Overexpression of cellular Src in fibroblasts enhances endocytic internalization of epidermal growth factor receptor

Unveiling the therapeutic prospects of EGFR inhibition in rotenone-mediated parkinsonism in rats: Modulation of dopamine D3 receptor

Parkinson's disease (PD) is characterized by the progressive loss of dopaminergic neurons in the substantia nigra. The dopamine D3 receptor (D3R) plays a significant role in the pathogenesis and treatment of PD. Activation of receptor tyrosine kinases (RTKs) inhibits signaling mediated by G protein-coupled receptor (GPCR). Epidermal growth factor receptors (EGFRs) and dopamine D3 receptors in the brain are directly associated with PD, both in terms of its development and potential treatment. Therefore, we investigated the impact of modulating the EGFR, a member of the RTKs family, and the dopamine D3R, a member of the GPCR family. In the present study, 100 mg/kg of lapatinib (LAP) was administered to rotenone-intoxicated rats for three weeks. Our findings indicate that LAP effectively alleviated motor impairment, improved histopathological abnormalities, and restored dopaminergic neurons in the substantia nigra. This restoration was achieved through the upregulation of dopamine D3R and increase of tyrosine hydroxylase (TH) expression, as well as boosting dopamine levels. Furthermore, LAP inhibited the activity of p-EGFR, GRK2, and SCR. Additionally, LAP exhibited antioxidant properties by inhibiting the 4-hydroxynonenal (4-HNE) and PLCγ/PKCβII pathway, while enhancing the antioxidant defense mechanism by increasing GSH-GPX4 pathway. The current study offers insights into the potential repositioning of LAP as a disease-modifying drug for PD. This could be achieved by modulating the dopaminergic system and curbing oxidative stress.

HER2-CDH1 Interaction via Wnt/B-Catenin Is Associated with Patients' Survival in HER2-Positive Metastatic Gastric Adenocarcinoma

Simple Summary

A deeper understanding of the molecular mechanisms involved in gastric cacner (GC) pathologenesis would help the identification of prognostic biomarkers and the development of new treatments. Human epidermal growth factor receptor 2 (HER2/ErbB2), a membrane-bound receptor of the EGFR family, may be overexpressed in GC. Trastuzumab is a HER2 inhibitor used to treat HER2+ metastatic gastric cancer (mGC). The present study aims to investigate the relationship between CDH1 mRNA expression and HER2-positivity in mGC using a multiplexed gene expression profile in two series of GC patients: 38 HER2+ and HER2- mGC and 36 HER2- GC with and without metastasis. Our results revealed the relationship between CDH1 and HER2 mRNA expression in mGC via the canonical WNT/β-catenin pathway and identified EGF as an independent prognostic biomarker for survival.

Abstract

Trastuzumab is a human epidermal growth factor receptor 2 (HER2) inhibitor used to treat HER2+ metastatic gastric cancer (mGC). The present study aims to investigate the relationship between CDH1 mRNA expression and HER2-positivity in mGC using a multiplexed gene expression profile in two series of gastric cancer (GC): Series 1 (n = 38): HER2+ and HER2- mGC; Series 2 (n = 36) HER2- GC with and without metastasis. To confirm the results, the same expression profiles were analyzed in 354 GC from The Cancer Genome Atlas (TCGA) dataset. The difference in gene expression connected HER2 overexpression with canonical wingless-type (Wnt)/β-catenin pathway and immunohistochemical (IHC) expression loss of E-cadherin (E-CAD). CDH1 mRNA expression was simultaneously associated with the rs16260-A variant and an increase in E-CAD expression. Differences in retinoic acid receptor alfa (RARA), RPL19 (coding for the 60S ribosomal L19 protein), catenin delta 1 (CTNND1), and epidermal growth factor (EGF) mRNA levels—all included in the Wnt/β-catenin pathway—were found associated with overall survival (OS). RARA, CTNND1, and EGF resulted in independent OS prognostic factors. EGF was confirmed as an independent factor along with TNM stage in HER2-overpressed mGC from TCGA collection. Our study highlighted factors involved in the WNT/β-catenin pathway that interconnected E-CAD with HER2 overexpression and patient survival.

The dual role of c-src in cell-to-cell transmission of α-synuclein

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons located in the substantia nigra pars compacta and the presence of proteinaceous inclusions called Lewy bodies and Lewy neurites in numerous brain regions. Increasing evidence indicates that Lewy pathology progressively involves additional regions of the nervous system as the disease advances, and the prion-like propagation of α-synuclein (α-syn) pathology promotes PD progression. Accordingly, the modulation of α-syn transmission may be important for the development of disease-modifying therapies in patients with PD. Here, we demonstrate that α-syn fibrils induce c-src activation in neurons, which depends on the FcγRIIb-SHP-1/-2-c-src pathway and enhances signals for the uptake of α-syn into neurons. Blockade of c-src activation inhibits the uptake of α-syn and the formation of Lewy body-like inclusions. Furthermore, the blockade of c-src activation also inhibits the release of α-syn via activation of autophagy. The brain-permeable c-src inhibitor, saracatinib, efficiently reduces α-syn propagation into neighboring regions in an in vivo model system. These results suggest a new therapeutic target against progressive PD.

The Na/K-ATPase Signaling: From Specific Ligands to General Reactive Oxygen Species

The signaling function of the Na/K-ATPase has been established for 20 years and is widely accepted in the field, with many excellent reports and reviews not cited here. Even though there is debate about the underlying mechanism, the signaling function is unquestioned. This short review looks back at the evolution of Na/K-ATPase signaling, from stimulation by cardiotonic steroids (also known as digitalis-like substances) as specific ligands to stimulation by reactive oxygen species (ROS) in general. The interplay of cardiotonic steroids and ROS in Na/K-ATPase signaling forms a positive-feedback oxidant amplification loop that has been implicated in some pathophysiological conditions.

Molecular mechanisms involved in epidermal growth factor receptor-mediated inhibition of dopamine D3 receptor signaling

The phenomenon wherein the signaling by a given receptor is regulated by a different class of receptors is termed transactivation or crosstalk. Crosstalk between receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs) is highly diverse and has unique functional implications because of the distinct structural features of the receptors and the signaling pathways involved. The present study used the epidermal growth factor receptor (EGFR) and dopamine D₃ receptor (D₃R), which are both associated with schizophrenia, as the model system to study crosstalk between RTKs and GPCRs. Loss-of-function approaches were used to identify the cellular components involved in the tyrosine phosphorylation of G protein-coupled receptor kinase 2 (GRK2), which is responsible for EGFR-induced regulation of the functions of D₃R. SRC proto-oncogene (Src, non-receptor tyrosine kinase), heterotrimeric G protein Gβγ subunit, and endocytosis of EGFR were involved in the tyrosine phosphorylation of GRK2. In response to EGF treatment, Src interacted with EGFR in a Gβγ-dependent manner, resulting in the endocytosis of EGFR. Internalized EGFR in the cytosol mediated Src/Gβγ-dependent tyrosine phosphorylation of GRK2. The binding of tyrosine-phosphorylated GRK2 to the T142 residue of D₃R resulted in uncoupling from G proteins, endocytosis, and lysosomal downregulation. This study identified the molecular mechanisms involved in the EGFR-mediated regulation of the functions of D₃R, which can be extended to the crosstalk between other RTKs and GPCRs.

Targeting Na/K-ATPase Signaling: A New Approach to Control Oxidative Stress

Renal and cardiac function are greatly affected by chronic oxidative stress which can cause many pathophysiological states. The Na/K-ATPase is well-described as an ion pumping enzyme involved in maintaining cellular ion homeostasis; however, in the past two decades, extensive research has been done to understand the signaling function of the Na/K-ATPase and determine its role in physiological and pathophysiological states. Our lab has shown that the Na/K-ATPase signaling cascade can function as an amplifier of reactive oxygen species (ROS) which can be initiated by cardiotonic steroids or increases in ROS. Regulation of systemic oxidative stress by targeting Na/K-ATPase signaling mediated oxidant amplification improves 5/6th partial nephrectomy (PNx) mediated uremic cardiomyopathy, renal sodium handling, as well as ameliorates adipogenesis. This review will present this new concept of Na/K-ATPase signaling mediated oxidant amplification loop and its clinic implication.

Syntenin mediates SRC function in exosomal cell-to-cell communication

Viral-like nanovesicles of endosomal origin, or “exosomes,” are newly recognized vehicles of signals that cells use to communicate, in various systemic diseases, including cancer. Yet the molecular mechanisms that regulate the biogenesis and activity of exosomes remain obscure. Here, we establish that the oncogenic protein SRC stimulates the secretion of exosomes loaded with syntenin and syndecans, known co-receptors for a plethora of signaling and adhesion molecules. SRC phosphorylates conserved tyrosine residues in the syndecans and syntenin and stimulates their endosomal budding. Moreover, SRC-dependent exosomes have a promigratory activity that strictly depends on syntenin expression. This work sheds light on a function of SRC in cell-to-cell communication and mechanisms of exosome biogenesis and activity, with potential broad impact for physiopathology.

The cytoplasmic tyrosine kinase SRC controls cell growth, proliferation, adhesion, and motility. The current view is that SRC acts primarily downstream of cell-surface receptors to control intracellular signaling cascades. Here we reveal that SRC functions in cell-to-cell communication by controlling the biogenesis and the activity of exosomes. Exosomes are viral-like particles from endosomal origin that can reprogram recipient cells. By gain- and loss-of-function studies, we establish that SRC stimulates the secretion of exosomes having promigratory activity on endothelial cells and that syntenin is mandatory for SRC exosomal function. Mechanistically, SRC impacts on syndecan endocytosis and on syntenin–syndecan endosomal budding, upstream of ARF6 small GTPase and its effector phospholipase D2, directly phosphorylating the conserved juxtamembrane DEGSY motif of the syndecan cytosolic domain and syntenin tyrosine 46. Our study uncovers a function of SRC in cell–cell communication, supported by syntenin exosomes, which is likely to contribute to tumor–host interactions.

Intracellular Dynamics and Fate of a Humanized Anti-Interleukin-6 Receptor Monoclonal Antibody, Tocilizumab

Tocilizumab (TCZ), a humanized anti-interleukin-6 (IL-6) receptor (IL-6R) monoclonal antibody, abrogates signal transducer protein gp130-mediated IL-6 signaling by competitively inhibiting the binding of IL-6 to the receptor, and shows clinical efficacy in autoimmune and inflammatory diseases. Despite accumulating evidence for therapeutic efficacy, the behavior and fate of TCZ at the cellular level remain largely unknown. To address this, we evaluated the endocytosis and intracellular trafficking of IL-6R in HeLa cells. The results of our study provide evidence that IL-6R is constitutively internalized from the cell surface by ligand or TCZ binding and the expression of gp130 in an independent manner and is targeted via endosomes without being significantly directed to the recycling pathway to, and degraded in, lysosomes. Furthermore, the cytoplasmic tail of IL-6R is required for constitutive endocytosis of the receptor, which is mediated by the clathrin and AP-2 complex. We further demonstrate that FcRn, whose function is to regulate the serum persistence of IgG, is confined primarily to early/recycling endosomes and rapidly transits between these compartments and late endosomes/lysosomes without being degraded. Importantly, the expression of FcRn induces the segregation of TCZ from IL-6R, resulting in extensive colocalization of TCZ and FcRn in IL-6R-depleted endosomal compartments. Collectively, our results suggest that FcRn can accelerate the retrieval of the internalized TCZ, not only from endosomes but also from lysosomes. Our findings provide new insight into the mechanism by which the antibody internalized into cells is rescued from lysosomal degradation and into how its serum levels are maintained.

A kinase inhibitor screen reveals protein kinase C-dependent endocytic recycling of ErbB2 in breast cancer cells

ErbB2 overexpression drives oncogenesis in 20-30% cases of breast cancer. Oncogenic potential of ErbB2 is linked to inefficient endocytic traffic into lysosomes and preferential recycling. However, regulation of ErbB2 recycling is incompletely understood. We used a high-content immunofluorescence imaging-based kinase inhibitor screen on SKBR-3 breast cancer cells to identify kinases whose inhibition alters the clearance of cell surface ErbB2 induced by Hsp90 inhibitor 17-AAG. Less ErbB2 clearance was observed with broad-spectrum PKC inhibitor Ro 31-8220. A similar effect was observed with Go 6976, a selective inhibitor of classical Ca(2+)-dependent PKCs (α, β1, βII, and γ). PKC activation by PMA promoted surface ErbB2 clearance but without degradation, and ErbB2 was observed to move into a juxtanuclear compartment where it colocalized with PKC-α and PKC-δ together with the endocytic recycling regulator Arf6. PKC-α knockdown impaired the juxtanuclear localization of ErbB2. ErbB2 transit to the recycling compartment was also impaired upon PKC-δ knockdown. PMA-induced Erk phosphorylation was reduced by ErbB2 inhibitor lapatinib, as well as by knockdown of PKC-δ but not that of PKC-α. Our results suggest that activation of PKC-α and -δ mediates a novel positive feedback loop by promoting ErbB2 entry into the endocytic recycling compartment, consistent with reported positive roles for these PKCs in ErbB2-mediated tumorigenesis. As the endocytic recycling compartment/pericentrion has emerged as a PKC-dependent signaling hub for G-protein-coupled receptors, our findings raise the possibility that oncogenesis by ErbB2 involves previously unexplored PKC-dependent endosomal signaling.

Regulation of Src trafficking and activation by the endocytic regulatory proteins MICAL-L1 and EHD1

Localization of the non-receptor tyrosine kinase Src to the cell periphery is required for its activation and to mediate focal adhesion turnover, cell spreading and migration. Inactive Src localizes to a perinuclear compartment and the movement of Src to the plasma membrane is mediated by endocytic transport. However, the precise pathways and regulatory proteins that are responsible for SRC transport are incompletely understood. Here, we demonstrate that Src partially colocalizes with the endocytic regulatory protein MICAL-L1 (molecule interacting with CasL-like protein 1) in mammalian cells. Furthermore, MICAL-L1 is required for growth-factor- and integrin-induced Src activation and transport to the cell periphery in HeLa cells and human fibroblasts. Accordingly, MICAL-L1 depletion impairs focal adhesion turnover, cell spreading and cell migration. Interestingly, we find that the MICAL-L1 interaction partner EHD1 (EH domain-containing protein 1) is also required for Src activation and transport. Moreover, the MICAL-L1-mediated recruitment of EHD1 to Src-containing recycling endosomes is required for the release of Src from the perinuclear endocytic recycling compartment in response to growth factor stimulation. Our study sheds new light on the mechanism by which Src is transported to the plasma membrane and activated, and provides a new function for MICAL-L1 and EHD1 in the regulation of intracellular non-receptor tyrosine kinases.

Role of epidermal growth factor receptor signaling in the interaction of Neisseria meningitidis with endothelial cells

Neisseria meningitidis, the causative agent of meningitis and septicemia, attaches to and invades various cell types. Both steps induce and/or require tyrosine phosphorylation of host cell proteins. Here, we used a phospho array platform to identify active receptor tyrosine kinases (RTKs) and key signaling nodes in N. meningitidis-infected brain endothelial cells to decipher RTK-dependent signaling pathways necessary for bacterial uptake. We detected several activated RTKs, including the ErbB family receptors epidermal growth factor receptor (EGFR), ErbB2, and ErbB4. We found that pharmacological inhibition and genetic ablation of ErbB receptor tyrosine phosphorylation and expression resulted in decreased bacterial uptake and heterologous expression of EGFR, ErbB2, or ErbB4 in Chinese ovary hamster (CHO-K1) cells, which do not express of EGFR and ErbB4; the decrease caused a significant increase in meningococcal invasion. Activation of EGFR and ErbB4 was mediated by transactivation via the common ligand HB-EGF (heparin-binding EGF-like ligand), which was significantly elevated in infected cell culture supernatants. We furthermore determined that N. meningitidis induced phosphorylation of EGFR at Tyr845 independent of ligand binding, which required c-Src activation and was involved in mediating uptake of N. meningitidis into eukaryotic cells. Increased uptake was repressed by expression of EGFR Y845F, which harbored a point mutation in the kinase domain. In addition, activation of ErbB4 at its autophosphorylation site, Tyr1284, and phosphorylation of ErbB2 Thr686 were observed. Altogether, our results provide evidence that EGFR, ErbB2, and ErbB4 are activated in response to N. meningitidis infection and shed new light on the role of ErbB signaling in meningococcal infection biology.

Cellular functions regulated by phosphorylation of EGFR on Tyr845

The Src gene product (Src) and the epidermal growth factor receptor (EGFR) are prototypes of oncogene products and function primarily as a cytoplasmic non-receptor tyrosine kinase and a transmembrane receptor tyrosine kinase, respectively. The identification of Src and EGFR, and the subsequent extensive investigations of these proteins have long provided cutting edge research in cancer and other molecular and cellular biological studies. In 1995, we reported that the human epidermoid carcinoma cells, A431, contain a small fraction of Src and EGFR in which these two kinase were in physical association with each other, and that Src phosphorylates EGFR on tyrosine 845 (Y845) in the Src-EGFR complex. Y845 of EGFR is located in the activation segment of the kinase domain, where many protein kinases contain kinase-activating autophosphorylation sites (e.g., cAMP-dependent protein kinase, Src family kinases, transmembrane receptor type tyrosine kinases) or trans-phosphorylation sites (e.g., cyclin-dependent protein kinase, mitogen-activated protein kinase, Akt protein kinase). A number of studies have demonstrated that Y845 phosphorylation serves an important role in cancer as well as normal cells. Here we compile the experimental facts involving Src phosphorylation of EGFR on Y845, by which cell proliferation, cell cycle control, mitochondrial regulation of cell metabolism, gamete activation and other cellular functions are regulated. We also discuss the physiological relevance, as well as structural insights of the Y845 phosphorylation.

A comprehensive, multi-scale dynamical model of ErbB receptor signal transduction in human mammary epithelial cells

The non-receptor tyrosine kinase Src and receptor tyrosine kinase epidermal growth factor receptor (EGFR/ErbB1) have been established as collaborators in cellular signaling and their combined dysregulation plays key roles in human cancers, including breast cancer. In part due to the complexity of the biochemical network associated with the regulation of these proteins as well as their cellular functions, the role of Src in EGFR regulation remains unclear. Herein we present a new comprehensive, multi-scale dynamical model of ErbB receptor signal transduction in human mammary epithelial cells. This model, constructed manually from published biochemical literature, consists of 245 nodes representing proteins and their post-translational modifications sites, and over 1,000 biochemical interactions. Using computer simulations of the model, we find it is able to reproduce a number of cellular phenomena. Furthermore, the model predicts that overexpression of Src results in increased endocytosis of EGFR in the absence/low amount of the epidermal growth factor (EGF). Our subsequent laboratory experiments also suggest increased internalization of EGFR upon Src overexpression under EGF-deprived conditions, further supporting this model-generated hypothesis.

Cortactin is a substrate of activated Cdc42-associated kinase 1 (ACK1) during ligand-induced epidermal growth factor receptor downregulation

Background

Epidermal growth factor receptor (EGFR) internalization following ligand binding controls EGFR downstream pathway signaling activity. Internalized EGFR is poly-ubiquitinated by Cbl to promote lysosome-mediated degradation and signal downregulation. ACK1 is a non-receptor tyrosine kinase that interacts with ubiquitinated EGFR to facilitate EGFR degradation. Dynamic reorganization of the cortical actin cytoskeleton controlled by the actin related protein (Arp)2/3 complex is important in regulating EGFR endocytosis and vesicle trafficking. How ACK1-mediated EGFR internalization cooperates with Arp2/3-based actin dynamics during EGFR downregulation is unclear.

Methodology/Principal Findings

Here we show that ACK1 directly binds and phosphorylates the Arp2/3 regulatory protein cortactin, potentially providing a direct link to Arp2/3-based actin dynamics during EGFR degradation. Co-immunoprecipitation analysis indicates that the cortactin SH3 domain is responsible for binding to ACK1. In vitro kinase assays demonstrate that ACK1 phosphorylates cortactin on key tyrosine residues that create docking sites for adaptor proteins responsible for enhancing Arp2/3 nucleation. Analysis with phosphorylation-specific antibodies determined that EGFR-induced cortactin tyrosine phosphorylation is diminished coincident with EGFR degradation, whereas ERK1/2 cortactin phosphorylation utilized in promoting activation of the Arp2/3 regulator N-WASp is sustained during EGFR downregulation. Cortactin and ACK1 localize to internalized vesicles containing EGF bound to EGFR visualized by confocal microscopy. RNA interference and rescue studies indicate that ACK1 and the cortactin SH3 domain are essential for ligand-mediated EGFR internalization.

Conclusions/Significance

Cortactin is a direct binding partner and novel substrate of ACK1. Tyrosine phosphorylation of cortactin by ACK1 creates an additional means to amplify Arp2/3 dynamics through N-WASp activation, potentially contributing to the overall necessary tensile and/or propulsive forces utilized during EGFR endocytic internalization and trafficking involved in receptor degradation.

Reggies/flotillins regulate E-cadherin-mediated cell contact formation by affecting EGFR trafficking

In epithelial cells, the reggie/flotillin proteins regulate—in association with PrP—the formation of E-cadherin adherens junctions (AJs) via the EGFR. Reggies control the EGF-mediated phosphorylation and internalization of EGFR. EGF signaling at the plasma membrane induces the macropinocytosis of E-cadherin and thus the formation of AJs.

The reggie/flotillin proteins are implicated in membrane trafficking and, together with the cellular prion protein (PrP), in the recruitment of E-cadherin to cell contact sites. Here, we demonstrate that reggies, as well as PrP down-regulation, in epithelial A431 cells cause overlapping processes and abnormal formation of adherens junctions (AJs). This defect in cell adhesion results from reggie effects on Src tyrosine kinases and epidermal growth factor receptor (EGFR): loss of reggies reduces Src activation and EGFR phosphorylation at residues targeted by Src and c-cbl and leads to increased surface exposure of EGFR by blocking its internalization. The prolonged EGFR signaling at the plasma membrane enhances cell motility and macropinocytosis, by which junction-associated E-cadherin is internalized and recycled back to AJs. Accordingly, blockage of EGFR signaling or macropinocytosis in reggie-deficient cells restores normal AJ formation. Thus, by promoting EGFR internalization, reggies restrict the EGFR signaling involved in E-cadherin macropinocytosis and recycling and regulate AJ formation and dynamics and thereby cell adhesion.

Acute ligand-independent Src activation mimics low EGF-induced EGFR surface signalling and redistribution into recycling endosomes

Src, a non-receptor tyrosine kinase, is a key signal transduction partner of epidermal growth factor (EGF) receptor (EGFR). In human breast cancer, EGFR and Src are frequently over-expressed and/or over-activated. Although reciprocal activation is documented, mechanisms underlying Src:EGFR interactions are incompletely understood. We here exploited ts/v-Src thermo-activation in MDCK monolayers to test whether acute Src activation impacts on signalling and trafficking of non-liganded EGFR. We found that thermo-activation caused rapid Src recruitment to the plasma membrane, concomitant association with EGFR, and its phosphorylation at Y845 and Y1173 predominantly at the cell surface. Like low EGF concentrations, activated Src (i) decreased EGF surface binding without affecting the total EGFR pool; (ii) triggered EGFR endocytosis via clathrin-coated vesicles; (iii) and led to its sequestration in perinuclear/recycling endosomes with avoidance of multivesicular bodies and lysosomal degradation. Combined Src activation and EGF were synergistic for EGFR-Y845 and -Y1173 phosphorylation at some endosomes. We conclude that acute effects of Src in MDCK cells may mimic those of low EGF on EGFR activation and redistribution. Src:EGFR interactions may be sufficient to trigger EGFR activation and might contribute to its local signalling, without requiring either soluble extracellular signal or receptor over-expression.

A G{alpha}i-GIV molecular complex binds epidermal growth factor receptor and determines whether cells migrate or proliferate

Migrating cells do not proliferate and vice versa, but the mechanism involved remains unknown. Ghosh et al. reveal how this cellular decision is made by showing that a Gα_i–GIV molecular complex interacts with EGF receptor and programs growth factor signaling, triggering migration when assembled and favoring mitosis when assembly is prevented.

Cells respond to growth factors by either migrating or proliferating, but not both at the same time, a phenomenon termed migration-proliferation dichotomy. The underlying mechanism of this phenomenon has remained unknown. We demonstrate here that Gα_i protein and GIV, its nonreceptor guanine nucleotide exchange factor (GEF), program EGF receptor (EGFR) signaling and orchestrate this dichotomy. GIV directly interacts with EGFR, and when its GEF function is intact, a Gα_i–GIV–EGFR signaling complex assembles, EGFR autophosphorylation is enhanced, and the receptor's association with the plasma membrane (PM) is prolonged. Accordingly, PM-based motogenic signals (PI3-kinase-Akt and PLCγ1) are amplified, and cell migration is triggered. In cells expressing a GEF-deficient mutant, the Gαi–GIV-EGFR signaling complex is not assembled, EGFR autophosphorylation is reduced, the receptor's association with endosomes is prolonged, mitogenic signals (ERK 1/2, Src, and STAT5) are amplified, and cell proliferation is triggered. In rapidly growing, poorly motile breast and colon cancer cells and in noninvasive colorectal carcinomas in situ in which EGFR signaling favors mitosis over motility, a GEF-deficient splice variant of GIV was identified. In slow growing, highly motile cancer cells and late invasive carcinomas, GIV is highly expressed and has an intact GEF motif. Thus, inclusion or exclusion of GIV's GEF motif, which activates Gαi, modulates EGFR signaling, generates migration-proliferation dichotomy, and most likely influences cancer progression.

Epidermal growth factor receptor translocation to the mitochondria: regulation and effect

Co-overexpression of the epidermal growth factor (EGF) receptor (EGFR) and c-Src frequently occurs in human tumors and is linked to enhanced tumor growth. In experimental systems this synergistic growth requires EGF-dependent association of c-Src with the EGFR and phosphorylation of Tyr-845 of the receptor by c-Src. A search for signaling mediators of Tyr(P)-845 revealed that mitochondrial cytochrome c oxidase subunit II (CoxII) binds EGFR in a Tyr(P)-845- and EGF-dependent manner. In cells this association involves translocation of EGFR to the mitochondria, but regulation of this process is ill-defined. The current study demonstrates that c-Src translocates to the mitochondria with similar kinetics as EGFR and that the catalytic activity of EGFR and c-Src as well as endocytosis and a mitochondrial localization signal are required for these events. CoxII can be phosphorylated by EGFR and c-Src, and EGF stimulation reduces Cox activity and cellular ATP, an event that is dependent in large part on EGFR localized to the mitochondria. These findings suggest EGFR plays a novel role in modulating mitochondrial function via its association with, and modification of CoxII.

c-Src associates with ErbB2 through an interaction between catalytic domains and confers enhanced transforming potential

Previous studies have demonstrated that c-Src tyrosine kinase interacts specifically with ErbB2, but not with other members of the epidermal growth factor receptor (EGFR) family. To identify the site of interaction, we recently used a chimeric EGFR/ErbB2 receptor approach to show that c-Src requires the kinase region of ErbB2 for binding. Here, we demonstrate that retention of a conserved amino acid motif surrounding tyrosine 877 (referred to here as EGFR(YHAD)) is sufficient to confer binding to c-Src. Surprisingly the association of c-Src was not dependent on its SH2 or SH3 domain or on the phosphorylation or kinase activity of the receptor. We further show that the chimeric EGFRs that contain the Y877 motif are transforming in vitro and in vivo following ligand stimulation. Transformation was also partially dependent on sustained activation of Stat3. Finally, we demonstrate that EGFRs with mutations in the catalytic domain, originally identified in lung cancer and conferring increased sensitivity to gefitinib and erlotinib, two EGFR kinase inhibitors, gained the capacity to bind c-Src. Moreover, transformation by these EGFR mutants was inhibited by Src inhibitors regardless of their sensitivities to gefitinib and erlotinib. These observations have important implications for understanding the molecular basis for resistance to EGFR inhibitors and implicate c-Src as a critical signaling molecule in EGFR mutant-induced transformation.

Fyn tyrosine kinase regulates the surface expression of glycosylphosphatidylinositol-linked ephrin via the modulation of sphingomyelin metabolism

Glycosylphosphatidylinositol-linked ephrin-As play important roles in various biological events, such as neuronal development and immune responses. Because the surface amount of ephrin-As is critical in these events, the trafficking of ephrin-As must be regulated by intracellular machinery. In particular, Src family protein-tyrosine kinases regulate the intracellular trafficking of several membrane molecules and act downstream of ephrin-As; whether they affect the trafficking of ephrin-As, however, has remained unexplored. Here, we report that the activity of Src family protein-tyrosine kinases, particularly Fyn, negatively regulates the cell-surface amount of ephrin-As. The expression of constitutively active Fyn decreases the surface amount of ephrin-As. Conversely, the expression of dominant-negative Fyn or the application of a Src-family inhibitor increases the surface amount of ephrin-A2. The total cellular amount of ephrin-A is inversely correlated with its amount on the surface, suggesting that ephrin-As are more stable in the intracellular compartment. The expression of constitutively active Fyn increases the amount of sphingomyelin clusters on the plasma membrane, whereas inhibiting Fyn decreases it. Moreover, the inhibition of sphingomyelin synthesis greatly increases the surface amount of ephrin-As. Altogether, these results suggest that Fyn regulates the surface amount of ephrin-As by modulating the metabolism of sphingomyelin, which presumably inhibits the trafficking of ephrin-As from endosomes to the plasma membrane. The signaling cascade described here may function as part of the negative feedback loop of ephrin-A function.

Met and c-Src cooperate to compensate for loss of epidermal growth factor receptor kinase activity in breast cancer cells

Breast cancers are not responsive to epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKI), although 30% of breast cancers overexpress EGFR. The mechanism of intrinsic resistance to EGFR TKIs in breast cancer is the focus of current studies. Here, we observed that EGFR remains tyrosine phosphorylated in breast cancer cells that proliferate in the presence of EGFR TKIs. In one such cell line, SUM229, inhibiting c-Src kinase activity with either a dominant-negative c-Src or a c-Src TKI decreased EGFR phosphorylation on Tyr(845), Tyr(992), and Tyr(1086) in the presence of EGFR TKIs. Conversely, overexpressing wild-type (wt) c-Src in the EGFR TKI-sensitive breast cancer cell line SUM149 increased EGFR kinase-independent EGFR tyrosine phosphorylation. In addition, in the presence of EGFR TKIs, inhibiting c-Src kinase activity decreased cell growth in SUM229 cells, and overexpressing wt-c-Src increased cell growth in SUM149 cells. We identified the receptor tyrosine kinase Met to be responsible for activating c-Src in SUM229 cells. Inhibiting Met kinase activity with a small molecule inhibitor decreased c-Src phosphorylation and kinase activation. In addition, inhibiting Met kinase activity in SUM229 cells decreased EGFR tyrosine phosphorylation and growth in the presence of EGFR TKIs. Stimulating Met kinase activity in SUM149 cells with hepatocyte growth factor increased EGFR tyrosine phosphorylation and cell growth in the presence of EGFR TKIs. These data suggest a Met/c-Src-mediated signaling pathway as a mediator of EGFR tyrosine phosphorylation and cell growth in the presence of EGFR TKIs.

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

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

Regulation of sodium pump endocytosis by cardiotonic steroids: Molecular mechanisms and physiological implications

We have previously shown that ouabain and other cardiotonic steroids interact with the plasmalemmal Na/K-ATPase and cause a time and dose dependent endocytosis of the Na/K-ATPase. This endocytosis is demonstrable using fluorescence imaging as well as conventional biochemical and biophysical cell separation methods. In proximal tubule cells, this process appears to regulate the density of basolateral Na/K-ATPase expression directly as well as indirectly modulate transepithelial sodium transport. Work with genetic manipulations, as well as pharmacological agents with cell culture models, have demonstrated that the cardiotonic steroid stimulated endocytosis of the plasmalemmal Na/K-ATPase requires caveolin and clathrin as well as the activation of c-Src, transactivation of the EGFR and activation of PI3K. Interestingly c-Src, EGFR and ERK1/2 all appear to be endocytosed along with the plasmalemmal Na/K-ATPase. These observations suggest a close analogy between a subset of plasmalemmal Na/K-ATPase and signaling companions with conventional receptor tyrosine kinases. While further studies are necessary to delineate the role of this endocytosis in the generation as well as the limit of signal transduction through the Na/K-ATPase signal cascade, we propose that it has an important role in the regulation of renal sodium handling as well as other important processes.

Role of Src-family kinases in formation and trafficking of macropinosomes

Src-family kinases that localize to the cytoplasmic side of cellular membranes through lipid modification play a role in signaling events including membrane trafficking. Macropinocytosis is an endocytic process for solute uptake by large vesicles called macropinosomes. Although macropinosomes can be visualized following uptake of fluorescent macromolecules, little is known about the dynamics of macropinosomes in living cells. Here, we show that constitutive c-Src expression generates macropinosomes in a kinase-dependent manner. Live-cell imaging of GFP-tagged c-Src (Src-GFP) reveals that c-Src associates with macropinosomes via its N-terminus continuously from their generation at membrane ruffles, through their centripetal trafficking, to fusion with late endosomes and lysosomes. Fluorescence recovery after photobleaching (FRAP) of Src-GFP shows that Src-GFP is rapidly recruited to macropinosomal membranes from the plasma membrane and intracellular organelles through vesicle transport even in the presence of a protein synthesis inhibitor. Furthermore, using a HeLa cell line overexpressing inducible c-Src, we show that following stimulation with epidermal growth factor (EGF), high levels of c-Src kinase activity promote formation of macropinosomes associated with the lysosomal compartment. Unlike c-Src, Lyn and Fyn, which are palmitoylated Src kinases, only minimally induce macropinosomes, although a Lyn mutant in which the palmitoylation site is mutated efficiently induces macropinocytosis. We conclude that kinase activity of nonpalmitoylated Src kinases including c-Src may play an important role in the biogenesis and trafficking of macropinosomes.

The role of Src in solid and hematologic malignancies

c-Src was the first protooncogene described and was among the first molecules in which tyrosine kinase activity was documented. c-Src has been defined as a common modular structure that participates in much of the crosstalk between the cytoplasmic protein tyrosine kinases and tyrosine kinase receptors. Understanding the structure and function of this important class of protein kinases and elucidating the molecular signaling events mediated by c-Src are important not only for identifying the critical pathways but also for designing new strategies to block or inhibit the action of these kinases. Despite the large amount of information available on c-Src, its precise functions in cancer remain to be elucidated. Recently, there has been renewed interest in c-Src as a molecular target for cancer therapy, and multiple c-Src inhibitors are entering clinical trials. In this review, the authors describe the function and expression of c-Src in human malignancies and the novel c-Src inhibitors and their potential applications for cancer treatment.

Ouabain-induced endocytosis of the plasmalemmal Na/K-ATPase in LLC-PK1 cells requires caveolin-1

BACKGROUND

We have demonstrated that ouabain causes dose- and time-dependent decreases in (86)Rb uptake in pig renal proximal tubule cell line (LLC-PK1) cells; and ouabain induces endocytosis of plasmalemmal Na/K-ATPase in LLC-PK1 cells in a clathrin-dependent pathway. Our data also suggest a role of endocytosis in both ouabain-induced signal transduction and proximal tubule sodium handling. The present study addresses the molecular mechanisms involved in this process.

METHODS

Studies were performed with cultured LLC-PK1 and a stable-expressed caveolin-1 knockdown LLC-PK1 cell line by SiRNA method.

RESULTS

In wild-type LLC-PK1 cells, depletion of cholesterol by methyl beta-cyclodextrin reduced ouabain-induced accumulation of Na/K-ATPase alpha-1 subunit, EGFR, Src, and MAPKs in clathrin-coated vesicles, as well as in endosomes. Depletion of cholesterol also significantly reduced the protein-protein interaction among alpha-1 subunit, AP2, PI-3K, and clathrin heavy chain. In LLC-PK1 cells expressing mock-vehicle and caveolin-1 siRNA, depletion of caveolin-1 abolished ouabain-induced decrease in Rb uptake and decrease in the plasmalemmal Na/K-ATPase content. Depletion of caveolin-1 also significantly reduced the ouabain-induced accumulation of Na/K-ATPase alpha-1 subunit, EGFR, Src, and MAPKs in clathrin-coat vesicles, as well as early and late endosomes. In addition, depletion of caveolin-1 also significantly reduced the protein-protein interaction among alpha-1 subunit, AP2, PI-3K, and clathrin heavy chain. These data suggest that caveolae are involved in ouabain-induced endocytosis and signal transduction by initiating assembly of signaling cascades through the caveolar Na/K-ATPase and/or the interaction with clathrin-mediated endocytosis of the Na/K-ATPase.

Ouabain induces endocytosis of plasmalemmal Na/K-ATPase in LLC-PK1 cells by a clathrin-dependent mechanism

BACKGROUND

We have demonstrated that ouabain causes dose- and time-dependent decreases in (86)Rb uptake in porcine proximal tubular (LLC-PK1) cells. The present study addresses the molecular mechanisms involved in this process.

METHODS

Studies were performed with cultured LLC-PK1 and Src family kinase deficient (SYF) cells.

RESULTS

We found that 50 nmol/L ouabain applied to the basal, but not apical, aspect for 12 hours caused decreases in the plasmalemmal Na/K-ATPase. This loss of plasmalemmal Na/K-ATPase reverses completely within 12 to 24 hours after removal of ouabain. Ouabain also increased the Na/K-ATPase content in both early and late endosomes, activated phosphatidylinositol 3-kinase (PI(3)K), and also caused a translocation of some Na/K-ATPase to the nucleus. Immunofluorescence demonstrated that the Na/K-ATPase colocalized with clathrin both before and after exposure to ouabain, and immunoprecipitation experiments confirmed that ouabain stimulated interactions among the Na/K-ATPase, adaptor protein-2 (AP-2), and clathrin. Potassium (K) depletion, chlorpromazine, or PI(3)K inhibition all significantly attenuated this ouabain-induced endocytosis. Inhibition of the ouabain-activated signaling process through Src by 4-Amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) significantly attenuated ouabain-induced endocytosis. Moreover, experiments performed in SYF cells demonstrated that ouabain induced increases in the endocytosis of the Na/K-ATPase when Src was reconstituted (SYF+), but not in the Src-deficient (SYF-) cells.

CONCLUSION

These data demonstrate that ouabain stimulates a clathrin-dependent endocytosis pathway that translocates the Na/K-ATPase to intracellular compartments, thus suggesting a potential role of endocytosis in ouabain-induced signal transduction as well as proximal tubule sodium handling.

Normal and Oncogenic Forms of the Receptor Tyrosine Kinase Kit

Kit is a receptor tyrosine kinase (RTK) that binds stem cell factor. This receptor ligand combination is important for normal hematopoiesis, as well as pigmentation, gut function, and reproduction. Structurally, Kit has both an extracellular and intracellular region. Theintra-cellular region is comprised of a juxtamembrane domain (JMD), a kinase domain, a kinase insert, and a carboxyl tail. Inappropriate expression or activation of Kit is associated with a variety of diseases in humans. Activating mutations in Kit have been identified primarily in the JMD and the second part of the kinase domain and have been associated with gastrointestinal stromal cell tumors and mastocytosis, respectively. There are also reports of activating mutations in some forms of germ cell tumors and core binding factor leukemias. Since the cloning of the Kit ligand in the early 1990s, there has been an explosion of information relating to the mechanism of action of normal forms of Kit as well as activated mutants. This is important because understanding this RTK at the biochemical level could assist in the development of therapeutics to treat primary and secondary defects in the tissues that require Kit. Furthermore, understanding the mechanisms mediating transformation of cells by activated Kit mutants will help in the design of interventions for human disease associated with these mutations. The objective of this review is to summarize what is known about normal and oncogenic forms of Kit. We will place particular emphasis on recent developments in understanding the mechanisms of action of normal and activated forms of this RTK and its association with human disease, particularly in hematopoietic cells.

The interplay between Src family kinases and receptor tyrosine kinases

Src family tyrosine kinases (SFKs) are involved in a diverse array of physiological processes, as highlighted in this review. An overview of how SFKs interact with, and participate in signaling from, receptor tyrosine kinases (RTKs) is discussed. And also, how SFKs are activated by RTKs, and how SFKs, in turn, can activate RTKs, as well as how SFKs can promote signaling from growth factor receptors in a number of ways including participation in signaling pathways required for DNA synthesis, control of receptor turnover, actin cytoskeleton rearrangements and motility, and survival are discussed.

Phosphorylation of Y845 on the epidermal growth factor receptor mediates binding to the mitochondrial protein cytochrome c oxidase subunit II

When co-overexpressed, the epidermal growth factor receptor (EGFR) and c-Src cooperate to cause synergistic increases in EGF-induced DNA synthesis, soft agar colony growth, and tumor formation in nude mice. This synergy is dependent upon c-Src-mediated phosphorylation of a unique tyrosine on the EGFR, namely, tyrosine 845 (Y845). Phenylalanine substitution of Y845 (Y845F) was found to inhibit EGF-induced DNA synthesis without affecting the catalytic activity of the receptor or its ability to phosphorylate Shc or activate mitogen-activated protein kinase. These results suggest that synergism may occur through alternate signaling pathways mediated by phosphorylated Y845 (pY845). One such pathway involves the transcription factor Stat5b. Here we describe another pathway that involves cytochrome c oxidase subunit II (CoxII). CoxII was identified as a specific binding partner of a pY845-containing peptide in a phage display screen. EGF-dependent binding of CoxII to the wild type but not to the mutant Y845F-EGFR was confirmed by coimmunoprecipitation experiments. This association also required the kinase activity of c-Src. Confocal microscopy, as well as biochemical fractionation, indicated that the EGFR translocates to the mitochondria after EGF stimulation, where it colocalizes with CoxII. Such translocation required the catalytic activity of the receptor but not phosphorylation of Y845. However, ectopic expression of the Y845F-EGFR prevented the EGF from protecting MDA-MB-231 breast cancer cells from adriamycin-induced apoptosis, whereas two mutants of Stat5b, a dominant-interfering mutant (DNstat5b) and a tyrosine mutation at 699 (Y699F-Stat5b) did not. Taken together, these data suggest that, through the ability of EGFR to translocate to the mitochondria, the binding of proteins such as CoxII to pY845 on the EGFR may positively regulate survival pathways that contribute to oncogenesis.

Apc deficiency is associated with increased Egfr activity in the intestinal enterocytes and adenomas of C57BL/6J-Min/+ mice

Overexpression of the epidermal growth factor receptor (EGFR) and its increased tyrosine kinase activity are implicated in colorectal cancer (CRC) development and malignant progression. The C57BL/6J-Min/+ (Min/+) mouse is a model for CRC and develops numerous intestinal adenomas. We analyzed the normal mucosa of Min/+ and Apc+/+ (WT) littermate mice together with Apc-null adenomas to gain insight into the roles of Egfr in these intestinal tissues. Protein analyses showed that Egfr activity was highest in the tumors, and also up-regulated in Min/+ relative to WT enterocytes. Expression of ubiquitylated Egfr (Egfr-Ub) was increased in Min/+ enterocytes and tumors. Tumors exhibited increased association of Egfr with clathrin heavy chain (CHC), Gab1, and p85alpha, the regulatory subunit of phosphoinositide 3-kinase (PI3K), and tumors also overexpressed c-Src, PDK1, and Akt. Immunohistochemistry for Akt-p-Ser473 revealed a low level of this active kinase in Min/+ and WT enterocytes and its strong presence in tumors. Prostaglandin E2 (PGE2) is a product of cyclooxygenase-2 (Cox-2) activity that is up-regulated in Min/+ tumors and transactivates Egfr. PGE2 expression was significantly higher in untreated Min/+ tumors and reduced by treatment with the Cox-2 inhibitor, celecoxib. Dietary administration of this NSAID also inhibited Egfr activity in tumors. Increased activation of the EGFR-PI3K-Akt signaling pathway in tumors relative to Apc+/+ and ApcMin/+ enterocytes provides potential opportunities for therapeutic interventions to differentially suppress tumor formation, promotion, progression, and/or recurrence.

The C terminus of c-Src inhibits breast tumor cell growth by a kinase-independent mechanism

Overexpression or increased activity of cellular Src (c-Src) is frequently detected in human breast cancer, implicating involvement of c-Src in the etiology of breast carcinomas. Curiously, overexpression of c-Src in tissue culture cells results in a weakly or non-transforming phenotype, indicating that it alone is not sufficient for oncogenesis. However, the protein has been demonstrated to potentiate mitogenic signals from transmembrane receptors. This report investigates the requirement for c-Src in breast cancer as a transducer and integrator of anchorage-dependent and -independent growth signals by utilizing the Src family pharmacological inhibitors, PP1 and PP2, or stable overexpression of the catalytically inactive c-Src mutant (K- c-Src). Both methods of inhibiting endogenous c-Src diminished formation of soft agar colonies and tumors in nude mice. The majority of the dominant-negative activity of K- c-Src was mapped to the Src homology 2 (SH2) domain and C-terminal half of the molecule, but not to the Unique domain, Src homology 3 (SH3) domain, or the N-terminal half of K- c-Src. Further analysis of the C terminus revealed that its ability to inhibit growth localized to the N-terminal lobe (N-lobe) of the catalytic region. These results underscore the requirement for c-Src to maintain the oncogenic phenotype of breast cancer cells and suggest that c-Src may be manipulated to inhibit cell growth by the direct disruption of its catalytic activity or the introduction of either the SH2 domain or the N-lobe of K- c-Src.

The tissue engineeting puzzle: a molecular perspective

The inability of biomaterial scaffolds to functionally integrate into surrounding tissue is one of the major roadblocks to developing new biomaterials and tissue-engineering scaffolds. Despite considerable advances, current approaches to engineering cell-surface interactions fall short in mimicking the complexity of signals through which surrounding tissue regulates cell behavior. Cells adhere and interact with their extracellular environment via integrins, and their ability to activate associated downstream signaling pathways depends on the character of adhesion complexes formed between cells and their extracellular matrix. In particular, alpha5beta1 and alphavbeta3 integrins are central to regulating downstream events, including cell survival and cell-cycle progression. In contrast to previous findings that alphavbeta3 integrins promote angiogenesis, recent evidence argues that alphavbeta3 integrins may act as negative regulators of proangiogenic integrins such as alpha5beta1. This suggests that fibronectin is critical for scaffold vascularization because it is the only mammalian adhesion protein that binds and activates alpha5beta1 integrins. Cells are furthermore capable of stretching fibronectin matrices such that the protein partially unfolds, and recent computational simulations provide structural models of how mechanical stretching affects fibronectin function. We propose a model whereby excessive tension generated by cells in contact to biomaterials may in fact render fibronectin fibrils nonangiogenic and potentially inhibit vascularization. The model could explain why current biomaterials independent of their surface chemistries and textures fail to vascularize.

Mechanism of Csk-mediated down-regulation of Src family tyrosine kinases in epidermal growth factor signaling

The Src family tyrosine kinases (SFKs) play pivotal roles as molecular switches that link a variety of extracellular cues to intracellular signaling pathway. The function of SFK is regulated by phosphorylation at the C-terminal regulatory site mediated by Csk. Recently a novel SFK target Cbp (or PAG) was identified as a membrane-anchored scaffold protein for Csk. To establish the mechanism of Csk/Cbp-mediated regulation of SFK in vivo, we observed dynamic changes in the interaction of Csk with Cbp by utilizing fusion proteins with modified green fluorescent proteins: cyan fluorescent protein (CFP) or yellow fluorescent protein (YFP). Upon SFK activation induced by epidermal growth factor stimulation, fluorescent resonance energy transfer (FRET) response was detected transiently at membrane ruffles in COS1 cells co-expressing CFP-Csk and Cbp-YFP and in cells expressing a single-molecule FRET indicator consisting of CskSH2 and Cbp. Suppression of SFK by PP2 or use of a mutant Cbp that lacks the Csk binding site abolished the FRET response, although a dominant-negative form of Csk enhanced and sustained the FRET response, demonstrating that the FRET response is dependent upon the SFK activity. These observations show that Csk/Cbp-mediated down-regulation of SFK takes place at membrane ruffles in an early stage of epidermal growth factor signaling and suggest that the Csk/Cbp-based FRET indicators are useful for monitoring the status of SFK in living cells.

Src promotes destruction of c-Cbl: implications for oncogenic synergy between Src and growth factor receptors

Cellular Src and epidermal growth factor receptor (EGFR) collaborate in the progression of certain human malignancies, and their cooverexpression characterizes relatively aggressive animal tumors. Our study addressed the mode of oncogenic cooperation and reports that overexpression of c-Src in model cellular systems results in the accumulation of EGFR at the cell surface. The underlying mechanism involves inhibition of the normal, c-Cbl-regulated process of ligand-induced receptor down-regulation. In response to activation of c-Src, c-Cbl proteins undergo tyrosine phosphorylation that promotes their ubiquitylation and proteasomal destruction. Consequently, ubiquitylation of EGFR by c-Cbl is restrained in Src-transformed cells, and receptor sorting to endocytosis is impaired. In conclusion, by promoting destruction of c-Cbl, c-Src enables EGFR to evade desensitization, which explains Src-EGFR collaboration in oncogenesis.

Role for cellular Src kinase in myoblast proliferation

In this study, a role for cellular Src in muscle cell proliferation and differentiation was investigated. Pharmacological inhibition of Src-class kinases repressed proliferation and promoted differentiation of the C2C12 muscle cell line, even when the cells were cultured under growth-inducing conditions of high serum. Pharmacological inhibition of Src-class kinases also affected cellular components that regulate proliferation and differentiation in muscle; cyclin D1 levels were reduced while, myogenin was increased. Suppression of cyclin D1 and enhancement of myogenin levels also occurred upon expression of a dominant negative Src, corroborating a role for Src kinases in regulating proliferation and differentiation. Inhibition of Src-family kinases also blocked fibroblast growth factor (FGF) induced proliferation but, notably, did not reverse the effect of FGF to inhibit differentiation. Evidence for the Src-class kinase Src in myoblast mitogenesis was obtained by determining the pattern of protein expression and activity for this kinase. Under all conditions examined, Src's expression and enzymatic activity were high in cultures of myoblasts and down-regulated during differentiation. Importantly, Src's activity was rapidly stimulated by mitogen-containing serum and attenuated when myoblasts were switched to low serum-containing differentiation medium. These data indicate that Src is important for maintaining muscle cell proliferation.

Src-dependent tyrosine phosphorylation regulates dynamin self-assembly and ligand-induced endocytosis of the epidermal growth factor receptor

Endocytosis of ligand-activated receptors requires dynamin-mediated GTP hydrolysis, which is regulated by dynamin self-assembly. Here, we demonstrate that phosphorylation of dynamin I by c-Src induces its self-assembly and increases its GTPase activity. Electron microscopic analyses reveal that tyrosine-phosphorylated dynamin I spontaneously self-assembles into large stacks of rings. Tyrosine 597 was identified as being phosphorylated both in vitro and in cultured cells following epidermal growth factor receptor stimulation. The replacement of tyrosine 597 with phenylalanine impairs Src kinase-induced dynamin I self-assembly and GTPase activity in vitro. Expression of Y597F dynamin I in cells attenuates agonist-driven epidermal growth factor receptor internalization. Thus, c-Src-mediated tyrosine phosphorylation is required for the function of dynamin in ligand-induced signaling receptor internalization.

Regulation of ROMK channels by protein tyrosine kinase and tyrosine phosphatase

Renal outer medulla K (ROMK) channels play an important role in K recycling in the thick ascending limb and in K secretion in the cortical collecting duct. ROMK1, a member of the ROMK family, has been shown to be a substrate for protein tyrosine kinase (PTK). The tyrosine phosphorylation of ROMK channels increases with low dietary K intake and decreases with high dietary K intake. Moreover, the stimulation of tyrosine phosphorylation of ROMK1 channels decreases the number of K channels by facilitating endocytosis. In contrast, the stimulation of tyrosine dephosphorylation increases the number of ROMK1 channels in the cell membrane by enhancing membrane insertion. PTK and tyrosine phosphatase-induced regulation of ROMK1 channels play a key role in mediating the effect of the dietary K intake on renal K secretion.

Decrease in K-ras p21 and increase in Raf1 and activated Erk 1 and 2 in murine lung tumors initiated by N-nitrosodimethylamine and promoted by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Recent evidence suggests that K-ras protooncogene protein p21 may have a tumor-suppressive role in the context of development of lung adenocarcinoma. Levels of K-ras p21, raf-1, mitogen-activated protein kinases Erk 1 and 2, the phosphorylated-activated forms of Erk 1 and 2 (Erk 1P and 2P), and proliferating cell nuclear antigen (PCNA) were measured by immunoblotting in mouse lung tumors (5 to 9 mm in size) caused by N-nitrosodimethylamine (NDMA) and in control lungs. In tumors compared with normal lung, cell membrane-associated K-ras p21 was significantly decreased and cytosolic K-ras p21 increased. Total, membrane, and cytosolic raf-1 and Erk 1P and 2P were increased in tumors compared with normal lung. A single dose of 5 nmol/kg 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) given after NDMA resulted in a significant 2.4-fold increase in tumor multiplicity. A significantly greater decrease in membrane-associated K-ras p21 and increase in total and membrane associated raf-1 occurred in the NDMA/TCDD tumors compared with the NDMA-only tumors. PCNA levels increased in tumors, a finding confirmed by immunohistochemistry, and correlated with tumor size after NDMA/TCDD treatment but not after NDMA only. The increase in raf-1 in the tumors was confirmed by immunohistochemistry, which also revealed an increase in raf-1-positive alveolar macrophages specifically associating with tumors from the earliest stages. These results suggest a possible tumor-suppressive function for K-ras p21 in lung and a positive role for raf-1 and Erk 1/2 in lung tumorigenesis. TCDD may promote tumors by contributing to downregulation of K-ras and stimulation of raf-1.

'Srcasm: a novel Src activating and signaling molecule

The Src family tyrosine kinase, Fyn, can facilitate regulation of cell proliferation and differentiation. Mice with mutations in the fyn gene have defects in the brain, immune system, and epidermal differentiation. To identify molecules that may interact with Fyn in the epidermis, we performed a yeast two-hybrid interaction screen of a murine keratinocyte library. A novel adaptor-like molecule was isolated and termed Srcasm for Src activating and signaling molecule. Murine Srcasm is a 52.7-kDa protein that contains a VHS membrane association domain and a number of tyrosine motifs suggesting that it may be a substrate for Src family kinases and serve as an adaptor protein. Northern blot analysis of murine tissues demonstrates that Srcasm expression is highest in brain and kidney. In situ hybridization analysis reveals that srcasm mRNA is expressed in regions of the epidermis and hair follicle where keratinocyte differentiation occurs. In the brain, srcasm mRNA distribution correlates with that of fyn, with both being highly expressed in the hippocampal and cerebellar Purkinje neurons. Fyn can phosphorylate Srcasm, and association of these molecules relies on cooperative binding between the SH2 and SH3 domains of Fyn and corresponding canonical binding sites in Srcasm. Srcasm is capable of interacting with Grb2 and the regulatory subunit of phosphoinositide 3-kinase, p85, in a phosphorylation-dependent manner. The evidence suggests that Srcasm may help promote Src family kinase signaling in cells.

c-src tyrosine kinase is associated with the asialoglycoprotein receptor in human hepatoma cells

The asialoglycoprotein (ASGP) receptor is expressed on hepatocytes and liver-derived cell lines and is responsible for the endocytosis of galactose-terminal glycoproteins via the coated pit pathway. Prior data showed that tyrosine kinase activity plays an important role in this endocytic process, though the critical kinase(s) responsible for this effect are unknown. We have detected a 60-kDa protein which coprecipitates with ASGP receptor in detergent-solubilized lysates of HepG2 cells. This protein autophosphorylates and binds radioactive ATP. It comigrates with authentic pp60 c-src and is recognized by a specific anti-src monoclonal antibody. The kinase associated with the ASGP receptor retains the ability to phosphorylate exogenous substrates on tyrosine. In conclusion, the tyrosine kinase c-src associates with the ASGP receptor, a protein of the coated pit pathway of endocytosis.

Transforming growth factor beta receptor signaling and endocytosis are linked through a COOH terminal activation motif in the type I receptor

Transforming growth factor beta (TGF-beta) coordinates a number of biological events important in normal and pathophysiological growth. In this study, deletion and substitution mutations were used to identify receptor motifs modulating TGF-beta receptor activity. Initial experiments indicated that a COOH-terminal sequence between amino acids 482-491 in the kinase domain of the type I receptor was required for ligand-induced receptor signaling and down-regulation. These 10 amino acids are highly conserved in mammalian, Xenopus, and Drosophila type I receptors. Although mutation or deletion of the region (referred to as the NANDOR BOX, for nonactivating non-down-regulating) abolishes TGF-beta-dependent mitogenesis, transcriptional activity, type I receptor phosphorylation, and down-regulation in mesenchymal cultures, adjacent mutations also within the kinase domain are without effect. Moreover, a kinase-defective type I receptor can functionally complement a mutant BOX expressing type I receptor, documenting that when the BOX mutant is activated, it has kinase activity. These results indicate that the sequence between 482 and 491 in the type I receptor provides a critical function regulating activation of the TGF-beta receptor complex.

c-Cbl ubiquitinates the EGF receptor at the plasma membrane and remains receptor associated throughout the endocytic route

Cbl family members have an evolutionarily conserved role in attenuating receptor tyrosine kinase function. Their negative regulatory capacity depends on a Ring finger domain that interacts with ubiquitin conjugating enzymes. Cbl molecules constitute a novel type of E3 or ubiquitin ligase family that is recruited to phosphotyrosine motifs. Ubiquitination of the receptor system is coupled to its downregulation, but it is unclear at which point in the endocytic pathway Cbl molecules come into play. Using low temperature and a dynamin mutant, we find that c-Cbl associates with and ubiquitinates the activated epidermal growth factor (EGF) receptor at the plasma membrane in the absence of endocytosis. With the aid of confocal microscopy and immunogold electron microscopy, we could demonstrate that c-Cbl associates with the EGF receptor at the plasma membrane prior to receptor recruitment into clathrin-coated pits and remains associated throughout the clathrin-mediated endocytic pathway. c-Cbl and the EGF receptor also colocalize in internal vesicles of multivesicular endosomes. Our data are consistent with a role for c-Cbl in clathrin-mediated endocytosis of tyrosine kinase receptors, as well as their intracellular sorting.

The tyrosine kinase ACK1 associates with clathrin-coated vesicles through a binding motif shared by arrestin and other adaptors

One target for the small GTPase Cdc42 is the nonreceptor tyrosine kinase activated Cdc42-associated kinase (ACK), which binds selectively to Cdc42.GTP. We report that ACK1 can associate directly with the heavy chain of clathrin. A central region in ACK1 containing a conserved motif behaves as a clathrin adaptor and competes with beta-arrestin for a common binding site on the clathrin N-terminal head domain. Overexpressed ACK1 perturbs clathrin distribution, an activity dependent on the presence of C-terminal "adaptor" sequences that are also present in the related nonkinase gene 33. ACK1 interacts with the adaptor Nck via SH3 interactions but does not form a trimeric complex with p21-activated serine/threonine kinase, which also binds Nck. Stable low level expression of green fluorescent protein-ACK1 in NIH 3T3 cells has been used to localize ACK1 to clathrin-containing vesicles. The co-localization of ACK1 in vivo with clathrin and AP-2 indicates that it participates in trafficking, underlying an ability to increase receptor-mediated transferrin uptake.

Mammalian Abp1, a signal-responsive F-actin-binding protein, links the actin cytoskeleton to endocytosis via the GTPase dynamin

The actin cytoskeleton has been implicated in endocytosis, yet few molecular links to the endocytic machinery have been established. Here we show that the mammalian F-actin-binding protein Abp1 (SH3P7/HIP-55) can functionally link the actin cytoskeleton to dynamin, a GTPase that functions in endocytosis. Abp1 binds directly to dynamin in vitro through its SH3 domain. Coimmunoprecipitation and colocalization studies demonstrated the in vivo relevance of this interaction. In neurons, mammalian Abp1 and dynamin colocalized at actin-rich sites proximal to the cell body during synaptogenesis. In fibroblasts, mAbp1 appeared at dynamin-rich sites of endocytosis upon growth factor stimulation. To test whether Abp1 functions in endocytosis, we overexpressed several Abp1 constructs in Cos-7 cells and assayed receptor-mediated endocytosis. While overexpression of Abp1's actin-binding modules did not interfere with endocytosis, overexpression of the SH3 domain led to a potent block of transferrin uptake. This implicates the Abp1/dynamin interaction in endocytic function. The endocytosis block was rescued by cooverexpression of dynamin. Since the addition of the actin-binding modules of Abp1 to the SH3 domain construct also fully restored endocytosis, Abp1 may support endocytosis by combining its SH3 domain interactions with cytoskeletal functions in response to signaling cascades converging on this linker protein.

Regulation of ROMK1 channels by protein-tyrosine kinase and -tyrosine phosphatase

We have used the two-electrode voltage clamp technique and the patch clamp technique to investigate the regulation of ROMK1 channels by protein-tyrosine phosphatase (PTP) and protein-tyrosine kinase (PTK) in oocytes coexpressing ROMK1 and cSrc. Western blot analysis detected the presence of the endogenous PTP-1D isoform in the oocytes. Addition of phenylarsine oxide (PAO), an inhibitor of PTP, reversibly reduced K(+) current by 55% in oocytes coinjected with ROMK1 and cSrc. In contrast, PAO had no significant effect on K(+) current in oocytes injected with ROMK1 alone. Moreover, application of herbimycin A, an inhibitor of PTK, increased K(+) current by 120% and completely abolished the effect of PAO in oocytes coexpressing ROMK1 and cSrc. The effects of herbimycin A and PAO were absent in oocytes expressing the ROMK1 mutant R1Y337A in which the tyrosine residue at position 337 was mutated to alanine. However, addition of exogenous cSrc had no significant effect on the activity of ROMK1 channels in inside-out patches. Moreover, the effect of PAO was completely abolished by treatment of oocytes with 20% sucrose and 250 microg/ml concanavalin A, agents that inhibit the endocytosis of ROMK1 channels. Furthermore, the effect of herbimycin A is absent in the oocytes pretreated with either colchicine, an inhibitor of microtubules, or taxol, an agent that freezes microtubules. We conclude that PTP and PTK play an important role in regulating ROMK1 channels. Inhibiting PTP increases the internalization of ROMK1 channels, whereas blocking PTK stimulates the insertion of ROMK1 channels.

Molecular mechanisms underlying endocytosis and sorting of ErbB receptor tyrosine kinases

The major process that regulates the amplitude and kinetics of signal transduction by tyrosine kinase receptors is endocytic removal of active ligand-receptor complexes from the cell surface, and their subsequent sorting to degradation or to recycling. Using the ErbB family of receptor tyrosine kinases we exemplify the diversity of the down regulation process, and concentrate on two sorting steps whose molecular details are emerging. These are the Eps15-mediated sorting to clathrin-coated regions of the plasma membrane and the c-Cbl-mediated targeting of receptors to lysosomal degradation. Like in yeast cells, sorting involves not only protein phosphorylation but also conjugation of ubiquitin molecules. The involvement of other molecules is reviewed and recent observations that challenge the negative regulatory role of endocytosis are described. Finally, we discuss the relevance of receptor down regulation to cancer therapy.