for the identification of novel therapeutic strategies for the treatment of neurodevelopmental and neurodegenerative disorders with AP-2 complex dysfunction.

EGFR trafficking: effect of dimerization, dynamics, and mutation

Spontaneous dimerization of EGF receptors (EGFR) and dysregulation of EGFR signaling has been associated with the development of different cancers. Under normal physiological conditions and to maintain homeostatic cell growth, once EGFR signaling occurs, it needs to be attenuated. Activated EGFRs are rapidly internalized, sorted through early endosomes, and ultimately degraded in lysosomes by a process generally known as receptor down-regulation. Through alterations to EGFR trafficking, tumors develop resistance to current treatment strategies, thus highlighting the necessity for combination treatment strategies that target EGFR trafficking. This review covers EGFR structure, trafficking, and altered surface expression of EGFR receptors in cancer, with a focus on how therapy targeting EGFR trafficking may aid tyrosine kinase inhibitor treatment of cancer.

An integrated mechanistic and data-driven computational model predicts cell responses to high- and low-affinity EGFR ligands

The biophysical properties of ligand binding heavily influence the ability of receptors to specify cell fates. Understanding the rules by which ligand binding kinetics impact cell phenotype is challenging, however, because of the coupled information transfers that occur from receptors to downstream signaling effectors and from effectors to phenotypes. Here, we address that issue by developing an integrated mechanistic and data-driven computational modeling platform to predict cell responses to different ligands for the epidermal growth factor receptor (EGFR). Experimental data for model training and validation were generated using MCF7 human breast cancer cells treated with the high- and low-affinity ligands epidermal growth factor (EGF) and epiregulin (EREG), respectively. The integrated model captures the unintuitive, concentration-dependent abilities of EGF and EREG to drive signals and phenotypes differently, even at similar levels of receptor occupancy. For example, the model correctly predicts the dominance of EREG over EGF in driving a cell differentiation phenotype through AKT signaling at intermediate and saturating ligand concentrations and the ability of EGF and EREG to drive a broadly concentration-sensitive migration phenotype through cooperative ERK and AKT signaling. Parameter sensitivity analysis identifies EGFR endocytosis, which is differentially regulated by EGF and EREG, as one of the most important determinants of the alternative phenotypes driven by different ligands. The integrated model provides a new platform to predict how phenotypes are controlled by the earliest biophysical rate processes in signal transduction and may eventually be leveraged to understand receptor signaling system performance depends on cell context.

One-sentence summary

Integrated kinetic and data-driven EGFR signaling model identifies the specific signaling mechanisms that dictate cell responses to EGFR activation by different ligands.

Control of phosphatidylinositol-3-kinase signaling by nanoscale membrane compartmentalization

Phosphatidylinositol-3-kinases (PI3Ks) are lipid kinases that produce 3-phosphorylated derivatives of phosphatidylinositol upon activation by various cues. These 3-phosphorylated lipids bind to various protein effectors to control many cellular functions. Lipid phosphatases such as phosphatase and tensin homolog (PTEN) terminate PI3K-derived signals and are critical to ensure appropriate signaling outcomes. Many lines of evidence indicate that PI3Ks and PTEN, as well as some specific lipid effectors are highly compartmentalized, either in plasma membrane nanodomains or in endosomal compartments. We examine the evidence for specific recruitment of PI3Ks, PTEN, and other related enzymes to membrane nanodomains and endocytic compartments. We then examine the hypothesis that scaffolding of the sources (PI3Ks), terminators (PTEN), and effectors of these lipid signals with a common plasma membrane nanodomain may achieve highly localized lipid signaling and ensure selective activation of specific effectors. This highlights the importance of spatial regulation of PI3K signaling in various physiological and disease contexts.

Regulation of EGF-stimulated activation of the PI-3K/AKT pathway by exocyst-mediated exocytosis

The phosphoinositide-3 kinase (PI-3K)/AKT cell survival pathway is an important pathway activated by EGFR signaling. Here we show, that in addition to previously described critical components of this pathway, i.e., the docking protein Gab1, the PI-3K/AKT pathway in epithelial cells is regulated by the exocyst complex, which is a vesicle tether that is essential for exocytosis. Using live-cell imaging, we demonstrate that PI(3,4,5)P₃ levels fluctuate at the membrane on a minutes time scale and that these fluctuations are associated with local PI(3,4,5)P₃ increases at sites where recycling vesicles undergo exocytic fusion. Supporting a role for exocytosis in PI(3,4,5)P₃ generation, acute promotion of exocytosis by optogenetically driving exocyst-mediated vesicle tethering up-regulates PI(3,4,5)P₃ production and AKT activation. Conversely, acute inhibition of exocytosis using Endosidin2, a small-molecule inhibitor of the exocyst subunit Exo70 (also designated EXOC7), or inhibition of exocyst function by siRNA-mediated knockdown of the exocyst subunit Sec15 (EXOC6), impairs PI(3,4,5)P₃ production and AKT activation induced by EGF stimulation of epithelial cells. Moreover, prolonged inhibition of EGF signaling by EGFR tyrosine kinase inhibitors results in spontaneous reactivation of AKT without a concomitant relief of EGFR inhibition. However, this reactivation can be negated by acutely inhibiting the exocyst. These experiments demonstrate that exocyst-mediated exocytosis-by regulating PI(3,4,5)P₃ levels at the plasma membrane-subserves activation of the PI-3K/AKT pathway by EGFR in epithelial cells.

1-deoxysphingolipid synthesis compromises anchorage-independent growth and plasma membrane endocytosis in cancer cells

Serine palmitoyltransferase (SPT) predominantly incorporates serine and fatty acyl-CoAs into diverse sphingolipids that serve as structural components of membranes and signaling molecules within or amongst cells. However, SPT also uses alanine as a substrate in the contexts of low serine availability, alanine accumulation, or disease-causing mutations in hereditary sensory neuropathy type I (HSAN1), resulting in the synthesis and accumulation of 1-deoxysphingolipids. These species promote cytotoxicity in neurons and impact diverse cellular phenotypes, including suppression of anchorage-independent cancer cell growth. While altered serine and alanine levels can promote 1-deoxysphingolipid synthesis, they impact numerous other metabolic pathways important for cancer cells. Here we combined isotope tracing, quantitative metabolomics, and functional studies to better understand the mechanistic drivers of 1-deoxysphingolipid toxicity in cancer cells. We determined that both alanine treatment and SPTLC1^C133W expression induce 1-deoxy(dihydro)ceramide synthesis and accumulation but fail to broadly impact intermediary metabolism, abundances of other lipids, or growth of adherent cells. However, we found spheroid culture and soft agar colony formation were compromised when endogenous 1-deoxysphingolipid synthesis was induced via SPTLC1^C133W expression. Consistent with these impacts on anchorage-independent cell growth, we observed that 1-deoxysphingolipid synthesis reduced plasma membrane endocytosis. These results highlight a potential role for SPT promiscuity in linking altered amino acid metabolism to plasma membrane endocytosis.

Can EGFR be a therapeutic target in breast cancer?

Epidermal growth factor receptor (EGFR) is highly expressed in certain cancer types and is involved in regulating the biological characteristics of cancer progression, including proliferation, metastasis, and drug resistance. Various medicines targeting EGFR have been developed and approved for several cancer types, such as lung and colon cancer. To date, however, EGFR inhibitors have not achieved satisfactory clinical results in breast cancer, which continues to be the most serious malignant tumor type in females. Therefore, clarifying the underlying mechanisms related to the ineffectiveness of EGFR inhibitors in breast cancer and developing new EGFR-targeted strategies (e.g., combination therapy) remain critical challenges. Various studies have demonstrated aberrant expression and maintenance of EGFR levels in breast cancer. In this review, we summarize the regulatory mechanisms underlying EGFR protein expression in breast cancer cells, including EGFR mutations, amplification, endocytic dysfunction, recycling acceleration, and degradation disorders. We also discuss potential therapeutic strategies that act directly or indirectly on EGFR, including reducing EGFR protein expression, treating the target protein to mediate precise clearance, and inhibiting non-EGFR signaling pathways. This review should provide new therapeutic perspectives for breast cancer patients with high EGFR expression.

Dynamin-Independent Mechanisms of Endocytosis and Receptor Trafficking

Endocytosis is a fundamental mechanism by which cells perform housekeeping functions. It occurs via a variety of mechanisms and involves many regulatory proteins. The GTPase dynamin acts as a “molecular scissor” to form endocytic vesicles and is a critical regulator among the proteins involved in endocytosis. Some GTPases (e.g., Cdc42, arf6, RhoA), membrane proteins (e.g., flotillins, tetraspanins), and secondary messengers (e.g., calcium) mediate dynamin-independent endocytosis. These pathways may be convergent, as multiple pathways exist in a single cell. However, what determines the specific path of endocytosis is complex and challenging to comprehend. This review summarizes the mechanisms of dynamin-independent endocytosis, the involvement of microRNAs, and factors that contribute to the cellular decision about the specific route of endocytosis.

Critical roles for EGFR and EGFR-HER2 clusters in EGF binding of SW620 human carcinoma cells

Epidermal growth factor (EGF) signalling regulates normal epithelial and other cell growth, with EGF receptor (EGFR) overexpression reported in many cancers. However, the role of EGFR clusters in cancer and their dependence on EGF binding is unclear. We present novel single-molecule total internal reflection fluorescence microscopy of (i) EGF and EGFR in living cancer cells, (ii) the action of anti-cancer drugs that separately target EGFR and human EGFR2 (HER2) on these cells and (iii) EGFR–HER2 interactions. We selected human epithelial SW620 carcinoma cells for their low level of native EGFR expression, for stable transfection with fluorescent protein labelled EGFR, and imaged these using single-molecule localization microscopy to quantify receptor architectures and dynamics upon EGF binding. Prior to EGF binding, we observe pre-formed EGFR clusters. Unexpectedly, clusters likely contain both EGFR and HER2, consistent with co-diffusion of EGFR and HER2 observed in a different model CHO-K1 cell line, whose stoichiometry increases following EGF binding. We observe a mean EGFR : EGF stoichiometry of approximately 4 : 1 for plasma membrane-colocalized EGFR–EGF that we can explain using novel time-dependent kinetics modelling, indicating preferential ligand binding to monomers. Our results may inform future cancer drug developments.

Endocytosis at the Crossroad of Polarity and Signaling Regulation: Learning from Drosophila melanogaster and Beyond

Cellular trafficking through the endosomal–lysosomal system is essential for the transport of cargo proteins, receptors and lipids from the plasma membrane inside the cells and across membranous organelles. By acting as sorting stations, vesicle compartments direct the fate of their content for degradation, recycling to the membrane or transport to the trans-Golgi network. To effectively communicate with their neighbors, cells need to regulate their compartmentation and guide their signaling machineries to cortical membranes underlying these contact sites. Endosomal trafficking is indispensable for the polarized distribution of fate determinants, adaptors and junctional proteins. Conversely, endocytic machineries cooperate with polarity and scaffolding components to internalize receptors and target them to discrete membrane domains. Depending on the cell and tissue context, receptor endocytosis can terminate signaling responses but can also activate them within endosomes that act as signaling platforms. Therefore, cell homeostasis and responses to environmental cues rely on the dynamic cooperation of endosomal–lysosomal machineries with polarity and signaling cues. This review aims to address advances and emerging concepts on the cooperative regulation of endocytosis, polarity and signaling, primarily in Drosophila melanogaster and discuss some of the open questions across the different cell and tissue types that have not yet been fully explored.

Fyn and TOM1L1 are recruited to clathrin-coated pits and regulate Akt signaling

The epidermal growth factor (EGF) receptor (EGFR) controls many aspects of cell physiology. EGF binding to EGFR elicits the membrane recruitment and activation of phosphatidylinositol-3-kinase, leading to Akt phosphorylation and activation. Concomitantly, EGFR is recruited to clathrin-coated pits (CCPs), eventually leading to receptor endocytosis. Previous work uncovered that clathrin, but not receptor endocytosis, is required for EGF-stimulated Akt activation, and that some EGFR signals are enriched in CCPs. Here, we examine how CCPs control EGFR signaling. The signaling adaptor TOM1L1 and the Src-family kinase Fyn are enriched within a subset of CCPs with unique lifetimes and protein composition. Perturbation of TOM1L1 or Fyn impairs EGF-stimulated phosphorylation of Akt2 but not Akt1. EGF stimulation also triggered the TOM1L1- and Fyn-dependent recruitment of the phosphoinositide 5-phosphatase SHIP2 to CCPs. Thus, the recruitment of TOM1L1 and Fyn to a subset of CCPs underlies a role for these structures in the support of EGFR signaling leading to Akt activation.

Spatial-proteomics reveals phospho-signaling dynamics at subcellular resolution

Dynamic change in subcellular localization of signaling proteins is a general concept that eukaryotic cells evolved for eliciting a coordinated response to stimuli. Mass spectrometry-based proteomics in combination with subcellular fractionation can provide comprehensive maps of spatio-temporal regulation of protein networks in cells, but involves laborious workflows that does not cover the phospho-proteome level. Here we present a high-throughput workflow based on sequential cell fractionation to profile the global proteome and phospho-proteome dynamics across six distinct subcellular fractions. We benchmark the workflow by studying spatio-temporal EGFR phospho-signaling dynamics in vitro in HeLa cells and in vivo in mouse tissues. Finally, we investigate the spatio-temporal stress signaling, revealing cellular relocation of ribosomal proteins in response to hypertonicity and muscle contraction. Proteomics data generated in this study can be explored through https://SpatialProteoDynamics.github.io .

Role of Endocytosis Proteins in Gefitinib-Mediated EGFR Internalisation in Glioma Cells

EGFR (epidermal growth factor receptor), a member of the ErbB tyrosine kinase receptor family, is a clinical therapeutic target in numerous solid tumours. EGFR overexpression in glioblastoma (GBM) drives cell invasion and tumour progression. However, clinical trials were disappointing, and a molecular basis to explain these poor results is still missing. EGFR endocytosis and membrane trafficking, which tightly regulate EGFR oncosignaling, are often dysregulated in glioma. In a previous work, we showed that EGFR tyrosine kinase inhibitors, such as gefitinib, lead to enhanced EGFR endocytosis into fused early endosomes. Here, using pharmacological inhibitors, siRNA-mediated silencing, or expression of mutant proteins, we showed that dynamin 2 (DNM2), the small GTPase Rab5 and the endocytosis receptor LDL receptor-related protein 1 (LRP-1), contribute significantly to gefitinib-mediated EGFR endocytosis in glioma cells. Importantly, we showed that inhibition of DNM2 or LRP-1 also decreased glioma cell responsiveness to gefitinib during cell evasion from tumour spheroids. By highlighting the contribution of endocytosis proteins in the activity of gefitinib on glioma cells, this study suggests that endocytosis and membrane trafficking might be an attractive therapeutic target to improve GBM treatment.

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.

Migration cues interpretation by clathrin-coated structures

Cell migration is oriented by cues from the environment. Such cues are read and interpreted by the cell and translated into a reorganization of the migration machinery to steer migration. Receptors at the cell surface are central to detect these cues. These receptors can be internalized and this plays an important role in the decision-making process leading to choosing a migration direction. Independently of endocytosis, recent findings suggest that regulation of these receptors and translation of the information they carry into a phenotype is facilitated by their clustering at discrete locations of the plasma membrane. Clathrin-coated structures are archetypal clustering assemblies and thus provide the cell with a finely tunable mechanism for controlling receptor availability. In addition, clathrin-coated structures can be regulated by many factors playing a role in cell migration and thus take part in feedback loop mechanisms that are instrumental in defining a migration direction.

Two-Pore Channels Regulate Expression of Various Receptors and Their Pathway-Related Proteins in Multiple Ways

Two-pore channels (TPCs) constitute a small family of ion channels within membranes of intracellular acidic compartments, such as endosomes and lysosomes. They were shown to provide transient and locally restricted Ca²⁺-currents, likely responsible for fusion and/or fission events of endolysosomal membranes and thereby for intracellular vesicle trafficking. Genetic deletion of TPCs not only affects endocytosis, recycling, and degradation of various surface receptors but also uptake and impact of bacterial protein toxins and entry and intracellular processing of some types of viruses. This review points to important examples of these trafficking defects on one part but mainly focuses on the resulting impact of the TPC inactivation on receptor expression and receptor signaling. Thus, a detailed RNA sequencing analysis using TPC1-deficient fibroblasts uncovered a multitude of changes in the expression levels of surface receptors and their pathway-related signaling proteins. We refer to several classes of receptors such as EGF, TGF, and insulin as well as proteins involved in endocytosis.

RAL GTPases mediate EGFR-driven intestinal stem cell proliferation and tumourigenesis

RAS-like (RAL) GTPases function in Wnt signalling-dependent intestinal stem cell proliferation and regeneration. Whether RAL proteins work as canonical RAS effectors in the intestine and the mechanisms of how they contribute to tumourigenesis remain unclear. Here, we show that RAL GTPases are necessary and sufficient to activate EGFR/MAPK signalling in the intestine, via induction of EGFR internalisation. Knocking down Drosophila RalA from intestinal stem and progenitor cells leads to increased levels of plasma membrane-associated EGFR and decreased MAPK pathway activation. Importantly, in addition to influencing stem cell proliferation during damage-induced intestinal regeneration, this role of RAL GTPases impacts on EGFR-dependent tumourigenic growth in the intestine and in human mammary epithelium. However, the effect of oncogenic RAS in the intestine is independent from RAL function. Altogether, our results reveal previously unrecognised cellular and molecular contexts where RAL GTPases become essential mediators of adult tissue homeostasis and malignant transformation.

ZNRF1 Mediates Epidermal Growth Factor Receptor Ubiquitination to Control Receptor Lysosomal Trafficking and Degradation

Activation of the epidermal growth factor receptor (EGFR) is crucial for development, tissue homeostasis, and immunity. Dysregulation of EGFR signaling is associated with numerous diseases. EGFR ubiquitination and endosomal trafficking are key events that regulate the termination of EGFR signaling, but their underlying mechanisms remain obscure. Here, we reveal that ZNRF1, an E3 ubiquitin ligase, controls ligand-induced EGFR signaling via mediating receptor ubiquitination. Deletion of ZNRF1 inhibits endosome-to-lysosome sorting of EGFR, resulting in delayed receptor degradation and prolonged downstream signaling. We further demonstrate that ZNRF1 and Casitas B-lineage lymphoma (CBL), another E3 ubiquitin ligase responsible for EGFR ubiquitination, mediate ubiquitination at distinct lysine residues on EGFR. Furthermore, loss of ZNRF1 results in increased susceptibility to herpes simplex virus 1 (HSV-1) infection due to enhanced EGFR-dependent viral entry. Our findings identify ZNRF1 as a novel regulator of EGFR signaling, which together with CBL controls ligand-induced EGFR ubiquitination and lysosomal trafficking.

Gefitinib induces EGFR and α5β1 integrin co-endocytosis in glioblastoma cells

Overexpression of EGFR drives glioblastomas (GBM) cell invasion but these tumours remain resistant to EGFR-targeted therapies such as tyrosine kinase inhibitors (TKIs). Endocytosis, an important modulator of EGFR function, is often dysregulated in glioma cells and is associated with therapy resistance. However, the impact of TKIs on EGFR endocytosis has never been examined in GBM cells. In the present study, we showed that gefitinib and other tyrosine kinase inhibitors induced EGFR accumulation in early-endosomes as a result of an increased endocytosis. Moreover, TKIs trigger early-endosome re-localization of another membrane receptor, the fibronectin receptor alpha5beta1 integrin, a promising therapeutic target in GBM that regulates physiological EGFR endocytosis and recycling in cancer cells. Super-resolution dSTORM imaging showed a close-proximity between beta1 integrin and EGFR in intracellular membrane compartments of gefitinib-treated cells, suggesting their potential interaction. Interestingly, integrin depletion delayed gefitinib-mediated EGFR endocytosis. Co-endocytosis of EGFR and alpha5beta1 integrin may alter glioma cell response to gefitinib. Using an in vitro model of glioma cell dissemination from spheroid, we showed that alpha5 integrin-depleted cells were more sensitive to TKIs than alpha5-expressing cells. This work provides evidence for the first time that EGFR TKIs can trigger massive EGFR and alpha5beta1 integrin co-endocytosis, which may modulate glioma cell invasiveness under therapeutic treatment.

Two-pore channels affect EGF receptor signaling by receptor trafficking and expression

Two-pore channels (TPCs) are key components for regulating Ca²⁺ current from endosomes and lysosomes to the cytosol. This locally restricted Ca²⁺ current forms the basis for fusion and fission events between endolysosomal membranes and thereby for intracellular trafficking processes. Here, we study the function of TPC1 and TPC2 for uptake, recycling, and degradation of epidermal growth factor receptor (EGFR) using a set of TPC knockout cells. RNA sequencing analysis revealed multiple changes in the expression levels of EGFR pathway-related genes in TPC1-deficient cells. We propose that a prolonged presence of activated EGFRs in endolysosomal signaling platforms, caused by genetic inactivation of TPCs, does not only affect EGFR signaling pathways but also increases de novo synthesis of EGFR. Increased basal phospho-c-Jun levels contribute to the high EGFR expression in TPC-deficient cells. Our data point to a role of TPCs not only as important regulators for the EGFR transportation network but also for EGFR-signaling and expression.

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Two-pore channels differently affect intracellular trafficking of EGF receptors

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Deletion of TPCs prolongs EGFR signaling in endolysosomal platforms

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TPCs affect expression of EGFR downstream signaling components

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JNK signaling contributes to increased EGFR expression in TPC-deficient cells

Dysfunctional EGFR and oxidative stress-induced PKD1 signaling drive formation of DCLK1+ pancreatic stem cells

Doublecortin-like kinase 1 (DCLK1)-positive pancreatic cancer stem cells develop at a precancerous stage and may contribute to the lack of efficacy of pancreatic cancer therapy. Although PanIN cells express oncogenic KRas and have an increased activity of epidermal growth factor receptor (EGFR), we demonstrate that, in DCLK1⁺ PanIN cells, EGFR signaling is not propagated to the nucleus. Mimicking blockage of EGFR with erlotinib in PanIN organoid culture or in p48^cre;Kras^G12D mice led to a significant increase in DCLK1⁺ PanIN cells. As a mechanism of how EGFR inhibition leads to formation of DCLK1⁺ cells, we identify an increase in hydrogen peroxide contributing to activation of Protein Kinase D1 (PKD1). Active PKD1 then drives stemness and abundance of DCLK1⁺ cells in lesions. Our data suggest a signaling mechanism that leads to the development of DCLK1⁺ pancreatic cancer stem cells, which can be exploited to target this population in potential therapeutic approaches.

NDRG1 enhances the sensitivity of cetuximab by modulating EGFR trafficking in colorectal cancer

N-myc downstream-regulated gene 1 (NDRG1) is a key regulator that interacts with many classic tumor signaling pathways, including some molecules downstream of the epidermal growth factor receptor (EGFR). However, whether NDRG1 is involved in the mechanism of resistance to cetuximab (CTX), the first monoclonal antibody targeting the EGFR has not been reported. Here, we found that NDRG1 enhanced the sensitivity of CTX in colorectal cancer (CRC) cell lines. Afterwards, we determined the underlying mechanism of this phenomenon. We demonstrated that NDRG1 inhibited the expression of EGFR; blocked EGFR phosphorylation and reduced the EGFR distribution in the cell membrane, cytoplasm and nucleus. And then, NDRG1 suppressed the EGFR downstream signaling: RAS/RAF/ERK and PI3k/AKT/mTOR pathways. Moreover, we discovered that NDRG1 attenuated the endocytosis and degradation of EGFR induced by caveolin-1 (Cav1). Additionally, our findings were further observed in an animal model and human tissues. Our results represent a potentially significant discovery that explains the mechanisms of NDRG1 in CTX resistance. NDRG1 could be a promising biomarker to predict optimum responses to CTX, and a key target to enhance CTX activity in the treatment of metastatic CRC (mCRC).

Specialised endocytic proteins regulate diverse internalisation mechanisms and signalling outputs in physiology and cancer

Although endocytosis was first described as the process mediating macromolecule or nutrient uptake through the plasma membrane, it is now recognised as a critical component of the cellular infrastructure involved in numerous processes, ranging from receptor signalling, proliferation and migration to polarity and stem cell regulation. To realise these varying roles, endocytosis needs to be finely regulated. Accordingly, multiple endocytic mechanisms exist that require specialised molecular machineries and an array of endocytic adaptor proteins with cell-specific functions. This review provides some examples of specialised functions of endocytic adaptors and other components of the endocytic machinery in different cell physiological processes, and how the alteration of these functions is linked to cancer. In particular, we focus on: (i) cargo selection and endocytic mechanisms linked to different adaptors; (ii) specialised functions in clathrin-mediated versus non-clathrin endocytosis; (iii) differential regulation of endocytic mechanisms by post-translational modification of endocytic proteins; (iv) cell context-dependent expression and function of endocytic proteins. As cases in point, we describe two endocytic protein families, dynamins and epsins. Finally, we discuss how dysregulation of the physiological role of these specialised endocytic proteins is exploited by cancer cells to increase cell proliferation, migration and invasion, leading to anti-apoptotic or pro-metastatic behaviours.

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.

Conserved roles for receptor tyrosine kinase extracellular regions in regulating receptor and pathway activity

Receptor Tyrosine Kinases (RTKs) comprise a diverse group of cell-surface receptors that mediate key signaling events during animal development and are frequently activated in cancer. We show here that deletion of the extracellular regions of 10 RTKs representing 7 RTK classes or their substitution with the dimeric immunoglobulin Fc region results in constitutive receptor phosphorylation but fails to result in phosphorylation of downstream signaling effectors Erk or Akt. Conversely, substitution of RTK extracellular regions with the extracellular region of the Epidermal Growth Factor Receptor (EGFR) results in increases in effector phosphorylation in response to EGF. These results indicate that the activation signal generated by the EGFR extracellular region is capable of activating at least seven different RTK classes. Failure of phosphorylated Fc-RTK chimeras or RTKs with deleted extracellular regions to stimulate phosphorylation of downstream effectors indicates that either dimerization and receptor phosphorylation per se are insufficient to activate signaling or constitutive dimerization leads to pathway inhibition.

Clathrin- and dynamin-dependent endocytosis limits canonical NF-κB signaling triggered by lymphotoxin β receptor

Background

Lymphotoxin β receptor (LTβR) is a member of tumor necrosis factor receptor (TNFR) superfamily which regulates the immune response. At the cellular level, upon ligand binding, the receptor activates the pro-inflammatory NF-κB and AP-1 pathways. Yet, the intracellular distribution of LTβR, the routes of its endocytosis and their connection to the signaling activation are not characterized. Here, we investigated the contribution of LTβR internalization to its signaling potential.

Methods

Intracellular localization of LTβR in unstimulated and stimulated cells was analyzed by confocal microscopy. Endocytosis impairment was achieved through siRNA- or CRISPR/Cas9-mediated depletion, or chemical inhibition of proteins regulating endocytic routes. The activation of LTβR-induced signaling was examined. The levels of effector proteins of the canonical and non-canonical branches of the NF-κB pathway, and the phosphorylation of JNK, Akt, ERK1/2, STAT1 and STAT3 involved in diverse signaling cascades, were measured by Western blotting. A transcriptional response to LTβR stimulation was assessed by qRT-PCR analysis.

Results

We demonstrated that LTβR was predominantly present on endocytic vesicles and the Golgi apparatus. The ligand-bound pool of the receptor localized to endosomes and was trafficked towards lysosomes for degradation. Depletion of regulators of different endocytic routes (clathrin-mediated, dynamin-dependent or clathrin-independent) resulted in the impairment of LTβR internalization, indicating that this receptor uses multiple entry pathways. Cells deprived of clathrin and dynamins exhibited enhanced activation of canonical NF-κB signaling represented by increased degradation of IκBα inhibitor and elevated expression of LTβR target genes. We also demonstrated that clathrin and dynamin deficiency reduced to some extent LTβR-triggered activation of the non-canonical branch of the NF-κB pathway.

Conclusions

Our work shows that the impairment of clathrin- and dynamin-dependent internalization amplifies a cellular response to LTβR stimulation. We postulate that receptor internalization restricts responsiveness of the cell to subthreshold stimuli.

Pediatric Soft Tissue Tumors With BCOR ITD Express EGFR but Not OLIG2

INTRODUCTION

Somatic internal tandem duplication of 3' of BCOR (BCOR ITD) has been found in clear cell sarcomas of the kidney (CCSK), soft tissue undifferentiated round cell sarcomas/primitive myxoid mesenchymal tumors of infancy (URCS/PMMTI), and a subgroup of central nervous system high-grade neuroepithelial tumors (CNS-HGNET). BCOR ITD+ tumors share morphologic features. Expression of OLIG2 and epidermal growth factor receptor (EGFR) has been reported in CNS-HGNET with BCOR ITD. Here, we characterize OLIG2 and EGFR expression in URCS/PMMTI with BCOR ITD.

METHODS

Paraffin blocks of 9 polymerase chain reaction-confirmed soft tissue BCOR ITD+ tumors (URCS/PMMTI) were immunophenotyped for OLIG2 and EGFR expression and scored semiquantitatively by percentage of positive cells and intensity of staining as negative, 1+, 2+, and 3+. Fluorescence in situ hybridization (FISH) for EGFR amplification was performed (amplification EGFR/CEP7 ratio ≥2.0).

RESULTS

All 9 tumors showed membrane/cytoplasmic expression of EGFR, strong and diffuse (3+) in 8 cases; weak (+2) in 1. FISH detected no EGFR amplification. OLIG2 was negative in all.

CONCLUSIONS

EGFR is overexpressed in pediatric URCS/PMMTI with BCOR ITD and may be related to transcriptional upregulation of EGFR by BCOR ITD. OLIG2 negative staining differentiates URCS/PMMTI from CNS-HGNET. This finding may further support the possibility that these tumors have a different stem cell of origin.

Integration of JAK/STAT receptor-ligand trafficking, signalling and gene expression in Drosophila melanogaster cells

The JAK/STAT pathway is an essential signalling cascade required for multiple processes during development and for adult homeostasis. A key question in understanding this pathway is how it is regulated in different cell contexts. Here, we have examined how endocytic processing contributes to signalling by the single cytokine receptor in Drosophila melanogaster cells, Domeless. We identify an evolutionarily conserved di-leucine (di-Leu) motif that is required for Domeless internalisation and show that endocytosis is required for activation of a subset of Domeless targets. Our data indicate that endocytosis both qualitatively and quantitatively regulates Domeless signalling. STAT92E, the single STAT transcription factor in Drosophila, appears to be the target of endocytic regulation, and our studies show that phosphorylation of STAT92E on Tyr704, although necessary, is not always sufficient for target transcription. Finally, we identify a conserved residue, Thr702, which is essential for Tyr704 phosphorylation. Taken together, our findings identify previously unknown aspects of JAK/STAT pathway regulation likely to play key roles in the spatial and temporal regulation of signalling in vivo.

C-Cbl regulates c-MPL receptor trafficking and its internalization

Thrombocyte formation from megakaryocyte and their progenitor cells is tightly regulated by thrombopoietin (TPO) and its receptor c‐MPL, thereby maintaining physiological functionality and numbers of circulating platelets. In patients, dysfunction of this regulation could cause thrombocytopenia or myeloproliferative syndromes. Since regulation of this pathway is still not completely understood, we investigated the role of the ubiquitin ligase c‐Cbl which was previously shown to negatively regulated c‐MPL signalling. We developed a new conditional mouse model using c‐Cbl^fl/flPf4^Cre mice and demonstrated that platelet‐specific knockout of c‐Cbl led to severe microthrombocytosis and impaired uptake of TPO and c‐MPL receptor internalization. Furthermore, we characterized a constitutive STAT5 activation c‐Cbl KO platelets. This study identified c‐Cbl as a potential player in causing megakaryocytic and thrombocytic disorders.

NAADP-regulated two-pore channels drive phagocytosis through endo-lysosomal Ca2+ nanodomains, calcineurin and dynamin

Macrophages clear pathogens by phagocytosis and lysosomes that fuse with phagosomes are traditionally regarded as to a source of membranes and luminal degradative enzymes. Here, we reveal that endo-lysosomes act as platforms for a new phagocytic signalling pathway in which FcγR activation recruits the second messenger NAADP and thereby promotes the opening of Ca²⁺ -permeable two-pore channels (TPCs). Remarkably, phagocytosis is driven by these local endo-lysosomal Ca²⁺ nanodomains rather than global cytoplasmic or ER Ca²⁺ signals. Motile endolysosomes contact nascent phagosomes to promote phagocytosis, whereas endo-lysosome immobilization prevents it. We show that TPC-released Ca²⁺ rapidly activates calcineurin, which in turn dephosphorylates and activates the GTPase dynamin-2. Finally, we find that different endo-lysosomal Ca²⁺ channels play diverse roles, with TPCs providing a universal phagocytic signal for a wide range of particles and TRPML1 being only required for phagocytosis of large targets.

EGFR Promotes the Development of Triple Negative Breast Cancer Through JAK/STAT3 Signaling

Purpose

To investigate the role of EGFR and STAT3 in breast cancer development and progression.

Methods

Through bioinformatics analysis differently expressed genes (DEGs) including EGFR and STAT3 were identified in breast cancer tissue. QRT-PCR and Western blot analysis were used to investigate EGFR and STAT3 levels in breast cancer tissues and cells. The influence of EGFR and STAT3 on the breast cancer cell proliferation (CCK-8 assay, clone formation assays), migration (wound healing assays) and invasion (transwell assays) were investigated. The influence of EGFR on breast cancer in vivo was examined by Nude mouse transplantation tumor experiments and immunohistochemistry (IHC) staining. The effects of EGFR on breast cancer signaling were assessed via Western blot.

Results

Both EGFR and p-STAT3 were up-regulated in breast cancer tissues and cell lines. EGFR expression was positively associated with p-STAT3. Moreover, EGFR and p-STAT3 activity enhanced the proliferation and invasion of tumor cells. Breast cancer cell growth was dramatically inhibited by EGFR silencing in vivo.

Conclusion

EGFR promotes breast cancer progression via STAT3 phosphorylation and JAK/STAT3 signaling.

Pseudomonas aeruginosa lectin LecB impairs keratinocyte fitness by abrogating growth factor signalling

Lectins are glycan-binding proteins with no catalytic activity and ubiquitously expressed in nature. Numerous bacteria use lectins to efficiently bind to epithelia, thus facilitating tissue colonisation. Wounded skin is one of the preferred niches for Pseudomonas aeruginosa, which has developed diverse strategies to impair tissue repair processes and promote infection. Here, we analyse the effect of the P. aeruginosa fucose-binding lectin LecB on human keratinocytes and demonstrate that it triggers events in the host, upon binding to fucosylated residues on cell membrane receptors, which extend beyond its role as an adhesion molecule. We found that LecB associates with insulin-like growth factor-1 receptor and dampens its signalling, leading to the arrest of cell cycle. In addition, we describe a novel LecB-triggered mechanism to down-regulate host cell receptors by showing that LecB leads to insulin-like growth factor-1 receptor internalisation and subsequent missorting towards intracellular endosomal compartments, without receptor activation. Overall, these data highlight that LecB is a multitask virulence factor that, through subversion of several host pathways, has a profound impact on keratinocyte proliferation and survival.

Spoiled for Choice: Diverse Endocytic Pathways Function at the Cell Surface

Endocytosis has long been identified as a key cellular process involved in bringing in nutrients, in clearing cellular debris in tissue, in the regulation of signaling, and in maintaining cell membrane compositional homeostasis. While clathrin-mediated endocytosis has been most extensively studied, a number of clathrin-independent endocytic pathways are continuing to be delineated. Here we provide a current survey of the different types of endocytic pathways available at the cell surface and discuss a new classification and plausible molecular mechanisms for some of the less characterized pathways. Along with an evolutionary perspective of the origins of some of these pathways, we provide an appreciation of the distinct roles that these pathways play in various aspects of cellular physiology, including the control of signaling and membrane tension.

SGLT1 is required for the survival of triple-negative breast cancer cells via potentiation of EGFR activity

Sodium/glucose cotransporter 1 (SGLT1), an essential active glucose transport protein that helps maintain high intracellular glucose levels, was previously shown to interact with epidermal growth factor receptor (EGFR); the SGLT1-EGFR interaction maintains intracellular glucose levels to promote survival of cancer cells. Here, we explore the role of SGLT1 in triple-negative breast cancer (TNBC), which is the most aggressive type of breast cancer. We performed TCGA analysis coupled to in vitro experiments in TNBC cell lines as well as in vivo xenografts established in the mammary fat pad of female nude mice. Tissue microarrays of TNBC patients with information of clinical-pathological parameters were also used to investigate the expression and function of SGLT1 in TNBC. We show that high levels of SGLT1 are associated with greater tumour size in TNBC. Knockdown of SGLT1 compromises cell growth in vitro and in vivo. We further demonstrate that SGLT1 depletion results in decreased levels of phospho-EGFR, and as a result, the activity of downstream signalling pathways (such as AKT and ERK) is inhibited. Hence, targeting SGLT1 itself or the EGFR-SGLT1 interaction may provide novel therapeutics against TNBC.

Network medicine-travelling with the insulin receptor: Encounter of the second type

Important progress has been made in understanding many aspects of insulin action in the last 10 years. Attention will be focused here on the physical protein interaction network of the internalized insulin receptor (IR) and its relationships with the genetic architecture of type 2 diabetes mellitus (T2D). The IR recognizes signals from the outside (circulating insulin) and engages the insulin signaling response. Within seconds, the IR is also involved in insulin internalization and its subsequent degradation in endosomes (physiological clearance of insulin). A T2D disease module sharing functional similarities with insulin secretion in pancreatic islets was recently identified in the close neighborhood of the internalized IR in liver. This module brought a new light on the apparent functional heterogeneity of numerous genes at risk to T2D by linking them to a few noncanonical layers of signaling feedback loops. These findings should be translated into a better understanding of the primary mechanisms of the disease and consequently a more precise sub-classification of T2D, ultimately leading to precision medicine and the development of new therapeutical drugs.

Herpes Simplex Virus 1 Can Enter Dynamin 1 and 2 Double-Knockout Fibroblasts

Dynamin GTPases, best known for their role in membrane fission of endocytic vesicles, provide a target for viruses to be exploited during endocytic uptake. Recently, we found that entry of herpes simplex virus 1 (HSV-1) into skin cells depends on dynamin, although our results supported that viral internalization occurs via both direct fusion with the plasma membrane and via endocytic pathways. To further explore the role of dynamin for efficient HSV-1 entry, we utilized conditional dynamin 1 and dynamin 2 double-knockout (DKO) fibroblasts as an experimental tool. Strikingly, HSV-1 entered control and DKO fibroblasts with comparable efficiencies. For comparison, we infected DKO cells with Semliki Forest virus, which is known to adopt clathrin-mediated endocytosis as its internalization pathway, and observed efficient virus entry. These results support the notion that the DKO cells provide alternative pathways for viral uptake. Treatment of cells with the dynamin inhibitor dynasore confirmed that HSV-1 entry depended on dynamin in the control fibroblasts. As expected, dynasore did not interfere with viral entry into DKO cells. Electron microscopy of HSV-1-infected cells suggests viral entry after fusion with the plasma membrane and by endocytosis in both dynamin-expressing and dynamin-deficient cells. Infection at low temperatures where endocytosis is blocked still resulted in HSV-1 entry, although at a reduced level, which suggests that nonendocytic pathways contribute to successful entry. Overall, our results strengthen the impact of dynamin for HSV-1 entry, as only cells that adapt to the lack of dynamin allow dynamin-independent entry.IMPORTANCE The human pathogen herpes simplex virus 1 (HSV-1) can adapt to a variety of cellular pathways to enter cells. In general, HSV-1 is internalized by fusion of its envelope with the plasma membrane or by endocytic pathways, which reflects the high adaptation to differences in its target cells. The challenges are to distinguish whether multiple or only one of these internalization pathways leads to successful entry and, furthermore, to identify the mode of viral uptake. In this study, we focused on dynamin, which promotes endocytic vesicle fission, and explored how the presence and absence of dynamin can influence viral entry. Our results support the idea that HSV-1 entry into mouse embryonic fibroblasts depends on dynamin; however, depletion of dynamin still allows efficient viral entry, suggesting that alternative pathways present upon dynamin depletion can accomplish viral internalization.

Quantitative proteomics reveals reduction of endocytic machinery components in gliomas

Background

Gliomas are the most frequent and aggressive malignancies of the central nervous system. Decades of molecular analyses have demonstrated that gliomas accumulate genetic alterations that culminate in enhanced activity of receptor tyrosine kinases and downstream mediators. While the genetic alterations, like gene amplification or loss, have been well characterized, little information exists about changes in the proteome of gliomas of different grades.

Methods

We performed unbiased quantitative proteomics of human glioma biopsies by mass spectrometry followed by bioinformatic analysis.

Findings

Various pathways were found to be up- or downregulated. In particular, endocytosis as pathway was affected by a vast and concomitant reduction of multiple machinery components involved in initiation, formation, and scission of endocytic carriers. Both clathrin-dependent and -independent endocytosis were changed, since not only clathrin, AP-2 adaptins, and endophilins were downregulated, but also dynamin that is shared by both pathways. The reduction of endocytic machinery components caused increased receptor cell surface levels, a prominent phenotype of defective endocytosis. Analysis of additional biopsies revealed that depletion of endocytic machinery components was a common trait of various glioma grades and subclasses.

Interpretation

We propose that impaired endocytosis creates a selective advantage in glioma tumor progression due to prolonged receptor tyrosine kinase signaling from the cell surface.

Fund

This work was supported by Grants 316030-164105 (to P. Jenö), 31003A-162643 (to M. Spiess) and PP00P3-176974 (to G. Hutter) from the Swiss National Science Foundation. Further funding was received by the Department of Surgery from the University Hospital Basel.

Dynamin 2 is required for GPVI signaling and platelet hemostatic function in mice

Receptor-mediated endocytosis, which contributes to a wide range of cellular functions, including receptor signaling, cell adhesion, and migration, requires endocytic vesicle release by the large GTPase dynamin 2. Here, the role of dynamin 2 was investigated in platelet hemostatic function using both pharmacological and genetic approaches. Dnm2^fl/fl Pf4-Cre (Dnm2^Plt−/−) mice specifically lacking dynamin 2 within the platelet lineage developed severe thrombocytopenia and bleeding diathesis and Dnm2^Plt−/− platelets adhered poorly to collagen under arterial shear rates. Signaling via the collagen receptor GPVI was impaired in platelets treated with the dynamin GTPase inhibitor dynasore, as evidenced by poor protein tyrosine phosphorylation, including that of the proximal tyrosine kinase Lyn on its activating tyrosine 396 residue. Platelet stimulation via GPVI resulted in a slight decrease in GPVI, which was maintained by dynasore treatment. Dynasore-treated platelets had attenuated function when stimulated via GPVI, as evidenced by reduced GPIbα downregulation, α-granule release, integrin αIIbβ3 activation, and spreading onto immobilized fibrinogen. By contrast, responses to the G-protein coupled receptor agonist thrombin were minimally affected by dynasore treatment. GPVI expression was severely reduced in Dnm2^Plt−/− platelets, which were dysfunctional in response to stimulation via GPVI, and to a lesser extent to thrombin. Dnm2^Plt−/− platelets lacked fibrinogen in their α-granules, but retained von Willebrand factor. Taken together, the data show that dynamin 2 plays a proximal role in signaling via the collagen receptor GPVI and is required for fibrinogen uptake and normal platelet hemostatic function.

Faraway, so close! Functions of Endoplasmic reticulum-Endosome contacts

Eukaryotic cells are partitioned into functionally distinct organelles. Long considered as independent units in the cytosol, organelles are actually in constant and direct interaction with each other, mostly through the establishment of physical connections named membrane contact sites. Membrane contact sites constitute specific active regions involved in organelle dynamics, inter-organelle exchanges and communications. The endoplasmic reticulum (ER), which spreads throughout the cytosol, forms an extensive network that has many connections with the other organelles of the cell. Ample connections between the ER and endocytic organelles are observed in many cell types, highlighting their prominent physiological roles. Even though morphologically similar - a contact is a contact -, the identity of ER-Endosome contacts is defined by their specific molecular composition, which in turn determines the function of the contact. Here, we review the molecular mechanisms of ER-Endosome contact site formation and their associated cellular functions. This article is part of a Special Issue entitled Endoplasmic reticulum platforms for lipid dynamics edited by Shamshad Cockcroft and Christopher Stefan.

Akt-ing Up Just About Everywhere: Compartment-Specific Akt Activation and Function in Receptor Tyrosine Kinase Signaling

The serine/threonine kinase Akt is a master regulator of many diverse cellular functions, including survival, growth, metabolism, migration, and differentiation. Receptor tyrosine kinases are critical regulators of Akt, as a result of activation of phosphatidylinositol-3-kinase (PI3K) signaling leading to Akt activation upon receptor stimulation. The signaling axis formed by receptor tyrosine kinases, PI3K and Akt, as well as the vast range of downstream substrates is thus central to control of cell physiology in many different contexts and tissues. This axis must be tightly regulated, as disruption of PI3K-Akt signaling underlies the pathology of many diseases such as cancer and diabetes. This sophisticated regulation of PI3K-Akt signaling is due in part to the spatial and temporal compartmentalization of Akt activation and function, including in specific nanoscale domains of the plasma membrane as well as in specific intracellular membrane compartments. Here, we review the evidence for localized activation of PI3K-Akt signaling by receptor tyrosine kinases in various specific cellular compartments, as well as that of compartment-specific functions of Akt leading to control of several fundamental cellular processes. This spatial and temporal control of Akt activation and function occurs by a large number of parallel molecular mechanisms that are central to regulation of cell physiology.

MICAL-L2 potentiates Cdc42-dependent EGFR stability and promotes gastric cancer cell migration

Enhanced migration potential is a common characteristic of cancer cells induced by mechanisms that are incompletely defined. The present study was designed to investigate relationship of a new discovered cytoskeleton regulator MICAL‐L2 and the endogenous epidermal growth factor receptor (EGFR) signalling pathways in gastric cancer cell migration. Increased expression of MICAL‐L2 in gastric cancer cells up‐regulated EGFR protein level, accompanied by the increase of cell migration, whereas silencing MICAL‐L2 down‐regulated EGFR and inhibited cell migration. Expression of MICAL‐L2 was also shown positively correlated with the activation of HSP27/cytoskeleton and HSP27/β‐catenin signalling pathways that provide key mechanisms controlling cell migration. The up‐regulating effect of MICAL‐L2 on EGFR is mediated through a transcription‐independent mechanism that involves inhibiting EGFR protein degradation in lysosome. Further analysis indicated that Cdc42 activation contributed in maintaining the effect of MICAL‐L2 on EGFR stability. Furthermore analysis of clinic specimens revealed increased expression of MICAL‐L2 in carcinoma tissues and a positive correlation between MICAL‐L2 and EGFR expression levels. The above results indicate that MICAL‐L2 potentiates gastric cell migration via inhibiting EGFR degradation in lysosome via a Cdc42‐dependent manner that leads to the activation of EGFR/HSP27 signalling pathways.

Localization dynamics of endogenous fluorescently labeled RAF1 in EGF-stimulated cells

Activation of the epidermal growth factor (EGF) receptor (EGFR) at the cell surface initiates signaling through the RAS-RAF-MAPK/ERK1/2 pathway and receptor endocytosis. Whether this signaling continues from endosomes remains unclear, because RAS is predominantly located on the plasma membrane, and the localization of endogenous RAF kinases, downstream effectors of RAS, is not defined. To examine RAF localization, we labeled endogenous RAF1 with mVenus using gene editing. From 10 to 15% of RAF1-mVenus (<2000 molecules/cell), which was initially entirely cytosolic, transiently translocated to the plasma membrane after EGF stimulation. Following an early burst of translocation, the membrane-associated RAF1-mVenus was undetectable by microscopy or subcellular fractionation, and this pool was estimated to be <200 molecules per cell. In contrast, persistent EGF-dependent translocation of RAF1-mVenus to the plasma membrane was driven by the RAF inhibitor sorafenib, which increases the affinity of Ras-GTP:RAF1 interactions. RAF1-mVenus was not found in EGFR-containing endosomes under any conditions. Computational modeling of RAF1 dynamics revealed that RAF1 membrane abundance is controlled most prominently by association and dissociation rates from RAS-GTP and by RAS-GTP concentration. The model further suggested that the relatively protracted activation of the RAF-MEK1/2-ERK1/2 module, in comparison with RAF1 membrane localization, may involve multiple rounds of cytosolic RAF1 rebinding to active RAS at the membrane.

Implication of calcium activated RasGRF2 in Annexin A6-mediated breast tumor cell growth and motility

The role of AnxA6 in breast cancer and in particular, the mechanisms underlying its contribution to tumor cell growth and/or motility remain poorly understood. In this study, we established the tumor suppressor function of AnxA6 in triple negative breast cancer (TNBC) cells by showing that loss of AnxA6 is associated with early onset and rapid growth of xenograft TNBC tumors in mice. We also identified the Ca²⁺ activated RasGRF2 as an effector of AnxA6 mediated TNBC cell growth and motility. Activation of Ca²⁺ mobilizing oncogenic receptors such as epidermal growth factor receptor (EGFR) in TNBC cells or pharmacological stimulation of Ca²⁺ influx led to activation, subsequent degradation and altered effector functions of RasGRF2. Inhibition of Ca²⁺ influx or overexpression of AnxA6 blocked the activation/degradation of RasGRF2. We also show that AnxA6 acts as a scaffold for RasGRF2 and Ras proteins and that its interaction with RasGRF2 is modulated by GTP and/or activation of Ras proteins. Meanwhile, down-regulation of RasGRF2 and treatment of cells with the EGFR-targeted tyrosine kinase inhibitor (TKI) lapatinib strongly attenuated the growth of otherwise EGFR-TKI resistant AnxA6 high TNBC cells. These data not only suggest that AnxA6 modulated Ca²⁺ influx and effector functions of RasGRF2 underlie at least in part, the AnxA6 mediated TNBC cell growth and/or motility, but also provide a rationale to target Ras-driven TNBC with EGFR targeted therapies in combination with inhibition of RasGRF2.

The crossroads of receptor-mediated signaling and endocytosis in plants

Plants deploy numerous plasma membrane receptors to sense and rapidly react to environmental changes. Correct localization and adequate protein levels of the cell-surface receptors are critical for signaling activation and modulation of plant development and defense against pathogens. After ligand binding, receptors are internalized for degradation and signaling attenuation. However, one emerging notion is that the ligand-induced endocytosis of receptor complexes is important for the signal duration, amplitude, and specificity. Recently, mutants of major endocytosis players, including clathrin and dynamin, have been shown to display defects in activation of a subset of signal transduction pathways, implying that signaling in plants might not be solely restricted to the plasma membrane. Here, we summarize the up-to-date knowledge of receptor complex endocytosis and its effect on the signaling outcome, in the context of plant development and immunity.

The Endosomal Protein CEMIP Links WNT Signaling to MEK1-ERK1/2 Activation in Selumetinib-Resistant Intestinal Organoids

MAPK signaling pathways are constitutively active in colon cancer and also promote acquired resistance to MEK1 inhibition. Here, we demonstrate that BRAF^V600E -mutated colorectal cancers acquire resistance to MEK1 inhibition by inducing expression of the scaffold protein CEMIP through a β-catenin- and FRA-1-dependent pathway. CEMIP was found in endosomes and bound MEK1 to sustain ERK1/2 activation in MEK1 inhibitor-resistant BRAF^V600E-mutated colorectal cancers. The CEMIP-dependent pathway maintained c-Myc protein levels through ERK1/2 and provided metabolic advantage in resistant cells, potentially by sustaining amino acids synthesis. CEMIP silencing circumvented resistance to MEK1 inhibition, partly, through a decrease of both ERK1/2 signaling and c-Myc. Together, our data identify a cross-talk between Wnt and MAPK signaling cascades, which involves CEMIP. Activation of this pathway promotes survival by potentially regulating levels of specific amino acids via a Myc-associated cascade. Targeting this node may provide a promising avenue for treatment of colon cancers that have acquired resistance to targeted therapies.Significance: MEK1 inhibitor-resistant colorectal cancer relies on the scaffold and endosomal protein CEMIP to maintain ERK1/2 signaling and Myc-driven transcription. Cancer Res; 78(16); 4533-48. ©2018 AACR.

Distinct functions of dynamin isoforms in tumorigenesis and their potential as therapeutic targets in cancer

Dynamins and their related proteins participate in the regulation of neurotransmission, antigen presentation, receptor internalization, growth factor signalling, nutrient uptake, and pathogen infection. Recently, emerging findings have shown dynamin proteins can also contribute to the genesis of cancer. This up-to-date review herein focuses on the functionality of dynamin in cancer development. Dynamin 1 and 2 both enhance cancer cell proliferation, tumor invasion and metastasis, whereas dynamin 3 has tumor suppression role. Antisense RNAs encoded on the DNA strand opposite a dynamin gene regulate the function of dynamin, and manipulate oncogenes and tumor suppressor genes. Certain dynamin-related proteins are also upregulated in distinct cancer conditions, resulting in apoptotic resistance, cell migration and poor prognosis. Altogether, dynamins are potential biomarkers as well as representing promising novel therapeutic targets for cancer treatment. This study also summarizes the current available dynamin-targeted therapeutics and suggests the potential strategy based on signalling pathways involved, providing important information to aid the future development of novel cancer therapeutics by targeting these dynamin family members.