Cbl-CIN85-endophilin complex mediates ligand-induced downregulation of EGF receptors

CIN85 modulates the down-regulation of Fc gammaRIIa expression and function by c-Cbl in a PKC-dependent manner in human neutrophils

We previously described a non-classical mechanism that arrests FcγRIIa signaling in human neutrophils once engaged by immune complexes or opsonized pathogens. The engagement of FcγRIIa leads to its ubiquitination by the ubiquitin ligase c-Cbl and degradation by the proteasome. Herein, we further examined some of the events regulating this novel pathway. The adaptor protein CIN85 was described in other systems to be involved in the regulation of the c-Cbl-dependent pathway. We found that CIN85 is expressed in human neutrophils and that it translocates like c-Cbl from the cytosol to the plasma membrane following receptor cross-linking. CIN85 was also recruited to the same subset of high density detergent-resistant membrane fractions in which stimulated FcγRIIa partitioned with c-Cbl. The integrity of these microdomains is essential to the FcγRIIa degradation process because the cholesterol-depleting agent methyl-β-cyclodextrin inhibits this event. Silencing the expression of CIN85 by siRNA in dibutyryl cyclic AMP-differentiated PLB 985 cells prevented FcγRIIa degradation and increased IgG-mediated phagocytosis. Confocal microscopy revealed that the presence of CIN85 is essential to the proper sorting of FcγRIIa during endocytosis. We also provide direct evidence that CIN85 is a substrate of serine/threonine kinase PKCs. Classical PKCs positively regulate FcγRIIa ubiquitination and degradation because these events were inhibited by Gö6976, a classical PKC inhibitor. We conclude that the ubiquitination and degradation of stimulated FcγRIIa mediated by c-Cbl are positively regulated by the adaptor protein CIN85 in a PKC-dependent manner and that these events contribute to the termination of FcγRIIa signaling.

The interactome of a PTB domain-containing adapter protein, Odin, revealed by SILAC

Signal transduction pathways are tightly controlled by positive and negative regulators. We have previously identified Odin (also known as ankyrin repeat and sterile alpha motif domain-containing 1A; gene symbol ANKS1A) as a negative regulator of growth factor signaling; however, the mechanisms through which Odin regulates these pathways remain to be elucidated. To determine how Odin negatively regulates growth factor signaling, we undertook a proteomic approach to systematically identify proteins that interact with Odin using the SILAC strategy. In this study, we identified 18 molecules that were specifically associated in a protein complex with Odin. Our study established that the complete family of 14-3-3 proteins occur in a protein complex with Odin, which is also supported by earlier reports that identified a few members of the 14-3-3 family as Odin interactors. Among the novel protein interactors of Odin were CD2-associated protein, SH3 domain kinase binding protein 1 and DAB2 interacting protein. We confirmed 8 of the eighteen interactions identified in the Odin protein complex by co-immunoprecipitation experiments. Finally, a literature-based network analysis revealed that Odin interacting partners are involved in various cellular processes, some of which are key molecules in regulating receptor endocytosis.

Hrs recognizes a hydrophobic amino acid cluster in cytokine receptors during ubiquitin-independent endosomal sorting

Hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs) is a component of the ESCRT-0 protein complex that captures ubiquitylated cargo proteins and sorts them to the lysosomal pathway. Although Hrs acts as a key transporter for ubiquitin-dependent endosomal sorting, we previously reported that Hrs is also involved in ubiquitin-independent endosomal sorting of interleukin-2 receptor β (IL-2Rβ). Here, we show direct interactions between bacterially expressed Hrs and interleukin-4 receptor α (IL-4Rα), indicating that their binding is not required for ubiquitylation of the receptors, similar to the case for IL-2Rβ. Examinations of the Hrs binding regions of the receptors reveal that a hydrophobic amino acid cluster in both IL-2Rβ and IL-4Rα is essential for the binding. Whereas the wild-type receptors are delivered to LAMP1-positive late endosomes, mutant receptors lacking the hydrophobic amino acid cluster are sorted to lysobisphosphatidic acid-positive late endosomes rather than LAMP1-positive late endosomes. We also show that the degradation of these mutant receptors is attenuated. Accordingly, Hrs functions during ubiquitin-independent endosomal sorting of the receptors by recognizing the hydrophobic amino acid cluster. These findings suggest the existence of a group of cargo proteins that have this hydrophobic amino acid cluster as a ubiquitin-independent sorting signal.

D-Cbl binding to Drk leads to dose-dependent down-regulation of EGFR signaling and increases receptor-ligand endocytosis

Proper control of Epidermal Growth Factor Receptor (EGFR) signaling is critical for normal development and regulated cell behaviors. Abnormal EGFR signaling is associated with tumorigenic process of various cancers. Complicated feedback networks control EGFR signaling through ligand production, and internalization-mediated destruction of ligand-receptor complexes. Previously, we found that two isoforms of D-Cbl, D-CblS and D-CblL, regulate EGFR signaling through distinct mechanisms. While D-CblL plays a crucial role in dose-dependent down-regulation of EGFR signaling, D-CblS acts in normal restriction of EGFR signaling and does not display dosage effect. Here, we determined the underlying molecular mechanism, and found that Drk facilitates the dose-dependent regulation of EGFR signaling through binding to the proline-rich motif of D-CblL, PR. Furthermore, the RING finger domain of D-CblL is essential for promoting endocytosis of the ligand-receptor complex. Interestingly, a fusion protein of the two essential domains of D-CblL, RING- PR, is sufficient to down-regulate EGFR signal in a dose-dependent manner by promoting internalization of the ligand, Gurken. Besides, RING-SH2^Drk, a fusion protein of the RING finger domain of D-Cbl and the SH2 domain of Drk, also effectively down-regulates EGFR signaling in Drosophila follicle cells, and suppresses the effects of constitutively activated EGFR. The RING-SH2^Drk suppresses EGFR signaling by promoting the endosomal trafficking of ligand-receptor complexes, suggesting that Drk plays a negative role in EGFR signaling by enhancing receptor endocytosis through cooperating with the RING domain of D-Cbl. Interfering the recruitment of signal transducer, Drk, to the receptor by the RING-SH2^Drk might further reduces EGFR signaling. The fusion proteins we developed may provide alternative strategies for therapy of cancers caused by hyper-activation of EGFR signaling.

A critical role of c-Cbl-interacting protein of 85 kDa in the development and progression of head and neck squamous cell carcinomas through the ras-ERK pathway

Activation of the transforming growth factor (TGF) α/epidermal growth factor receptor (EGFR)-mediated signaling pathway is a common mechanism for dysregulated growth of head and neck squamous cell carcinoma (HNSCC). c-Cbl-interacting protein of 85 kDa (CIN85) is an adaptor protein that facilitates EGFR internalization. Little is known, however, about a role of CIN85 in EGFR signaling as well as its relevance to tumor development and progression of HNSCC. Here, we demonstrate that CIN85 is highly expressed in HNSCC tumor samples compared with adjacent normal tissues, and this overexpression is significantly correlated with advanced clinical stage. The experiments using CIN85-overexpressing and knockdown HNSCC cell lines showed that CIN85 promotes HNSCC growth and facilitates EGFR internalization without apparently affecting phosphorylation of EGFR. Moreover, CIN85 promoted TGF-α-induced activation of Ras and phosphorylation of downstream molecules such as c-Raf, MEK, and extracellular signal-regulated kinase, leading to expression of c-Myc that is critical for sustained proliferation of HNSCC. Taken together, these findings suggest that CIN85 not only controls EGFR internalization but also promotes the EGFR-mediated tumor development and progression, and thus, CIN85 may serve as a potential therapeutic target in a subset of HNSCC.

Primary resistance to cetuximab in a panel of patient-derived tumour xenograft models: activation of MET as one mechanism for drug resistance

Cetuximab (Erbitux®) targets the epidermal growth factor receptor (EGFR) and is approved for treatment of colorectal and head and neck cancer. Despite wide expression of EGFR, only a subgroup of cancer patients responds to cetuximab therapy. In the present study we assessed the cetuximab response in vivo of 79 human patient-derived xenografts originating from five tumour histotypes. We analysed basic tumour characteristics including EGFR expression and activation, mutational status of KRAS, BRAF and NRAS, the expression of EGFR ligands and the activation of HER3 (ErbB3) and the hepatocyte growth factor receptor MET. Based on these results, a cetuximab response score including positive and negative factors affecting therapeutic response is proposed. Positive factors are high expression and activation of EGFR and its ligands epiregulin or amphiregulin, negative factors are markers for downstream pathway activation independent of EGFR. In cetuximab resistant NSCL adenocarcinoma LXFA 526 and LXFA 1647, overexpression due to gene amplification and strong activation of MET was identified. Knock-down of MET by siRNA in the corresponding cell lines showed that anchorage-independent growth and migration are dependent on MET. MET knock down sensitized LXFA 526L and LXFA 1647L to EGF. Combined treatments of a MET inhibitor and cetuximab were additive. Therefore, combination therapy of cetuximab and a MET inhibitor in selected lung cancer patients could be of high clinical significance.

Additional serine/threonine phosphorylation reduces binding affinity but preserves interface topography of substrate proteins to the c-Cbl TKB domain

The E3-ubiquitin ligase, c-Cbl, is a multi-functional scaffolding protein that plays a pivotal role in controlling cell phenotype. As part of the ubiquitination and downregulation process, c-Cbl recognizes targets, such as tyrosine kinases and the Sprouty proteins, by binding to a conserved (NX/R)pY(S/T)XXP motif via its uniquely embedded SH2 domain (TKB domain). We previously outlined the mode of binding between the TKB domain and various substrate peptide motifs, including epidermal growth factor receptor (EGFR) and Sprouty2 (Spry2), and demonstrated that an intrapetidyl hydrogen bond forms between the (pY-1) arginine or (pY-2) asparagine and the phosphorylated tyrosine, which is crucial for binding. Recent reports demonstrated that, under certain types of stimulation, the serine/threonine residues at the pY+1 and/or pY+2 positions within this recognition motif of EGFR and Sprouty2 may be endogenously phosphorylated. Using structural and binding studies, we sought to determine whether this additional phosphorylation could affect the binding of the TKB domain to these peptides and consequently, whether the type of stimulation can dictate the degree to which substrates bind to c-Cbl. Here, we show that additional phosphorylation significantly reduces the binding affinity between the TKB domain and its target proteins, EGFR and Sprouty2, as compared to peptides bearing a single tyrosine phosphorylation. The crystal structure indicates that this is accomplished with minimal changes to the essential intrapeptidyl bond and that the reduced strength of the interaction is due to the charge repulsion between c-Cbl and the additional phosphate group. This obvious reduction in binding affinity, however, indicates that Cbl's interactions with its TKB-centered binding partners may be more favorable in the absence of Ser/Thr phosphorylation, which is stimulation and context specific in vivo. These results demonstrate the importance of understanding the environment in which certain residues are phosphorylated, and the necessity of including this in structural investigations.

K33-linked polyubiquitination of T cell receptor-zeta regulates proteolysis-independent T cell signaling

Tagging the cell surface receptor with ubiquitin is believed to provide a signal for the endocytic pathway. E3 ubiquitin ligases such as Cbl-b and Itch have been implicated in T cell activation and tolerance induction. However, the underlying mechanisms remain unclear. We describe that in mice deficient in the E3 ubiquitin ligases Cbl-b and Itch, T cell activation was augmented, accompanied by spontaneous autoimmunity. The double-mutant T cells exhibited increased phosphorylation of the T cell receptor-zeta (TCR-zeta) chain, whereas the endocytosis and stability of the TCR complex were not affected. TCR-zeta was polyubiquitinated via a K33-linkage, which affected its phosphorylation and association with the zeta chain-associated protein kinase Zap-70. The juxtamembrane K54 residue in TCR-zeta was identified to be a primary ubiquitin conjugation site, whose mutation increased its phosphorylation and association of TCR-zeta and Zap-70. Thus, the present study reveals unconventional K33-linked polyubiquitination in nonproteolytic regulation of cell-surface-receptor-mediated signal transduction.

A Dyn2-CIN85 complex mediates degradative traffic of the EGFR by regulation of late endosomal budding

The epidermal growth factor receptor (EGFR) is over-expressed in a variety of human cancers. Downstream signalling of this receptor is tightly regulated both spatially and temporally by controlling its internalization and subsequent degradation. Internalization of the EGFR requires dynamin 2 (Dyn2), a large GTPase that deforms lipid bilayers, leading to vesicle scission. The adaptor protein CIN85 (cbl-interacting protein of 85 kDa), which has been proposed to indirectly link the EGFR to the endocytic machinery at the plasma membrane, is also thought to be involved in receptor internalization. Here, we report a novel and direct interaction between Dyn2 and CIN85 that is induced by EGFR stimulation and, most surprisingly, occurs late in the endocytic process. Importantly, disruption of the CIN85-Dyn2 interaction results in accumulation of internalized EGFR in late endosomes that become aberrantly elongated into distended tubules. Consistent with the accumulation of this receptor is a sustention of downstream signalling cascades. These findings provide novel insights into a previously unknown protein complex that can regulate EGFR traffic at very late stages of the endocytic pathway.

Combined microdeletions and CHD7 mutation causing severe CHARGE/DiGeorge syndrome: clinical presentation and molecular investigation by array-CGH

Phenotypic variation in CHARGE syndrome remains unexplained. A subcategory of CHARGE patients show overlapping phenotypic characteristics with DiGeorge syndrome (thymic hypo/aplasia, hypocalcemia, T-cell immunodeficiency). Very few have been tested or reported to carry a mutation of the CHD7 (chromodomain helicase DNA-binding domain) gene detected in two-thirds of CHARGE patients. In an attempt to explore the genetic background of a severe CHARGE/DiGeorge phenotype, we performed comparative genomic array hybridization in an infant carrier of a CHD7 mutation. The high-resolution comparative genomic array hybridization revealed interesting findings, including a deletion distal to the DiGeorge region and disruptions in other chromosomal regions of genes implicated in immunological and other functions possibly contributing to the patient's severe phenotype and early death.

Multiple mechanisms collectively regulate clathrin-mediated endocytosis of the epidermal growth factor receptor

Four independent mechanisms for uptake of activated EGFR are identified by a combination of receptor mutagenesis and RNA interference approaches.

Endocytosis of the epidermal growth factor receptor (EGFR) is important for the regulation of EGFR signaling. However, EGFR endocytosis mechanisms are poorly understood, which precludes development of approaches to specifically inhibit EGFR endocytosis and analyze its impact on signaling. Using a combination of receptor mutagenesis and RNA interference, we demonstrate that clathrin-dependent internalization of activated EGFR is regulated by four mechanisms, which function in a redundant and cooperative fashion. These mechanisms involve ubiquitination of the receptor kinase domain, the clathrin adaptor complex AP-2, the Grb2 adaptor protein, and three C-terminal lysine residues (K1155, K1158, and K1164), which are acetylated, a novel posttranslational modification for the EGFR. Based on these findings, the first internalization-defective EGFR mutant with functional kinase and normal tyrosine phosphorylation was generated. Analysis of the signaling kinetics of this mutant revealed that EGFR internalization is required for the sustained activation of protein kinase B/AKT but not for the activation of mitogen-activated protein kinase.

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.

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.

Ubiquitin conjugase UBCH8 targets active FMS-like tyrosine kinase 3 for proteasomal degradation

The class III receptor tyrosine kinase FMS-like tyrosine kinase 3 (FLT3) regulates normal hematopoiesis and immunological functions. Nonetheless, constitutively active mutant FLT3 (FLT3-ITD) causally contributes to transformation and is associated with poor prognosis of acute myeloid leukemia (AML) patients. Histone deacetylase inhibitors (HDACi) can counteract deregulated gene expression profiles and decrease oncoprotein stability, which renders them candidate drugs for AML treatment. However, these drugs have pleiotropic effects and it is often unclear how they correct oncogenic transcriptomes and proteomes. We report here that treatment of AML cells with the HDACi LBH589 induces the ubiquitin-conjugating enzyme UBCH8 and degradation of FLT3-ITD. Gain- and loss-of-function approaches show that UBCH8 and the ubiquitin-ligase SIAH1 physically interact with and target FLT3-ITD for proteasomal degradation. These ubiquitinylating enzymes though have a significantly lesser effect on wild-type FLT3. Furthermore, physiological and pharmacological stimulation of FLT3 phosphorylation, inhibition of FLT3-ITD autophosphorylation and analysis of kinase-inactive FLT3-ITD revealed that tyrosine phosphorylation determines degradation of FLT3 and FLT3-ITD by the proteasome. These results provide novel insights into antileukemic activities of HDACi and position UBCH8, which have been implicated primarily in processes in the nucleus, as a previously unrecognized important modulator of FLT3-ITD stability and leukemic cell survival.

CIN85/RukL is a novel binding partner of nephrin and podocin and mediates slit diaphragm turnover in podocytes

Podocyte damage is the basis of many glomerular diseases with ultrastructural changes and decreased expression of components of the slit diaphragm such as nephrin and podocin. Under physiological conditions it is likely that the slit diaphragm underlies permanent renewal processes to indemnify its stability in response to changes in filtration pressure. This would require constant reorganization of the podocyte foot process and the renewal of slit diaphragm components. Thus far, the mechanisms underlying the turnover of slit diaphragm proteins are largely unknown. In this manuscript we examined a mechanism of nephrin endocytosis via CIN85/Ruk(L)-mediated ubiquitination. We can demonstrate that the loss of nephrin expression and onset of the proteinuria in CD2AP(-/-) mice correlates with an increased accumulation of ubiquitinated proteins and expression of CIN85/Ruk(L) in podocytes. In cultured murine podocytes CD2AP deficiency leads to an early ubiquitination of nephrin and podocin after stimulation with fibroblast growth factor-4. Binding assays with different CIN85/Ruk isoforms and mutants showed that nephrin and podocin are binding to the coiled-coil domain of CIN85/Ruk(L). We found that in the presence of CIN85/Ruk(L), which is involved in down-regulation of receptor-tyrosine kinases, nephrin is internalized after stimulation with fibroblast growth factor-4. Interestingly, coexpression of CIN85/Ruk(L) with CD2AP led to a decreased binding of CIN85/Ruk(L) to nephrin and podocin, which indicates a functional competition between CD2AP and CIN85/Ruk(L). Our results support a novel role for CIN85/Ruk(L) in slit diaphragm turnover and proteinuria.

I-BAR protein antagonism of endocytosis mediates directional sensing during guided cell migration

The I-BAR domain–containing protein, missing-in-metastasis, directs cell motility by opposing the endocytic activity of the endophilin–CD2AP–cortactin complex.

Although directed cellular migration facilitates the coordinated movement of cells during development and repair, the mechanisms regulating such migration remain poorly understood. Missing-in-metastasis (MIM) is a defining member of the inverse Bin/Amphiphysin/Rvs domain (I-BAR) subfamily of lipid binding, cytoskeletal regulators whose levels are altered in a number of cancers. Here, we provide the first genetic evidence that an I-BAR protein regulates directed cell migration in vivo. Drosophila MIM (dmim) is involved in Drosophila border cell migration, with loss of dmim function resulting in a lack of directional movement by the border cell cluster. In vivo endocytosis assays combined with genetic analyses demonstrate that the dmim product regulates directed cell movement by inhibiting endocytosis and antagonizing the activities of the CD2-associated protein/cortactin complex in these cells. These studies demonstrate that DMIM antagonizes pro-endocytic components to facilitate polarity and localized guidance cue sensing during directional cell migration.

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.

c-Cbl-mediated degradation of TRAIL receptors is responsible for the development of the early phase of TRAIL resistance

We previously reported two modes of development of acquired TRAIL resistance: early phase and late phase [1]. In these studies, we observed that greater Akt activity and the expression of Bcl-xL were related mainly to the late phase of acquired TRAIL resistance. Recently we became aware of a possible mechanism of early phase TRAIL resistance development through internalization and degradation of TRAIL receptors (DR4 and DR5). Our current studies demonstrate that TRAIL receptors rapidly diminish at the membrane as well as the cytoplasm within 4h after TRAIL exposure, but recover completely after one or two days. Our studies also reveal that Cbl, a ubiquitously expressed cytoplasmic adaptor protein, is responsible for the rapid degradation of TRAIL receptors; Cbl binds to them and induces monoubiquitination of these receptors concurrent with their degeneration soon after TRAIL exposure, creating the early phase of acquired TRAIL resistance.

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.

A mutational analysis of the endophilin-A N-BAR domain performed in living flies

Background

Endophilin is a cytoplasmic protein with an important function in clathrin-dependent endocytosis at synapses and elsewhere. Endophilin has a BAR (Bin/Amphiphysin/Rvs-homology) domain, which is implicated in the sensing and induction of membrane curvature. Previous structure-function studies of the endophilin-A BAR domain have almost exclusively been made in reduced systems, either in vitro or ex vivo in cultured cells. To extend and complement this work, we have analyzed the role played by the structural features of the endophilin-A BAR domain in Drosophila in vivo.

Methodology/Principal Findings

The study is based on genetic rescue of endophilin-A (endoA) null mutants with wild type or mutated endoA transgenes. We evaluated the viability of the rescuants, the locomotor behavior in adult flies and the neurotransmission at the larval neuromuscular junction. Whereas mutating the endophilin BAR domain clearly affected adult flies, larval endophilin function was surprisingly resistant to mutagenesis. Previous reports have stressed the importance of a central appendage on the convex BAR surface, which forms a hydrophobic ridge able to directly insert into the lipid bilayer. We found that the charge-negative substitution A66D, which targets the hydrophobic ridge and was reported to completely disrupt the ability of endophilin-BAR to tubulate liposomes in vitro, rescued viability and neurotransmission with the same efficiency as wild type endoA transgenes, even in adults. A similar discrepancy was found for the hydrophilic substitutions A63S/A66S and A63S/A66S/M70Q. The A66W mutation, which introduces a bulky hydrophobic side chain and induces massive vesiculation of liposomes in vitro, strongly impeded eye development, even in presence of the endogenous endoA gene. Substantial residual function was observed in larvae rescued with the EndoA(Arf) transgene, which encodes a form of endophilin-A that completely lacks the central appendage. Whereas a mutation (D151P) designed to increase the BAR curvature was functional, another mutation (P143A, ΔLEN) designed to decrease the curvature was not.

Conclusions/Significance

Our results provide novel insight into the structure/function relationship of the endophilin-A BAR domain in vivo, especially with relation to synaptic function.

VAV2 regulates epidermal growth factor receptor endocytosis and degradation

Vav proteins are guanine nucleotide exchange factors for Rho GTPases that regulate cell adhesion, motility, spreading and proliferation in response to growth factor signalling. In this work, we show that Vav2 expression delayed epidermal growth factor receptor (EGFR) internalization and degradation, and enhanced EGFR, ERK and Akt phosphorylations. This effect of Vav2 on EGFR degradation is dependent on its guanine nucleotide exchange function. Knockdown of Vav2 in HeLa cells enhanced EGFR degradation and reduced cell proliferation. epidermal growth factor stimulation led to co-localization of Vav2 with EGFR and Rab5 in endosomes. We further show that the effect of Vav2 on EGFR stability is modulated by its interaction with two endosome-associated proteins and require RhoA function. Thus, in this work, we report for the first time that Vav2 can regulate growth factors receptor signalling by slowing receptor internalization and degradation through its interaction with endosome-associated proteins.

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.

Potential predictive markers of chemotherapy resistance in stage III ovarian serous carcinomas

BACKGROUND

Chemotherapy resistance remains a major obstacle in the treatment of women with ovarian cancer. Establishing predictive markers of chemoresponse would help to individualize therapy and improve survival of ovarian cancer patients. Chemotherapy resistance in ovarian cancer has been studied thoroughly and several non-overlapping single genes, gene profiles and copy number alterations have been suggested as potential markers. The objective of this study was to explore genetic alterations behind chemotherapy resistance in ovarian cancer with the ultimate aim to find potential predictive markers.

METHODS

To create the best opportunities for identifying genetic alterations of importance for resistance, we selected a homogenous tumor material concerning histology, stage and chemotherapy. Using high-resolution whole genome array comparative genomic hybridization (CGH), we analyzed the tumor genomes of 40 fresh-frozen stage III ovarian serous carcinomas, all uniformly treated with combination therapy paclitaxel/carboplatin. Fisher's exact test was used to identify significant differences. Subsequently, we examined four genes in the significant regions (EVI1, MDS1, SH3GL2, SH3KBP1) plus the ABCB1 gene with quantitative real-time polymerase chain reaction (QPCR) to evaluate the impact of DNA alterations on the transcriptional level.

RESULTS

We identified gain in 3q26.2, and losses in 6q11.2-12, 9p22.3, 9p22.2-22.1, 9p22.1-21.3, Xp22.2-22.12, Xp22.11-11.3, and Xp11.23-11.1 to be significantly associated with chemotherapy resistance. In the gene expression analysis, EVI1 expression differed between samples with gain versus without gain, exhibiting higher expression in the gain group.

CONCLUSION

In conclusion, we detected specific genetic alterations associated with resistance, of which some might be potential predictive markers of chemotherapy resistance in advanced ovarian serous carcinomas. Thus, further studies are required to validate these findings in an independent ovarian tumor series.

Protein phosphatase 2A PR130/B''alpha1 subunit binds to the SH2 domain-containing inositol polyphosphate 5-phosphatase 2 and prevents epidermal growth factor (EGF)-induced EGF receptor degradation sustaining EGF-mediated signaling

To elucidate novel cell biological functions of specific protein phosphatase 2A (PP2A) holoenzymes, we identified and biochemically characterized a complex between the Src homology 2 (SH2) domain-containing inositol polyphosphate 5-phosphatase 2 (SHIP2) and a PP2A holoenzyme comprising PR130/B''alpha1 as a regulatory subunit (PP2A(T130)) in several mammalian cell lines. PR130/B''alpha1 and SHIP2 partially colocalize in untreated HeLa cells, and both translocate to the cell membrane on epidermal growth factor (EGF) stimulation. Concomitantly, a transient EGF-dependent interaction of PR130/B''alpha1 with the EGF receptor (EGFR) was observed, whereas the SHIP2-PR130 interaction remained constitutive. As previously reported for SHIP2, RNA interference-mediated knockdown of PR130 in COS-7 cells resulted in increased EGF-induced proteasome-dependent EGFR degradation, and an increased interaction of EGFR with the E3 ligase c-Cbl. In concordance with faster EGFR clearance or desensitization, intrinsic EGFR kinase activity (phospho-Tyr-1068) and downstream protein kinase B and extracellular signal-regulated kinase/mitogen-activated protein kinase pathways were more rapidly inactivated in PR130-knockdown cells. Notably, these effects could be rescued by reintroduction of RNA interference-resistant Myc-PR130, excluding any off-target effect. These data highlight a novel biological role of the PP2A(T130) holoenzyme in EGF signaling through interaction with EGFR and the phosphatidylinositol (3,4,5)-trisphosphate 5-phosphatase SHIP2. This interaction may be of clinical relevance as dysfunction of EGF-mediated signaling has been linked to various human cancers.

Oncogenic EGFR signaling networks in glioma

The epidermal growth factor receptor (EGFR) is a primary contributor to glioblastoma (GBM) initiation and progression. Here, we examine how EGFR and key downstream signaling networks contribute to the hallmark characteristics of GBM such as rapid cancer cell proliferation and diffused invasion. Additionally, we discuss current therapeutic options for GBM patients and elaborate on the mechanisms through which EGFR promotes chemoresistance. We conclude by offering a perspective on how the potential of integrative systems biology may be harnessed to develop safe and effective treatment strategies for this disease.

Adaptive regulation at the cell surface by N-glycosylation

The association of receptors and solute transporters with components of the endocytic machinery regulates their surface levels, and thereby cellular sensitivity to cytokines, ligands and nutrients in the extracellular environment. Most transmembrane receptors and solute transporters are glycoproteins, and the Asn (N)-linked oligosaccharides (N-glycans) can bind animal lectins, forming multivalent lattices or microdomains that regulate glycoprotein mobility in the plane of membrane. The N-glycan number (sequence-encoded NXS/T) and context-dependent Golgi N-glycan branching cooperate to regulate glycoprotein affinities for the galectin family of lectins. Galectin-3 binding reduces EGF receptor trafficking into clathrin-coated pits and caveolae lipid rafts, decreases ligand-independent receptor activation and promotes alpha5beta1 integrin remodelling in focal adhesions. N-glycan branching in the medial Golgi increases glycan affinity for galectins, and the Golgi pathway is sensitive to uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) supply, in turn hexosamine pathway metabolites (fructose-6-P, glutamine and acetyl-CoA). Thus, lattice avidity and cellular responsiveness to extracellular cues are regulated in an adaptive manner by metabolism and Golgi modification to glycoproteins. Computational modelling of the hexosamine/Golgi/lattice has provided new insight on cell surface adaptation in cancer and autoimmune disease.

Negative action of hepatocyte growth factor/c-Met system on angiotensin II signaling via ligand-dependent epithelial growth factor receptor degradation mechanism in vascular smooth muscle cells

RATIONALE

Neointimal hyperplasia contributes to atherosclerosis and restenosis after percutaneous coronary intervention. Vascular injury in each of these conditions results in the release of mitogenic growth factors and hormones that contribute to pathological vascular smooth muscle cell growth and inflammation. Hepatocyte growth factor (HGF) is known as an antiinflammatory growth factor, although it is downregulated in injured tissue. However, the precise mechanism how HGF reduces inflammation is unclear.

OBJECTIVE

To elucidate the mechanism how HGF and its receptor c-Met reduces angiotensin II (Ang II)-induced inflammation.

METHODS AND RESULTS

HGF reduced Ang II-induced vascular smooth muscle cell growth and inflammation by controlling translocation of SHIP2 (Src homology domain 2-containing inositol 5'-phosphatase 2), which led to Ang II-dependent degradation of epithelial growth factor receptor. Moreover, the present study also revealed a preventive effect of HGF on atherosclerotic change in an Ang II infusion and cuff HGF transgenic mouse model.

CONCLUSIONS

These data suggest that the HGF/c-Met system might regulate extrinsic factor signaling that maintains the homeostasis of organs.

Galectin-3 negatively regulates TCR-mediated CD4+ T-cell activation at the immunological synapse

We have investigated the function of endogenous galectin-3 in T cells. Galectin-3-deficient (gal3(-/-)) CD4(+) T cells secreted more IFN-gamma and IL-4 than gal3(+/+)CD4(+) T cells after T-cell receptor (TCR) engagement. Galectin-3 was recruited to the cytoplasmic side of the immunological synapse (IS) in activated T cells. In T cells stimulated on supported lipid bilayers, galectin-3 was primarily located at the peripheral supramolecular activation cluster (pSMAC). Gal3(+/+) T cells formed central SMAC on lipid bilayers less effectively and adhered to antigen-presenting cells less firmly than gal3(-/-) T cells, suggesting that galectin-3 destabilizes the IS. Galectin-3 expression was associated with lower levels of early signaling events and phosphotyrosine signals at the pSMAC. Additional data suggest that galectin-3 potentiates down-regulation of TCR in T cells. By yeast two-hybrid screening, we identified as a galectin-3-binding partner, Alix, which is known to be involved in protein transport and regulation of cell surface expression of certain receptors. Co-immunoprecipitation confirmed galectin-3-Alix association and immunofluorescence analysis demonstrated the translocation of Alix to the IS in activated T cells. We conclude that galectin-3 is an inhibitory regulator of T-cell activation and functions intracellularly by promoting TCR down-regulation, possibly through modulating Alix's function at the IS.

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.

Role of cell cycle in epidermal growth factor receptor inhibitor-mediated radiosensitization

Epidermal growth factor receptor (EGFR) inhibitors are increasingly used in combination with radiotherapy in the treatment of various EGFR-overexpressing cancers. However, little is known about the effects of cell cycle status on EGFR inhibitor-mediated radiosensitization. Using EGFR-overexpressing A431 and UMSCC-1 cells in culture, we found that radiation activated the EGFR and extracellular signal-regulated kinase pathways in quiescent cells, leading to progression of cells from G(1) to S, but this activation and progression did not occur in proliferating cells. Inhibition of this activation blocked S-phase progression and protected quiescent cells from radiation-induced death. To determine if these effects were caused by EGFR expression, we transfected Chinese hamster ovary (CHO) cells, which lack EGFR expression, with EGFR expression vector. EGFR expressed in CHO cells also became activated in quiescent cells but not in proliferating cells after irradiation. Moreover, quiescent cells expressing EGFR underwent increased radiation-induced clonogenic death compared with both proliferating CHO cells expressing EGFR and quiescent wild-type CHO cells. Our data show that radiation-induced enhancement of cell death in quiescent cells involves activation of the EGFR and extracellular signal-regulated kinase pathways. Furthermore, they suggest that EGFR inhibitors may protect quiescent tumor cells, whereas radiosensitization of proliferating cells may be caused by downstream effects such as cell cycle redistribution. These findings emphasize the need for careful scheduling of treatment with the combination of EGFR inhibitors and radiation and suggest that EGFR inhibitors might best be given after radiation in order to optimize clinical outcome.

Spinocerebellar ataxia 2 (SCA2)

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominantly inherited, neurodegenerative disease. It can manifest either with a cerebellar syndrome or as Parkinson's syndrome, while later stages involve mainly brainstem, spinal cord and thalamus. This particular atrophy pattern resembles sporadic multi-system-atrophy (MSA) and results in some clinical features indicative of SCA2, such as early saccade slowing, early hyporeflexia, severe tremor of postural or action type, and early myoclonus. For treatment, levodopa is temporarily useful for rigidity/bradykinesia and for tremor, magnesium for muscle cramps, but neuroprotective therapy will depend on the elucidation of pathogenesis. The disease cause lies in the polyglutamine domain of the protein ataxin-2, which can expand in families over successive generations resulting in earlier onset age and faster progression. Genetic testing in SCA2 and other polyglutamine disorders like the well-studied Huntington's disease is now readily available for family planning. Although these disorders differ clinically and in the affected neuron populations, it is not understood how the different polyglutamine proteins mediate such tissue specificity. The neuronal intranuclear inclusion bodies described in other polyglutamine disorders are not frequent in SCA2. For the quite ubiquitously expressed ataxin-2, a subcellular localization at the Golgi, the endoplasmic reticulum and the plasma membrane, in interaction with proteins of mRNA translation and of endocytosis have been observed. As a first victim of SCA2 degeneration, cerebellar Purkinje neurons may be preferentially susceptible to alterations of these subcellular pathways, and therefore our review aims to portray the particular profile of the SCA2 disease process and correlate it to the specific features of ataxin-2.

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.

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.

Endocytosis and intracellular trafficking of ErbBs

This review article describes the pathways and mechanisms of endocytosis and post-endocytic sorting of the EGF receptor (EGFR/ErbB1) and other members of the ErbB family. Growth factor binding to EGFR accelerates its internalization through clathrin-coated pits which is followed by the efficient lysosomal targeting of internalized receptors and results in receptor down-regulation. The role of EGFR interaction with the Grb2 adaptor protein and Cbl ubiquitin ligase, and receptor ubiquitination in the clathrin-dependent internalization and sorting of EGFR in multivesicular endosomes is discussed. Activation and phosphorylation of ErbB2, ErbB3 and ErbB4 also results in their ubiquitination. However, these ErbBs are internalized and targeted to lysosomes less efficiently than EGFR. When overexpressed endocytosis-impaired ErbBs may inhibit the internalization and degradation of EGFR.

Systems biological analysis of epidermal growth factor receptor internalization dynamics for altered receptor levels

Epidermal growth factor (EGF) receptor (EGFR) overexpression is a hallmark of many cancers. EGFR endocytosis is a critical step in signal attenuation, raising the question of how receptor expression levels affect the internalization process. Here we combined quantitative experimental and mathematical modeling approaches to investigate the role of the EGFR expression level on the rate of receptor internalization. Using tetramethylrhodamine-labeled EGF, we established assays for quantifying EGF-triggered EGFR internalization by both high resolution confocal microscopy and flow cytometry. We determined that the flow cytometry approach was more sensitive for examining large populations of cells. Mathematical modeling was used to investigate the relationship between EGF internalization kinetics, EGFR expression, and internalization machinery. We predicted that the standard parameter used to assess internalization kinetics, the temporal evolution r(t) of the ratio of internalized versus surface-located ligand.receptor complexes, does not describe a straight line, as proposed previously. Instead, a convex or concave curve occurs depending on whether initial receptor numbers or internalization adaptors are limiting the uptake reaction, respectively. To test model predictions, we measured EGF-EGFR binding and internalization in cells expressing different levels of green fluorescent protein-EGFR. As expected, surface binding of rhodamine-labeled EGF increased with green fluorescent protein-EGFR expression level. Unexpectedly, internalization of ligand. receptor complexes increased linearly with increasing receptor expression level, suggesting that receptors and not internalization adaptors were limiting the uptake in our experimental model. Finally, determining the ratio of internalized versus surface-located ligand.receptor complexes for this cell line confirmed that it follows a convex curve, supporting our model predictions.

Essential role of c-Cbl in amphiregulin-induced recycling and signaling of the endogenous epidermal growth factor receptor

The intracellular processing of the epidermal growth factor receptor (EGFR) induced by epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-alpha) has been studied meticulously, with the former resulting in EGFR degradation and the latter in EGFR recycling to the plasma membrane. However, little is known about how other EGF family growth factors affect the trafficking of the EGFR. Additionally, although both EGF and TGF-alpha have been shown to effectively induce initial c-Cbl (ubiquitin ligase)-mediated ubiquitination of the EGFR, limited information is available regarding the role of c-Cblin the trafficking and signaling of recycling EGFR. Thus, in this study, we investigated the roles of c-Cblin endogenous EGFR trafficking and signaling after stimulation with amphiregulin (AR). We demonstrated that a physiological concentration of AR induced recycling of the endogenous EGFR to the plasma membrane, which correlated closely with transient association of the EGFR with c-Cbl and transient EGFR ubiquitination. Most importantly, we used c-Cbl small interfering RNA (siRNA) duplexes and ac-Cbl dominant negative mutant to show that c-Cbl is critical for the efficient transition of the EGFR from early endosomes to a recycling pathway and that c-Cbl regulates the duration of extracellular signal regulated kinase 1/2 mitogen-activated protein kinase (ERK1/2 MAPK) phosphorylation. These data support novel functions of c-Cbl in mediating recycling of EGF receptors to the plasma membrane, as well as in mediating the duration of activation (transient vs sustained) of ERK1/2 MAPK phosphorylation.

CBL exon 8/9 mutants activate the FLT3 pathway and cluster in core binding factor/11q deletion acute myeloid leukemia/myelodysplastic syndrome subtypes

PURPOSE

CBL is a negative regulator of activated receptor tyrosine kinases (RTK). In this study, we determined the frequency of CBL mutations in acute leukemias and evaluated the oncogenic potential of mutant CBL.

EXPERIMENTAL DESIGN

The cDNA of 300 acute myeloid leukemia (AML)/myelodysplastic syndrome (MDS) and acute lymphoblastic leukemia (ALL) patients and 82 human leukemic cell lines was screened for aberrations in the linker and RING finger domain of CBL. The oncogenic potential of identified mutants was evaluated in hematopoietic cells.

RESULTS

We identified 3 of 279 AML/MDS patients expressing CBL exon 8/9 deletion mutants. Three of four cases at diagnosis expressed deleted transcripts missing exon 8 or exon 8/9. In remission samples a weak or no expression of mutant CBL was detected. No aberrations were found in normal hematopoietic tissues. One of 116 sequenced AML/MDS cases carried a R420G missense mutation. All AML/MDS patients with identified CBL mutants belonged to the core binding factor and 11q deletion AML subtypes. Functionally, CBL negatively regulated FMS-like tyrosine kinase 3 (FLT3) activity and interacted with human FLT3 via the autophosphorylation sites Y589 and Y599 and colocalized in vivo. Expression of CBLDeltaexon8 and CBLDeltaexon8+9 in FLT3-WT-Ba/F3 cells induced growth factor-independent proliferation associated with autophosphorylation of FLT3 and activated the downstream targets signal transducer and activator of transcription 5 (STAT5) and protein kinase B (AKT). FLT3 ligand-dependent hyperproliferation of CBL mutant cells could be abrogated by treatment with the FLT3 PTK inhibitor PKC412 (midostaurin).

CONCLUSION

CBL exon8/9 mutants occur in genetically defined AML/MDS subtypes and transform hematopoietic cells by constitutively activating the FLT3 pathway. This phenotype resembles the one of mutated RTKs and suggests that CBL mutant AML patients might benefit from treatment with FLT3 PTK inhibitors.

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.