The endophilin-CIN85-Cbl complex mediates ligand-dependent downregulation of c-Met

CD44-independent activation of the Met signaling pathway by HGF and InlB

Listeria monocytogenes is a facultative intracellular Gram-positive bacterium responsible for listeriosis. It is able to invade, survive and replicate in phagocytic and non-phagocytic cells. The L. monocytogenes surface protein InlB interacts with c-Met, the hepatocyte growth factor (HGF) receptor, inducing bacterial internalization in numerous non-phagocytic cells. As InlB and HGF are known to trigger similar signaling pathways upon c-Met activation, we investigated the role of CD44, and more specifically its isoform CD44v6, in bacterial internalization in non-phagocytic cells. Indeed, CD44, the hyaluronic acid transmembrane receptor, and more specifically its isoform CD44v6 have been reported as necessary for the activation of c-Met upon the interaction with either the endogenous ligand HGF or the L. monocytogenes surface protein InlB. Our results demonstrate that, in the cell lines that we used, CD44 receptors play no role in the activation of c-Met, neither during L. monocytogenes entry, nor upon HGF activation. Furthermore, none of the CD44 isoforms was recruited at the L. monocytogenes entry site, and depletion by siRNA of total CD44 or of CD44v6 isoform did not reduce bacterial infections. Conversely, the overexpression of CD44 or CD44v6 had no significant effect on L. monocytogenes internalization. Together our results reveal that the activation of c-Met can be largely CD44-independent.

Ubiquitination and endocytosis of the high affinity receptor for IgE

The high affinity receptor for IgE (FcvarepsilonRI) is constitutivelly expressed on the surface of mast cells and basophils as a multimeric complex. Upon antigen ligation to FcvarepsilonRI-bound IgE molecules, the receptor complex transduces intracellular signals leading to the release of preformed and newly synthesised pro-inflammatory mediators. FcvarepsilonRI engagement also generates negative intracellular signals involving the coordinated action of adapters, phosphatases and ubiquitin ligases that limits the intensity and duration of positive signals. Relevant to this, antigen-induced FcvarepsilonRI ubiquitination has become recognized as an important signal for the internalization and delivery of engaged receptor complexes to lysosomes for degradation. In this article, we review recent advances in our understanding of molecular mechanisms that guarantee the clearance of antigen-stimulated FcvarepsilonRI complexes from the cell surface. A particular emphasis will be given on how lipid rafts and the ubiquitin pathway cooperate to ensure receptor internalization and sorting along the endocytic compartments. A brief discussion regarding how ubiquitination regulates the endocytosis of Fc receptors other than FcvarepsilonRI will be included.

Novel oncogenic mutations of CBL in human acute myeloid leukemia that activate growth and survival pathways depend on increased metabolism

Acute myeloid leukemia (AML) is characterized by multiple mutagenic events that affect proliferation, survival, as well as differentiation. Recently, gain-of-function mutations in the α helical structure within the linker sequence of the E3 ubiquitin ligase CBL have been associated with AML. We identified four novel CBL mutations, including a point mutation (Y371H) and a putative splice site mutation in AML specimens. Characterization of these two CBL mutants revealed that coexpression with the receptor tyrosine kinases FLT3 (Fms-like tyrosine kinase 3) or KIT-induced ligand independent growth or ligand hyperresponsiveness, respectively. Growth of cells expressing mutant CBL required expression and kinase activity of FLT3. In addition to the CBL-dependent phosphorylation of FLT3 and CBL itself, transformation was associated with activation of Akt and STAT5 and required functional expression of the small GTPases Rho, Rac, and Cdc42. Furthermore, the mutations led to constitutively elevated intracellular reactive oxygen species levels, which is commonly linked to increased glucose metabolism in cancer cells. Inhibition of hexokinase with 2-deoxyglucose blocked the transforming activity of CBL mutants and reduced activation of signaling mechanisms. Overall, our data demonstrate that mutations of CBL alter cellular biology at multiple levels and require not only the activation of receptor proximal signaling events but also an increase in cellular glucose metabolism. Pathways that are activated by CBL gain-of-function mutations can be efficiently targeted by small molecule drugs.

CIN85 regulates dopamine receptor endocytosis and governs behaviour in mice

Despite extensive investigations of Cbl-interacting protein of 85 kDa (CIN85) in receptor trafficking and cytoskeletal dynamics, little is known about its functions in vivo. Here, we report the study of a mouse deficient of the two CIN85 isoforms expressed in the central nervous system, exposing a function of CIN85 in dopamine receptor endocytosis. Mice lacking CIN85 exon 2 (CIN85(Deltaex2)) show hyperactivity phenotypes, characterized by increased physical activity and exploratory behaviour. Interestingly, CIN85(Deltaex2) animals display abnormally high levels of dopamine and D2 dopamine receptors (D2DRs) in the striatum, an important centre for the coordination of animal behaviour. Importantly, CIN85 localizes to the post-synaptic compartment of striatal neurons in which it co-clusters with D2DRs. Moreover, it interacts with endocytic regulators such as dynamin and endophilins in the striatum. Absence of striatal CIN85 causes insufficient complex formation of endophilins with D2DRs in the striatum and ultimately decreased D2DR endocytosis in striatal neurons in response to dopamine stimulation. These findings indicate an important function of CIN85 in the regulation of dopamine receptor functions and provide a molecular explanation for the hyperactive behaviour of CIN85(Deltaex2) mice.

Role of ubiquitination in Na,K-ATPase regulation during lung injury

During acute lung injury edema accumulates in the alveolar space, resulting in hypoxemia due to intrapulmonary shunt. The alveolar Na,K-ATPase, by effecting active Na(+) transport, is essential for removing edema from the alveolar spaces. However, during hypoxia it is endocytosed and degraded, which results in decreased Na,K-ATPase function and impaired lung edema clearance. Na,K-ATPase endocytosis and degradation require the phosphorylation and subsequent ubiquitination of the Na,K-ATPase. These events are the results of cross-talk between post-translational modifications, and how ubiquitination of a specific protein can result from injurious extracellular stimuli. Here, we review current knowledge on the regulation of Na,K-ATPase activity during lung injury, focusing on the role of Na,K-ATPase ubiquitination during hypoxia. A better understanding of these signaling pathways can be of relevance for the design of novel treatments to ameliorate the deleterious effects of acute lung injury.

c-Cbl facilitates endocytosis and lysosomal degradation of cystic fibrosis transmembrane conductance regulator in human airway epithelial cells

Cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-activated Cl(-) channel expressed in the apical membrane of fluid-transporting epithelia. The apical membrane density of CFTR channels is determined, in part, by endocytosis and the postendocytic sorting of CFTR for lysosomal degradation or recycling to the plasma membrane. Although previous studies suggested that ubiquitination plays a role in the postendocytic sorting of CFTR, the specific ubiquitin ligases are unknown. c-Cbl is a multifunctional molecule with ubiquitin ligase activity and a protein adaptor function. c-Cbl co-immunoprecipitated with CFTR in primary differentiated human bronchial epithelial cells and in cultured human airway cells. Small interfering RNA-mediated silencing of c-Cbl increased CFTR expression in the plasma membrane by inhibiting CFTR endocytosis and increased CFTR-mediated Cl(-) currents. Silencing c-Cbl did not change the expression of the ubiquitinated fraction of plasma membrane CFTR. Moreover, the c-Cbl mutant with impaired ubiquitin ligase activity (FLAG-70Z-Cbl) did not affect the plasma membrane expression or the endocytosis of CFTR. In contrast, the c-Cbl mutant with the truncated C-terminal region (FLAG-Cbl-480), responsible for protein adaptor function, had a dominant interfering effect on the endocytosis and plasma membrane expression of CFTR. Moreover, CFTR and c-Cbl co-localized and co-immunoprecipitated in early endosomes, and silencing c-Cbl reduced the amount of ubiquitinated CFTR in early endosomes. In summary, our data demonstrate that in human airway epithelial cells, c-Cbl regulates CFTR by two mechanisms: first by acting as an adaptor protein and facilitating CFTR endocytosis by a ubiquitin-independent mechanism, and second by ubiquitinating CFTR in early endosomes and thereby facilitating the lysosomal degradation of CFTR.

The ORF3 protein of hepatitis E virus delays degradation of activated growth factor receptors by interacting with CIN85 and blocking formation of the Cbl-CIN85 complex

Hepatitis E virus (HEV) causes an acute self-limiting disease that is endemic in developing countries. Previous studies suggested that the ORF3 protein (pORF3) of HEV is required for infection in vivo and is likely to modulate the host response. Our previous work showed that pORF3 localizes to early and recycling endosomes and causes a delay in the postinternalization trafficking of epidermal growth factor receptor (EGFR) to late endosomes/lysosomes. Here we report that pORF3 also delays the trafficking and degradation of activated hepatocyte growth factor receptor (c-Met) and delineate the mechanistic details of these effects. A mutant ORF3 protein, which does not localize to endosomes, also showed similar effects on growth factor receptor trafficking, making this effect independent of the endosomal localization of pORF3. The ORF3 protein was found to interact with CIN85, a multidomain adaptor protein implicated in the Cbl-mediated downregulation of receptor tyrosine kinases. This interaction competed with the formation of the growth factor receptor-Cbl-CIN85 complex, resulting in the reduced ubiquitination of CIN85 and trafficking of the growth factor receptor complex toward late endosomes/lysosomes. We propose that through its effects on growth factor receptor trafficking, pORF3 prolongs endomembrane growth factor signaling and promotes cell survival to contribute positively to viral replication and pathogenesis.

Emerging roles of Ruk/CIN85 in vesicle-mediated transport, adhesion, migration and malignancy

Ruk/CIN85 is an adaptor protein. Similar to many other proteins of this type, Ruk/CIN85 is known to take part in multiple cellular processes including signal transduction, vesicle-mediated transport, cytoskeleton remodelling, programmed cell death and viral infection. Recent studies have also revealed the potential importance of Ruk/CIN85 in cancer cell invasiveness. In this review we summarize the various roles of this protein as well as the potential contribution of Ruk/CIN85 to malignancy and the invasiveness of cancer cells. In the last section of the paper we also speculate on the utility of Ruk/CIN85 as a target for novel anti-cancer therapies.

Phosphorylation of human tristetraprolin in response to its interaction with the Cbl interacting protein CIN85

Background

Tristetraprolin (TTP) is the prototype member of a family of CCCH tandem zinc finger proteins and is considered to be an anti-inflammatory protein in mammals. TTP plays a critical role in the decay of tumor necrosis factor alpha (TNF) mRNA, among others, by binding AU-rich RNA elements in the 3′-untranslated regions of this transcript and promoting its deadenylation and degradation.

Methodology/Principal Findings

We used yeast two-hybrid analysis to identify potential protein binding partners for human TTP (hTTP). Various regions of hTTP recovered 31 proteins that fell into 12 categories based on sequence similarities. Among these, the interactions between hTTP and CIN85, cytoplasmic poly (A) binding protein (PABP), nucleolin and heat shock protein 70 were confirmed by co-immunoprecipitation experiments. CIN85 and hTTP co-localized in the cytoplasm of cells as determined by confocal microscopy. CIN85 contains three SH3 domains that specifically bind a unique proline-arginine motif (PXXXPR) found in several CIN85 effectors. We found that the SH3 domains of CIN85 bound to a PXXXPR motif located near the C-terminus of hTTP. Co-expression of CIN85 with hTTP resulted in the increased phosphorylation of hTTP at serine residues in positions 66 and 93, possibly due in part to the demonstrated association of mitogen-activated protein kinase kinase kinase 4 (MEKK4) to both proteins. The presence of CIN85 did not appear to alter hTTP's binding to RNA probes or its stimulated breakdown of TNF mRNA.

Conclusions/Significance

These studies describe interactions between hTTP and nucleolin, cytoplasmic PABP, heat shock protein 70 and CIN85; these interactions were initially discovered by two-hybrid analysis, and confirmed by co-immunoprecipitation. We found that CIN85 binding to a C-terminal motif within hTTP led to the increased phosphorylation of hTTP, possibly through enhanced association with MEKK4. The functional consequences to each of the members of this putative complex remain to be determined.

Selection criteria for c-Met-targeted therapies: emerging evidence for biomarkers

Extensive development of targeted therapies emphasize the critical need for biomarkers and major efforts have been engaged to identify screening, prognostic, stratification and therapy-monitoring markers. One of the challenges in translating preclinical studies into effective clinical therapies remains the accurate identification of a responsive subsets of patients. Studies on trastuzumab demonstrated that patient response could be specifically correlated with the amplification of the Her2 gene. However, for the EGF receptor, it has been more difficult to find the right stratification biomarker and recent data demonstrate that genetic alterations for the EGF receptor have to be considered. Taken together, these data underline the need for a deeper understanding of both targeted receptor and human disease to determine pathways that might be investigated during early clinical trials in order to define relevant biomarkers for patient selection. This article, dealing with the c-Met tyrosine kinase receptor, provides an overview of c-Met alterations observed in cancer and proposes approaches for stratification biomarker selection.

Coordinated actions of actin and BAR proteins upstream of dynamin at endocytic clathrin-coated pits

The GTPase dynamin, a key player in endocytic membrane fission, interacts with numerous proteins that regulate actin dynamics and generate/sense membrane curvature. To determine the functional relationship between these proteins and dynamin, we have analyzed endocytic intermediates that accumulate in cells that lack dynamin (derived from dynamin 1 and 2 double conditional knockout mice). In these cells, actin-nucleating proteins, actin, and BAR domain proteins accumulate at the base of arrested endocytic clathrin-coated pits, where they support the growth of dynamic long tubular necks. These results, which we show reflect the sequence of events in wild-type cells, demonstrate a concerted action of these proteins prior to, and independent of, dynamin and emphasize similarities between clathrin-mediated endocytosis in yeast and higher eukaryotes. Our data also demonstrate that the relationship between dynamin and actin is intimately connected to dynamin's endocytic role and that dynamin terminates a powerful actin- and BAR protein-dependent tubulating activity.

The function, proteolytic processing, and histopathology of Met in cancer

The hepatocyte growth factor (HGF) and its receptor, the Met receptor tyrosine kinase, form a signaling network promoting cell proliferation, invasion, and survival in normal and cancer cells. Improper regulation of this pathway is attributed to many cancer types through overexpression, activating mutations, or autocrine loop formation. Many studies describe the localization of Met as membranous/cytoplasmic, but some studies using antibodies targeted to the C-terminal domain of Met report nuclear localization. This chapter seeks to highlight the histopathology and expression of Met in cancer and its association with clinicopathological characteristics. We also discuss recent studies of the proteolytic processing of Met and effects of the processing on the subcellular localization of Met. Finally, we comment on Met as a therapeutic target for cancer treatment.

A novel small molecule Met inhibitor, PF2362376, exhibits biological activity against osteosarcoma

The receptor tyrosine kinase Met is dysregulated in several human cancers including osteosarcoma (OSA) in which overexpression is a negative prognostic indicator and enforced Met expression in normal osteoblasts leads to genomic instability and malignant transformation. Met is also known to be inappropriately expressed in canine OSA tumour samples and cell lines. The purpose of this study was to evaluate the potential utility of an orally bioavailable small molecule Met inhibitor, PF2362376, against canine OSA cell lines as a prelude to future clinical work. PF2362376 inhibited phosphorylation of Met, Gab-1, Erk and Akt, but not of Src or STAT3. Furthermore, PF2362376 inhibited proliferation of canine OSA cell lines and induced cell death at biologically achievable concentrations. Last, activities associated with Met signalling including migration, invasion, branching morphogenesis and colony formation in soft agar were blocked by PF2362376. These studies support the notion that Met is a relevant target for therapeutic intervention in OSA.

Differential inhibition sensitivities of MET mutants to the small molecule inhibitor SU11274

Point mutations emerge as one of the rate-limiting steps in tumor response to small molecule inhibitors of protein kinases. Here we characterized the response of the MET mutated variants, V1110I, V1238I, V1206L and H1112L to the small molecule SU11274. Our results reveal a distinct inhibition pattern of the four mutations with IC(50) values for autophosphorylation inhibition ranging between 0.15 and 1.5muM. Differences were further seen on the ability of SU11274 to inhibit phosphorylation of downstream MET transducers such as AKT, ERK, PLCgamma and STAT3 and a variety of MET-dependent biological endpoints. In all the assays, H1112L was the most sensitive to SU11274, while V1206L was less affected under the used concentration range. The differences in responses to SU11274 are discussed based on a structural model of the MET kinase domain.

Loss of Cbl-b increases osteoclast bone-resorbing activity and induces osteopenia

Cbl proteins are multifunctional adaptor molecules that modulate cellular activity by targeting the ubiquitylating system, endocytic complexes, and other effectors to a wide variety of regulatory proteins, especially activated receptor and nonreceptor tyrosine kinases. Cbl and Cbl-b perform unique functions in various cells, in addition to redundant functions that are required for embryonic development. We previously showed that eliminating Cbl impaired osteoclast motility, which modestly delayed embryonic bone development. We now report that Cbl-b(-/-) mice are osteopenic, because of increased bone resorption with little compensating increase in bone formation. In vitro bone-resorbing activity and differentiation of osteoclast-like cells (OCLs) were increased, as were some RANKL-induced signaling events (activation of NF-kappaB and the mitogen-activated protein kinases extracellular signal-regulated kinase [ERK] and p38), suggesting that specific RANKL-activated mechanisms contribute to the increased rate of differentiation and bone-resorbing activity. Re-expressing Cbl-b in Cbl-b(-/-) OCLs normalized the increased bone-resorbing activity and overexpressing Cbl-b in wildtype OCLs inhibited bone resorption. Cbl was without effect in either wildtype or Cbl-b(-/-) OCLs. Functional tyrosine kinase binding (TKB) and RING finger domains were required for the rescue by Cbl-b. Thus, both Cbl and Cbl-b perform regulatory functions in osteoclasts that are unique to one or the other protein (i.e., functions that cannot be compensated by the other homolog). One of Cbl-b's unique functions in osteoclasts is to downregulate bone resorption.

Intermolecular interactions of Sprouty proteins and their implications in development and disease

Receptor tyrosine kinase (RTK) signaling is spatially and temporally regulated by a number of positive and negative regulatory mechanisms. These regulatory mechanisms control the amplitude and duration of the signals initiated at the cell surface to have a normal or aberrant biological outcome in development and disease, respectively. In the past decade, the Sprouty (Spry) family of proteins has been identified as modulators of RTK signaling in normal development and disease. This review summarizes recent advances concerning the biological activities modulated by Spry family proteins, their interactions with signaling proteins, and their involvement in cardiovascular diseases and cancer. The diversity of mechanisms in the regulation of Spry expression and activity in cell systems emphasizes the crucial role of Spry proteins in development and growth across the animal kingdom.

Functions of actin in endocytosis

Endocytosis is a fundamental eukaryotic process required for remodelling plasma-membrane lipids and protein to ensure appropriate membrane composition. Increasing evidence from a number of cell types reveals that actin plays an active, and often essential, role at key endocytic stages. Much of our current mechanistic understanding of the endocytic process has come from studies in budding yeast and has been facilitated by yeast's genetic amenability and by technological advances in live cell imaging. While endocytosis in metazoans is likely to be subject to a greater array of regulatory signals, recent reports indicate that spatiotemporal aspects of vesicle formation requiring actin are likely to be conserved across eukaryotic evolution. In this review we focus on the 'modular' model of endocytosis in yeast before highlighting comparisons with other cell types. Our discussion is limited to endocytosis involving clathrin as other types of endocytosis have not been demonstrated in yeast.

Decorin is a novel antagonistic ligand of the Met receptor

Decorin, a member of the small leucine-rich proteoglycan gene family, impedes tumor cell growth by down-regulating the epidermal growth factor receptor. Decorin has a complex binding repertoire, thus, we predicted that decorin would modulate the bioactivity of other tyrosine kinase receptors. We discovered that decorin binds directly and with high affinity (K_d = ∼1.5 nM) to Met, the receptor for hepatocyte growth factor (HGF). Binding of decorin to Met is efficiently displaced by HGF and less efficiently by internalin B, a bacterial Met ligand. Interaction of decorin with Met induces transient receptor activation, recruitment of the E3 ubiquitin ligase c-Cbl, and rapid intracellular degradation of Met (half-life = ∼6 min). Decorin suppresses intracellular levels of β-catenin, a known downstream Met effector, and inhibits Met-mediated cell migration and growth. Thus, by antagonistically targeting multiple tyrosine kinase receptors, decorin contributes to reduction in primary tumor growth and metastastic spreading.

Breakdown of endocytosis in the oncogenic activation of receptor tyrosine kinases

There is increasing evidence to support the concept that the malignant behavior of many tumors is sustained by the deregulated activation of growth factor receptors. Activation of receptor tyrosine kinases (RTKs) by their respective ligand(s) initiates cellular signals that tightly modulate cell proliferation, survival, differentiation and migration to ensure normal tissue patterning. Therefore, uncontrolled activation of such signals can have deleterious effects, leading to oncogenesis. To date, deregulation of most RTKs has been implicated in the development of cancer, although the mechanisms that lead to their deregulation are not yet fully understood (10). RTK endocytosis, the internalization and trafficking of receptors inside the cell, has long been established as a mechanism to attenuate RTK signaling. However, RTKs have been demonstrated to continue to signal along the endocytic pathway, which contributes to the spatio-temporal regulation of signal transduction. This review will focus on recent advances linking defective endocytosis of RTKs in the development of cancer.

Involvement of CD44v6 in InlB-dependent Listeria invasion

Listeria monocytogenes, a Gram-positive bacterium, is the causative agent for the disease called listeriosis. This pathogen utilizes host cell surface proteins such as E-cadherin or c-Met in order to invade eukaryotic cells. The invasion via c-Met depends on the bacterial protein InlB that activates c-Met phosphorylation and internalization mimicking in many regards HGF, the authentic c-Met ligand. In this paper, we demonstrate that the activation of c-Met induced by InlB is dependent on CD44v6, a member of the CD44 family of transmembrane glycoproteins. Inhibiting CD44v6 by means of a blocking peptide, a CD44v6 antibody or CD44v6-specific siRNA prevents the activation of c-Met induced by InlB. Subsequently, signalling, scattering and the entry of InlB-coated beads into host cells are also impaired by CD44v6 blocking reagents. For the entry process, ezrin, a protein that links the CD44v6 cytoplasmic domain to the cytoskeleton, is required as well. Most importantly, this collaboration between c-Met and CD44v6 contributes to the invasion of L. monocytogenes into target cells as demonstrated by a drastic decrease in bacterial invasion in the presence of blocking agents such as the CD44v6 peptide or antibody.

The MET axis as a therapeutic target

The MET receptor tyrosine kinase and its ligand hepatocyte growth factor (HGF) have been implicated in transformation of a variety of malignancies. Chronic or dysregulated activation of the MET/HGF pathway may lead to increased cell growth, invasion, angiogenesis, and metastasis, reduced apoptosis, altered cytoskeletal functions and other biological changes. It has been suggested that ligand activated MET stimulation can be sufficient for a transforming phenotype. In addition, amplification and activation mutations (germline and/or somatic) within the tyrosine kinase domain, juxtamembrane domain, or semaphorin domain have been identified for MET. MET gain-of-function mutations lead to either deregulated or prolonged tyrosine kinase activity, which are instrumental to its transforming activity. A number of therapeutic strategies targeting ligand-dependent activation or the kinase domain have been employed to inhibit MET. The different structural requirements for activation of signaling events and biological functions regulated by MET will be summarized. Therapeutic targets and current pre-clinical and clinical approaches will be described. Targeting the HGF/MET pathway, alone or in combination with standard therapies, is likely to improve present therapies in MET-dependent malignancies.

Down-regulation of the met receptor tyrosine kinase by presenilin-dependent regulated intramembrane proteolysis

Hepatocyte growth factor/scatter factor (HGF/SF) acts through the membrane-anchored Met receptor tyrosine kinase to induce invasive growth. Deregulation of this signaling is associated with tumorigenesis and involves, in most cases, overexpression of the receptor. We demonstrate that Met is processed in epithelial cells by presenilin-dependent regulated intramembrane proteolysis (PS-RIP) independently of ligand stimulation. The proteolytic process involves sequential cleavage by metalloproteases and the gamma-secretase complex, leading to generation of labile fragments. In normal epithelial cells, although expression of cleavable Met by PS-RIP is down-regulated, uncleavable Met displayed membrane accumulation and induced ligand-independent motility and morphogenesis. Inversely, in transformed cells, the Met inhibitory antibody DN30 is able to promote Met PS-RIP, resulting in down-regulation of the receptor and inhibition of the Met-dependent invasive growth. This demonstrates the original involvement of a proteolytic process in degradation of the Met receptor implicated in negative regulation of invasive growth.

Role of multivesicular bodies and their components in the egress of enveloped RNA viruses

As an enveloped virus buds, the nascent viral capsid becomes wrapped in a plasma membrane-derived lipid envelope, and a membrane fission event is thus necessary to separate the virion from the host cell. This membrane fission event is well characterised in the case of enveloped RNA viruses, where it is promoted by late assembly domains (L-domains) present at the level of specific viral structural proteins. Research conducted over the past 10 years has demonstrated that L-domains represent docking sites for cellular proteins essential for the biogenesis of a cellular organelle, the multivesicular body (MVB). In this way, enveloped RNA viruses hijack the MVB components to the cellular site where the budding is executed. This review will focus on the cellular machinery exploited by enveloped RNA viruses in order to be released from infected cells. The role of ubiquitin and lipids in viral budding will also be discussed.

ErbB2 and ErbB4 Cbl binding sites can functionally replace the ErbB1 Cbl binding site

Poor downregulation of ErbB receptors is associated with enhanced downstream signaling and tumorigenesis. It has been suggested that poor downregulation of ErbB-2, -3 and -4 receptors when compared to ErbB1 is due to decreased recruitment of Cbl E3 ligase proteins. However, a highly conserved Cbl binding site is not only present in ErbB1/EGFR (FLQRpY(1045)SSDP), but also in ErbB2 (PLQRpY(1091)SEDP) and ErbB4 (STQRpY(1103)SADP). We therefore replaced the ErbB1 Cbl binding site by that of ErbB2 and ErbB4. Whereas retrovirally infected NIH3T3 cells containing the EGFR Y1045F mutation showed dramatically impaired Cbl recruitment, EGFR ubiquitination and delayed EGFR degradation, replacement of the EGFR Cbl binding site by that of ErbB2 or ErbB4 did not affect Cbl recruitment, receptor-ubiquitination, -degradation, -downregulation or ligand degradation. We conclude that poor downregulation of ErbB2 and ErbB4 receptors is not due to sequence variations in the Cbl binding site of these receptors.

Intersectin 1 forms a complex with adaptor protein Ruk/CIN85 in vivo independently of epidermal growth factor stimulation

Intersectin 1 (ITSN1) is an adaptor protein involved in clathrin-mediated endocytosis, apoptosis, signal transduction and cytoskeleton organization. Here, we show that ITSN1 forms a complex with adaptor protein Ruk/CIN85, implicated in downregulation of receptor tyrosine kinases. The interaction is mediated by the SH3A domain of ITSN1 and the third or fourth proline-rich blocks of Ruk/CIN85, and does not depend on epidermal growth factor stimulation, suggesting a constitutive association of ITSN1 with Ruk/CIN85. Moreover, both proteins colocalize in MCF-7 cells with their common binding partner, the ubiquitin ligase c-Cbl. The possible biological role of the interaction between ITSN1 and Ruk/CIN85 is discussed.

Met receptor tyrosine kinase degradation is altered in response to the leucine-rich repeat of the Listeria invasion protein internalin B

Entry of the bacterial pathogen Listeria monocytogenes into host epithelial cells is critical for infection and virulence. One major pathway for Listeria entry involves binding of the bacterial protein Internalin B to the host receptor tyrosine kinase Met (hepatocyte growth factor receptor). Activation of Met and downstream signaling cascades is critical for Listeria entry. Internalin B is composed of several structural domains including an N-terminal leucine-rich repeat that is sufficient for binding Met and stimulating downstream signal transduction. Internalin B is monomeric, whereas the leucine-rich repeat is dimeric when expressed as an isolated fragment. The different quaternary states of Internalin B and the leucine-rich repeat suggest that these two Met ligands might cause distinct biological effects. Here we demonstrate that Internalin B and the leucine-rich repeat fragment exhibit agonist properties that differentially influence Met down-regulation in lysosomes. Specifically, Met stability is increased in response to the leucine-rich repeat fragment compared with Internalin B. Interestingly, Internalin B and the leucine-rich repeat stimulate equivalent rates of clathrin-mediated Met internalization. However, the leucine-rich repeat is defective in promoting lysosomal down-regulation of Met and instead enhances receptor recycling to the cell surface. In addition, the leucine-rich repeat causes prolonged Met activation (phosphorylation) and increased cell motility compared with Internalin B. Taken together, our findings indicate that individual domains of Internalin B differentially regulate Met trafficking. The ability of the leucine-rich repeat fragment to promote Met recycling could account for the increased cell motility induced by this ligand.

CD2AP and Cbl-3/Cbl-c constitute a critical checkpoint in the regulation of ret signal transduction

The glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) are critical for nervous system development and maintenance. GFLs promote survival and growth via activation of the receptor tyrosine kinase (RTK) Ret. In sympathetic neurons, the duration of Ret signaling is governed by how rapidly Ret is degraded after its activation. In an effort to elucidate mechanisms that control the half-life of Ret, we have identified two novel Ret interactors, CD2-associated protein (CD2AP) and Cbl-3. CD2AP, an adaptor molecule involved in the internalization of ubiquitinated RTKs, is associated with Ret under basal, unstimulated conditions in neurons. After Ret activation by GDNF, CD2AP dissociates. Similarly, the E3-ligase Cbl-3 interacts with unphosphorylated Ret and dissociates from Ret after Ret activation. In contrast to their dissociation from autophosphorylated Ret, an interaction between CD2AP and Cbl-3 is induced by GDNF stimulation of sympathetic neurons, suggesting that CD2AP and Cbl-3 dissociate from Ret as a complex. In neurons, the overexpression of CD2AP enhances the degradation of Ret and inhibits GDNF-dependent survival, and gene silencing of CD2AP blocks Ret degradation and promotes GDNF-mediated survival. Surprisingly, Cbl-3 overexpression dramatically stabilizes activated Ret and enhances neuronal survival, even though Cbl-family E3 ligases normally function to trigger RTK downregulation. In combination with CD2AP, however, Cbl-3 promotes Ret degradation rapidly and almost completely blocks survival promotion by GDNF, suggesting that Cbl-3 acts as a switch that is triggered by CD2AP and oscillates between inhibition and promotion of Ret degradation. Consistent with the hypothesis, Cbl-3 silencing in neurons only inhibited Ret degradation and enhanced neuronal survival in combination with CD2AP silencing. CD2AP and Cbl-3, therefore, constitute a checkpoint that controls the extent of Ret downregulation and, thereby, the sensitivity of neurons to GFLs.

MET as a target for treatment of chest tumors

The receptor tyrosine kinase MET has been studied of a large variety of human cancers, including lung and mesothelioma. The MET receptor and its ligand HGF (hepatocyte growth factor) play important roles in cell growth, survival and migration, and dysregulation of the HGF-MET pathway leads to oncogenic changes including tumor proliferation, angiogenesis and metastasis. In small cell lung cancer (SCLC), non-small cell lung cancer (NSCLC), and malignant pleural mesothelioma (MPM), MET is dysregulated via overexpression, constitutive activation, gene amplification, ligand-dependent activation, mutation or epigenetic mechanisms. New drugs targeted against MET and HGF are currently being investigated in vitro and in vivo, with promising results. These drugs function at a variety of steps within the HGF-MET pathway, including MET expression at the RNA or protein level, the ligand-receptor interaction, and tyrosine kinase function. This paper will review the structure, function, mechanisms of tumorigenesis, and potential for therapeutic inhibition of the MET receptor in lung cancer and mesothelioma.

Differential ubiquitin binding of the UBA domains from human c-Cbl and Cbl-b: NMR structural and biochemical insights

The Cbl proteins, RING-type E3 ubiquitin ligases, are responsible for ubiquitinating the activated tyrosine kinases and targeting them for degradation. Both c-Cbl and Cbl-b have a UBA (ubiquitin-associated) domain at their C-terminal ends, and these two UBA domains share a high sequence similarity (75%). However, only the UBA from Cbl-b, but not from c-Cbl, can bind ubiquitin (Ub). To understand the mechanism by which the UBA domains specifically interact with Ub with different affinities, we determined the solution NMR structures of these two UBA domains, cUBA from human c-Cbl and UBAb from Cbl-b. Their structures show that these two UBA domains share the same fold, a compact three-helix bundle, highly resembling the typical UBA fold. Chemical shift perturbation experiments reveal that the helix-1 and loop-1 of UBAb form a predominately hydrophobic surface for Ub binding. By comparing the Ub-interacting surface on UBAb and its counterpart on cUBA, we find that the hydrophobic patch on cUBA is interrupted by a negatively charged residue Glu12. Fluorescence titration data show that the Ala12Glu mutant of UBAb completely loses the ability to bind Ub, whereas the mutation disrupting the dimerization has no significant effect on Ub binding. This study provides structural and biochemical insights into the Ub binding specificities of the Cbl UBA domains, in which the hydrophobic surface distribution on the first helix plays crucial roles in their differential affinities for Ub binding. That is, the amino acid residue diversity in the helix-1 region, but not the dimerization, determines the abilities of various UBA domains binding with Ub.

Expression of Cbl-interacting protein of 85 kDa in MPTP mouse model of Parkinson's disease and 1-methyl-4-phenyl-pyridinium ion-treated dopaminergic SH-SY5Y cells

The newly discovered Cbl-interacting protein of 85 kDa (CIN85) is involved in many cellular processes, but its functions in the brain and in neurodegenerative diseases remain unclear. In this paper, we investigated the distribution of CIN85 protein in different regions of adult mouse brain using Western blot analysis and immunohistochemistry, and found that CIN85 was ubiquitously expressed in mouse brain. In the striatum and substantia nigra, two regions most deeply affected in Parkinson's disease, the level of CIN85 protein was relatively high. In the MPTP mouse model of Parkinson's disease, the expression of CIN85 in the striatum and substantia nigra was complicated. But in 1-methyl-4-phenyl-pyridinium ion-treated human dopaminergic SH-SY5Y cells, the expression of CIN85 increased dramatically. Knocking down of CIN85 by short hairpin RNA reduced SH-SY5Y cell death. Therefore, CIN85 might play different roles in the dopaminergic cell line and in the nigrostriatum of mouse brain under neurotoxin challenge.

Podosomes and Invadopodia: Related structures with Common Protein Components that May Promote Breast Cancer Cellular Invasion

A rate-limiting step in breast cancer progression is acquisition of the invasive phenotype, which can precede metastasis. Expression of cell-surface proteases at the leading edge of a migrating cell provides cells with a mechanism to cross tissue barriers. A newly appreciated mechanism that may be relevant for breast cancer cell invasion is the formation of invadopodia, well-defined structures that project from the ventral membrane and promote degradation of the extracellular matrix, allowing the cell to cross a tissue barrier. Recently, there has been some controversy and discussion as to whether invadopodia, which are associated with carcinoma cells, are related to a similar structure called podosomes, which are associated with normal cells. Invadopodia and podosomes share many common characteristics, including a similar size, shape, subcellular localization and an ability to promote invasion. These two structures also share many common protein components, which we outline herein. It has been speculated that podosomes may be precursors to invadopodia and by extension both structures may be relevant to cancer cell invasion. Here, we compare and contrast the protein components of invadopodia and podosomes and discuss a potential role for these proteins and the evidence that supports a role for invadopodia and podosomes in breast cancer invasion.

Invadopodia: at the cutting edge of tumour invasion

Invasion of tissues by malignant tumours is facilitated by tumour cell migration and degradation of extracellular matrix (ECM) barriers. Several invasive neoplasms, including head and neck squamous cell carcinoma, breast carcinoma, melanoma and glioma, contain tumour cells that can form actin-rich protrusions with ECM proteolytic activity called invadopodia. These dynamic organelle-like structures adhere to, and digest, collagens, laminins and fibronectin. Invadopodia are dependent on multiple transmembrane, cytoplasmic and secreted proteins engaged in cell adhesion, signal transduction, actin assembly, membrane regulation and ECM proteolysis. Strategies aimed at disrupting invadopodia could form the basis of novel anti-invasive therapies for treating patients. Here we review the molecular basis of invadopodia formation with particular emphasis on the intracellular signaling networks that are essential for invadopodia activity and examine the potential role of these structures in glioma invasion.

The gradient of Gurken, a long-range morphogen, is directly regulated by Cbl-mediated endocytosis

The asymmetric localization of gurken mRNA and post-translational sorting mechanisms are responsible for the polar distribution of Gurken protein in Drosophila. However, endocytosis of Egfr, the receptor for Gurken in the follicle cells, also plays a role in shaping the extracellular gradient of the Gurken morphogen. Previously, we have found that mutation in the Cbl gene caused elevated Egfr signaling along the dorsoventral axis, and resulted in dorsalization phenotypes in embryos and egg shells. Here, we report that overexpression of the Cbl long isoform significantly changed Gurken distribution. Using an HRP-Gurken fusion protein, we demonstrate that internalization of the Gurken-Egfr complex depends on the activity of Cbl. Increased levels of CblL promote the internalization of this complex, leading to the reduction of free ligands. The Gurken-Egfr complex trafficks through the Rab5/Rab7 associated endocytic pathway to the lysosomal degradation compartment for signaling termination. We observe endocytic Gurken not only in the dorsal but also in the ventral follicle cells, which is, to our knowledge, the first visualization of Gurken on the ventral side of egg chambers. Our results show that Gurken travels towards the lateral/posterior of the egg chamber in the absence of Cbl, suggesting that Cbl actively regulates Gurken distribution through promoting endocytosis and subsequent degradation.

A tale of two Cbls: interplay of c-Cbl and Cbl-b in epidermal growth factor receptor downregulation

The precise role of Cbl in epidermal growth factor (EGF) receptor (EGFR) endocytosis and trafficking remains to be fully uncovered. Here, we showed that mutant EGFR1044, which was truncated after residue 1044, did not associate with c-Cbl and was not ubiquitinated initially in response to EGF but was internalized with kinetics similar to those of wild-type EGFR. This finding indicates that c-Cbl-mediated ubiquitination is not required for EGF-induced EGFR endocytosis. We also showed that the previously identified internalization-deficient mutant receptor EGFR1010LL/AA bound to c-Cbl and was fully ubiquitinated in response to EGF, which indicates that c-Cbl binding and ubiquitination are not sufficient for EGFR internalization. We next investigated EGFR trafficking following EGFR internalization. We found that c-Cbl disassociation from EGFR occurred well in advance of EGFR degradation and that this event was concurrent with the selective dephosphorylation of EGFR at Y1045. This finding suggests that once EGFR is ubiquitinated, continual Cbl association is not required for EGFR degradation. Because EGFR1044 is ubiquitinated and degraded similarly to wild-type EGFR, we examined the role of another prominent Cbl homologue, Cbl-b, and found that Cbl-b was associated with both EGFR and EGFR1044. Further study showed that Cbl-b bound to EGFR at two regions: one in the C-terminal direction from residue 1044 and one in the N-terminal direction from residue 958. Moreover, Cbl-b association with EGFR rose markedly following a decrease in c-Cbl association, corresponding to a second peak of EGFR ubiquitination occurring later in EGFR trafficking. Using RNA interference to knock down both c-Cbl and Cbl-b, we were able to abolish EGFR downregulation. This knockdown had no affect on the rate of EGF-induced EGFR internalization. We found that the two Cbls accounted for total receptor ubiquitination and that while c-Cbl and Cbl-b are each alone sufficient to effect EGFR degradation, both are involved in the physiological, EGF-mediated process of receptor downregulation. Furthermore, these data ultimately reveal a previously unacknowledged temporal interplay of two major Cbl homologues with the trafficking of EGFR.

Systematic mutagenesis of the murine gammaherpesvirus 68 M2 protein identifies domains important for chronic infection

Murine gammaherpesvirus 68 (MHV68) infection of inbred mice represents a genetically tractable small-animal model for assessing the requirements for the establishment of latency, as well as reactivation from latency, within the lymphoid compartment. By day 16 postinfection, MHV68 latency in the spleen is found in B cells, dendritic cells, and macrophages. However, as with Epstein-Barr virus, by 3 months postinfection MHV68 latency is predominantly found in isotype-switched memory B cells. The MHV68 M2 gene product is a latency-associated antigen with no discernible homology to any known cellular or viral proteins. However, depending on experimental conditions, the M2 protein has been shown to play a critical role in both the efficient establishment of latency in splenic B cells and reactivation from latently infected splenic B cells. Inspection of the sequence of the M2 protein reveals several hallmarks of a signaling molecule, including multiple PXXP motifs and two potential tyrosine phosphorylation sites. Here, we report the generation of a panel of recombinant MHV68 viruses harboring mutations in the M2 gene that disrupt putative functional motifs. Subsequent analyses of the panel of M2 mutant viruses revealed a functionally important cluster of PXXP motifs in the C-terminal region of M2, which have previously been implicated in binding Vav proteins (P. A. Madureira, P. Matos, I. Soeiro, L. K. Dixon, J. P. Simas, and E. W. Lam, J. Biol. Chem. 280:37310-37318, 2005; L. Rodrigues, M. Pires de Miranda, M. J. Caloca, X. R. Bustelo, and J. P. Simas, J. Virol. 80:6123-6135, 2006). Further characterization of two adjacent PXXP motifs in the C terminus of the M2 protein revealed differences in the functions of these domains in M2-driven expansion of primary murine B cells in culture. Finally, we show that tyrosine residues 120 and 129 play a critical role in both the establishment of splenic latency and reactivation from latency upon explant of splenocytes into tissue culture. Taken together, these analyses will aide future studies for identifying M2 interacting partners and B-cell signaling pathways that are manipulated by the M2 protein.

The Nedd4-like family of E3 ubiquitin ligases and cancer

Accumulating evidence suggests that E3 ubiquitin ligases play important roles in cancer development. In this article, we provide a comprehensive summary of the roles of the Nedd4-like family of E3 ubiquitin ligases in human cancer. There are nine members of the Nedd4-like E3 family, all of which share a similar structure, including a C2 domain at the N-terminus, two to four WW domains in the middle of the protein, and a homologous to E6-AP COOH terminus domain at the C-terminus. The assertion that Nedd4-like E3s play a role in cancer is supported by the overexpression of Smurf2 in esophageal squamous cell carcinoma, WWP1 in prostate and breast cancer, Nedd4 in prostate and bladder cancer, and Smurf1 in pancreatic cancer. Because Nedd4-like E3s regulate ubiquitin-mediated trafficking, lysosomal or proteasomal degradation, and nuclear translocation of multiple proteins, they modulate important signaling pathways involved in tumorigenesis like TGFbeta, EGF, IGF, VEGF, SDF-1, and TNFalpha. Additionally, several Nedd4-like E3s directly regulate various cancer-related transcription factors from the Smad, p53, KLF, RUNX, and Jun families. Interestingly, multiple Nedd4-like E3s show ligase independent function. Furthermore, Nedd4-like E3s themselves are frequently regulated by phosphorylation, ubiquitination, translocation, and transcription in cancer cells. Because the regulation and biological output of these E3s is such a complex process, study of the role of these E3s in cancer development poses some challenges. However, understanding the oncogenic potential of these E3s may facilitate the identification and development of biomarkers and drug targets in human cancer.

Invasive and adherent bacterial pathogens co-Opt host clathrin for infection

Infection by the bacterium Listeria monocytogenes depends on host cell clathrin. To determine whether this requirement is widespread, we analyzed infection models using diverse bacteria. We demonstrated that bacteria that enter cells following binding to cellular receptors (termed "zippering" bacteria) invade in a clathrin-dependent manner. In contrast, bacteria that inject effector proteins into host cells in order to gain entry (termed "triggering" bacteria) invade in a clathrin-independent manner. Strikingly, enteropathogenic Escherichia coli (EPEC) required clathrin to form actin-rich pedestals in host cells beneath adhering bacteria, even though this pathogen remains extracellular. Furthermore, clathrin accumulation preceded the actin rearrangements necessary for Listeria entry. These data provide evidence for a clathrin-based entry pathway allowing internalization of large objects (bacteria and ligand-coated beads) and used by "zippering" bacteria as part of a general mechanism to invade host mammalian cells. We also revealed a nonendocytic role for clathrin required for extracellular EPEC infections.

c-Met must translocate to the nucleus to initiate calcium signals

Hepatocyte growth factor (HGF) is important for cell proliferation, differentiation, and related activities. HGF acts through its receptor c-Met, which activates downstream signaling pathways. HGF binds to c-Met at the plasma membrane, where it is generally believed that c-Met signaling is initiated. Here we report that c-Met rapidly translocates to the nucleus upon stimulation with HGF. Ca(2+) signals that are induced by HGF result from phosphatidylinositol 4,5-bisphosphate hydrolysis and inositol 1,4,5-trisphosphate formation within the nucleus rather than within the cytoplasm. Translocation of c-Met to the nucleus depends upon the adaptor protein Gab1 and importin beta1, and formation of Ca(2+) signals in turn depends upon this translocation. HGF may exert its particular effects on cells because it bypasses signaling pathways in the cytoplasm to directly activate signaling pathways in the nucleus.

The Cbl interactome and its functions

Cbl proteins are ubiquitin ligases and multifunctional adaptor proteins that are implicated in the regulation of signal transduction in various cell types and in response to different stimuli. Cbl-associated proteins can assemble together at a given time or space inside the cell, and such an interactome can form signal competent networks that control many physiological processes. Dysregulation of spatial or temporal constraints in the Cbl interactome results in the development of human pathologies such as immune diseases, diabetes and cancer.

Current knowledge of the large RhoGAP family of proteins

The Rho GTPases are implicated in almost every fundamental cellular process. They act as molecular switches that cycle between an active GTP-bound and an inactive GDP-bound state. Their slow intrinsic GTPase activity is greatly enhanced by RhoGAPs (Rho GTPase-activating proteins), thus causing their inactivation. To date, more than 70 RhoGAPs have been identified in eukaryotes, ranging from yeast to human, and based on sequence homology of their RhoGAP domain, we have grouped them into subfamilies. In the present Review, we discuss their regulation, biological functions and implication in human diseases.

The adaptor molecule CIN85 regulates Syk tyrosine kinase level by activating the ubiquitin-proteasome degradation pathway

Triggering of mast cells and basophils by IgE and Ag initiates a cascade of biochemical events that lead to cell degranulation and the release of allergic mediators. Receptor aggregation also induces a series of biochemical events capable of limiting FcepsilonRI-triggered signals and functional responses. Relevant to this, we have recently demonstrated that Cbl-interacting 85-kDa protein (CIN85), a multiadaptor protein mainly involved in the process of endocytosis and vesicle trafficking, regulates the Ag-dependent endocytosis of the IgE receptor, with consequent impairment of FcepsilonRI-mediated cell degranulation. The purpose of this study was to further investigate whether CIN85 could alter the FcepsilonRI-mediated signaling by affecting the activity and/or expression of molecules directly implicated in signal propagation. We found that CIN85 overexpression inhibits the FcepsilonRI-induced tyrosine phosphorylation of phospholipase Cgamma, thus altering calcium mobilization. This functional defect is associated with a substantial decrease of Syk protein levels, which are restored by the use of selective proteasome inhibitors, and it is mainly due to the action of the ubiquitin ligase c-Cbl. Furthermore, coimmunoprecipitation experiments demonstrate that CIN85 overexpression limits the ability of Cbl to bind suppressor of TCR signaling 1 (Sts1), a negative regulator of Cbl functions, while CIN85 knockdown favors the formation of Cbl/Sts1 complexes. Altogether, our findings support a new role for CIN85 in regulating Syk protein levels in RBL-2H3 cells through the activation of the ubiquitin-proteasome pathway and provide a mechanism for this regulation involving c-Cbl ligase activity.