Regulation of the association of p120cbl with Grb2 in Jurkat T cells

Regions outside of conserved PxxPxR motifs drive the high affinity interaction of GRB2 with SH3 domain ligands

SH3 domains are evolutionarily conserved protein interaction domains that control nearly all cellular processes in eukaryotes. The current model is that most SH3 domains bind discreet PxxPxR motifs with weak affinity and relatively low selectivity. However, the interactions of full-length SH3 domain-containing proteins with ligands are highly specific and have much stronger affinity. This suggests that regions outside of PxxPxR motifs drive these interactions. In this study, we observed that PxxPxR motifs were required for the binding of the adaptor protein GRB2 to short peptides from its ligand SOS1. Surprisingly, PxxPxR motifs from the proline rich region of SOS1 or CBL were neither necessary nor sufficient for the in vitro or in vivo interaction with full-length GRB2. Together, our findings show that regions outside of the consensus PxxPxR sites drive the high affinity association of GRB2 with SH3 domain ligands, suggesting that the binding mechanism for this and other SH3 domain interactions may be more complex than originally thought.

The ESCRT machinery: from the plasma membrane to endosomes and back again

The manipulation and reorganization of lipid bilayers are required for diverse cellular processes, ranging from organelle biogenesis to cytokinetic abscission, and often involves transient membrane disruption. A set of membrane-associated proteins collectively known as the endosomal sorting complex required for transport (ESCRT) machinery has been implicated in membrane scission steps, which transform a single, continuous bilayer into two distinct bilayers, while simultaneously segregating cargo throughout the process. Components of the ESCRT pathway, which include 5 distinct protein complexes and an array of accessory factors, each serve discrete functions. This review focuses on the molecular mechanisms by which the ESCRT proteins facilitate cargo sequestration and membrane remodeling and highlights their unique roles in cellular homeostasis.

T-cell receptor ligation causes Wiskott-Aldrich syndrome protein degradation and F-actin assembly downregulation

BACKGROUND

Wiskott-Aldrich syndrome protein (WASP) links T-cell receptor (TCR) signaling to the actin cytoskeleton. WASP is normally protected from degradation by the Ca(++)-dependent protease calpain and by the proteasome because of its interaction with the WASP-interacting protein.

OBJECTIVE

We investigated whether WASP is degraded after TCR ligation and whether its degradation downregulates F-actin assembly caused by TCR ligation.

METHODS

Primary T cells, Jurkat T cells, and transfected 293T cells were used in immunoprecipitation experiments. Intracellular F-actin content was measured in splenic T cells from wild-type, WASP-deficient, and c-Casitas B-lineage lymphoma (Cbl)-b-deficient mice by using flow cytometry. Calpeptin and MG-132 were used to inhibit calpain and the proteasome, respectively.

RESULTS

A fraction of WASP in T cells was degraded by calpain and by the ubiquitin-proteasome pathway after TCR ligation. The Cbl-b and c-Cbl E3 ubiquitin ligases associated with WASP after TCR signaling and caused its ubiquitination. Inhibition of calpain and lack of Cbl-b resulted in a significantly more sustained increase in F-actin content after TCR ligation in wild-type T cells but not in WASP-deficient T cells.

CONCLUSION

TCR ligation causes WASP to be degraded by calpain and to be ubiquitinated by Cbl family E3 ligases, which targets it for destruction by the proteasome. WASP degradation might provide a mechanism for regulating WASP-dependent TCR-driven assembly of F-actin.

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.

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.

Stromal-cell-derived factor-1/CXCL12-induced chemotaxis of a T cell line involves intracellular signaling through Cbl and Cbl-b and their regulation by Src kinases and CD45

Stromal-cell-derived factor-1alpha (SDF-1alpha/CXCL12) is a potent chemoattractant for T cells. We report that Cbl family members, Cbl and Cbl-b, are tyrosine-phosphorylated after SDF-1alpha/CXCL12 stimulation of Jurkat T cells. Enhanced phosphorylation of Cbl and Cbl-b was regulated by src family kinases, and perhaps Fyn. Activated Cbl and Cbl-b interacted with Crk-L, Zap-70, Nck, PLC-gamma and Fyb after SDF-1alpha/CXCL12 stimulation, implicating association of these proteins in SDF-1alpha/CXCL12 actions. SDF-1alpha/CXCL12 did not induce tyrosine phosphorylation of Cbl or Cbl-b in Lck-deficient T cell line J.CaM1.6 or CD45-deficient T cell line J45.01. Thus, Lck Src kinase and tyrosine phosphatase CD45 are likely involved in regulating activation of Cbl family members. A functional role for Cbl and Cbl-b in migration was demonstrated by the decrease in SDF-1/CXCL12-induced migration in a T cell line in which transfected small interfering RNA for Cbl and Cbl-b decreased expression of Cbl and Cbl-b, but not MAPK activity. SDF-1alpha/CXCL12-induced chemotaxis was greatly reduced in the CD45-deficient T cell line. Our results implicate CD45, Cbl, Cbl-b, src kinases and potentially other associated proteins as mediators of SDF-1alpha/CXCL12-induced cell migration of Jurkat T cells.

Recruitment and activation of PLCgamma1 in T cells: a new insight into old domains

Engagement of the T-cell antigen receptor leads to recruitment of phospholipase Cgamma1 (PLCgamma1) to the LAT-nucleated signaling complex and to PLCgamma1 activation in a tyrosine phosphorylation-dependent manner. The mechanism of PLCgamma1 recruitment and the role of PLCgamma1 Src homology (SH) domains in this process remain incompletely understood. Using a combination of biochemical methods and real-time fluorescent imaging, we show here that the N-terminal SH2 domain of PLCgamma1 is necessary but not sufficient for its recruitment. Either the SH3 or C-terminal SH2 domain of PLCgamma1, with the participation of Vav1, c-Cbl and Slp76, are required to stabilize PLCgamma1 recruitment. All three PLCgamma1 SH domains are required for phosphorylation of PLCgamma1 Y783, which is critical for enzyme activation. These novel findings entailed revision of the currently accepted model of PLCgamma1 recruitment and activation in T lymphocytes.

Nef enhances c-Cbl phosphorylation in HIV-infected CD4+ T lymphocytes

The multifunctional HIV-1 protein Nef possesses several motifs that interact with signaling molecules in infected T cells. In order to determine whether Nef influences T cell activation, cells were infected with Nef-positive and Nef-negative clones of HIV. CD28 expression and changes in tyrosine phosphorylation were monitored. We observed no Nef-dependent changes in CD28 expression or function. However, infection with Nef-positive virus led to changes in tyrosine phosphorylation. This Nef-induced phosphorylation was observed in unstimulated cells, and c-Cbl was identified as one of the proteins whose phosphorylation was upregulated by Nef. Furthermore, Lck is required for Nef-mediated c-Cbl tyrosine phosphorylation. These results suggest that Nef modifies T cell signaling in the absence of T cell receptor engagement and co-stimulation.

Distinct Turnover of Alternatively Spliced Isoforms of the RET Kinase Receptor Mediated by Differential Recruitment of the Cbl Ubiquitin Ligase

Alternative splicing of transcripts encoding the RET kinase receptor leads to isoforms differing in their cytoplasmic tail. Although in vitro studies have demonstrated a higher transforming activity of the long RET isoform (RET51), only the short isoform (RET9) can rescue the effects of a RET null mutation in the enteric nervous system and kidney development. The molecular basis underlying the distinct functions of the two RET isoforms is not understood. Here we demonstrated that activated RET51 associated more strongly with the ubiquitin ligase Cbl than did RET9, leading to increased ubiquitylation and faster turnover of RET51. The association of Cbl with RET was indirect and was mediated through Grb2. A constitutive complex of Grb2 and Cbl could be recruited to both receptor isoforms via docking of Shc to phosphorylated Tyr-1062 in RET. A mutant Shc protein unable to recruit the Grb2.Cbl complex decreased the turnover and prolonged the half-life of RET9, thus ascribing a previously unknown negative role to the Shc adaptor molecule. In addition, phosphorylation of Tyr-1096, which is present in RET51 but absent in RET9, endowed the longer isoform with a second route to recruit the Grb2.Cbl complex. These findings establish a mechanism for the differential down-regulation of RET9 and RET51 signaling that could explain the apparently paradoxical activities of these two RET isoforms. More generally, these results illustrate how alternative splicing can regulate the half-life and function of a growth factor receptor.

Coexistence of phosphotyrosine-dependent and -independent interactions between Cbl and Bcr-Abl

Cbl is one of the major tyrosine-phosphorylated proteins in Bcr-Abl-expressing cells. A direct association between the SH2 domain of Bcr-Abl and tyrosine-phosphorylated Cbl has been demonstrated. The purpose of this study was to determine if and how unphosphorylated Cbl and Bcr-Abl may associate. Interactions between Cbl and Bcr-Abl were investigated in yeast two- and three-hybrid systems, gel overlay assays, and immunoprecipitates from mammalian cells expressing wild-type and the Y177F mutant of Bcr-Abl. No direct interaction between Bcr-Abl and unphosphorylated Cbl was observed. Bcr-Abl did, however, associate with Grb2, an adaptor protein that binds tyrosine 177 of Bcr-Abl. Additionally, Grb2 interacted with Cbl. In a yeast three-hybrid assay, Grb2 mediated an interaction between Cbl and Bcr-Abl that was dependent on a functional Grb2 binding site. This interaction was confirmed in vitro using purified proteins. In cells expressing Bcr-Abl with a mutation in the Grb2 binding site, binding of Cbl to Bcr-Abl was significantly reduced, but Cbl tyrosine phosphorylation was maintained. Imatinib treatment of these cells further reduced but did not abrogate Cbl binding, reflecting residual kinase activity. Multiple phosphotyrosine-dependent and -independent interactions stabilize the interaction between Cbl and Abl. Grb2 or another, yet unidentified, protein may mediate an initial interaction between Cbl and Bcr-Abl that is independent of Cbl tyrosine phosphorylation. Following this initial interaction, Cbl can then become tyrosine phosphorylated and interact with the SH2 domain of Bcr-Abl, further stabilizing the complex.

Epidermal growth factor receptor internalization through clathrin-coated pits requires Cbl RING finger and proline-rich domains but not receptor polyubiquitylation

Cbl proteins have been implicated in the regulation of endocytic trafficking of epidermal growth factor receptor. However, the precise role of Cbl in epidermal growth factor receptor endocytosis is not defined. To directly visualize Cbl in cells and perform structure-function analysis of Cbl's role in epidermal growth factor receptor internalization, a yellow fluorescent protein-fusion of c-Cbl was constructed. Upon epidermal growth factor receptor activation, Cbl-yellow fluorescent protein moved with epidermal growth factor receptor to clathrin-coated pits and endosomes. Localization of Cbl-yellow fluorescent protein to these endocytic organelles was dependent on a proline-rich domain of c-Cbl that interacts with Grb2 as shown by fluorescence resonance energy transfer microscopy. In contrast, direct binding of Cbl to phosphotyrosine 1045 of the epidermal growth factor receptor was required for epidermal growth factor receptor polyubiquitination, but was not essential for Cbl-yellow fluorescent protein localization in epidermal growth factor receptor-containing compartments. These data suggest that the binding of Cbl to epidermal growth factor receptor through Grb2 is necessary and sufficient for Cbl function during clathrin-mediated endocytosis. Overexpression of c-Cbl mutants that are capable of Grb2 binding but defective in linker/RING finger domain function severely inhibited epidermal growth factor receptor internalization. The same dominant-negative mutants of Cbl did not block epidermal growth factor receptor recruitment into coated pits but retained receptors in coated pits, thus preventing receptor endocytosis and transport to endosomes. These data suggest that the linker and RING finger domain of Cbl may function during late steps of coated vesicle formation. We propose that the RING domain of Cbl facilitates endocytosis either by epidermal growth factor receptor monoubiquitylation or by ubiquitylation of proteins associated with the receptor.

Ephrin-A1 induces c-Cbl phosphorylation and EphA receptor down-regulation in T cells

Eph receptor tyrosine kinases are expressed by T lineage cells, and stimulation with their ligands, the ephrins, has recently been shown to modulate T cell behavior. We show that ephrin-A1 stimulation of Jurkat T cells induces tyrosine phosphorylation of EphA3 receptors and cytoplasmic proteins, including the c-cbl proto-oncogene. Cbl phosphorylation was also observed in peripheral blood T cells. In contrast, stimulation of Jurkat cells with the EphB receptor ligand ephrin-B1 does not cause Cbl phosphorylation. EphA activation also induced Cbl association with Crk-L and Crk-II adapters, but not the related Grb2 protein. Induction of Cbl phosphorylation upon EphA activation appeared to be dependent upon Src family kinase activity, as Cbl phosphorylation was selectively abrogated by the Src family inhibitor 4-amino-5(4-chlorophenyl-7-(tert-butyl)pyrazolo[3,4-d]pyrimidine, while EphA phosphorylation was unimpaired. Ephrin-A1 stimulation of Jurkat cells was also found to cause down-regulation of endogenous EphA3 receptors from the cell surface and their degradation. In accordance with the role of Cbl as a negative regulator of receptor tyrosine kinases, overexpression of wild-type Cbl, but not its 70-Z mutant, was found to down-regulate EphA receptor expression. Receptor down-regulation could also be inhibited by blockage of Src family kinase activity. Our findings show that EphA receptors can actively signal in T cells, and that Cbl performs multiple roles in this signaling pathway, functioning to transduce signals from the receptors as well as regulating activated EphA receptor expression.

Association of a new c-Cbl related protein with the very first stages of apoptosis induction

This study investigates the involvement of the c-cbl proto-oncogene during the first stages of the apoptotic process. We have already shown that a c-Cbl aptotosis-related protein of 90 kDa (CARP 90) is detected very rapidly in the cytoplasm as well as in the nucleus of murine thymocytes after hydrocortisone (HC) treatment. We report here that this protein appeared as well after in vivo treatment of mice by gamma-irradiation or injection of anti-CD3 monoclonal antibody, two potent thymic apoptosis inductors, providing a close relationship between the occurrence of apoptosis and the appearance of CARP 90. We showed that CARP 90 and p120(cbl) share numerous epitopes strikingly suggesting that CARP 90 is coded by c-cbl. In addition, KO mice do not sustain CARP 90 appearance. We finally showed that CARP 90 contains N- and C-terminal end epitopes of p120(cbl), which suggests that CARP 90 is an alternative spliced form of c-cbl. This protein was also observed under gamma-irradiation in tissues of different origin, which enlarges the physiological significance of this phenomenon. The very rapid CARP 90 appearance under apoptotic conditions in the nucleus of cells originating in different tissues makes this protein if not a possible new actor of the apoptotic process, at least an interesting marker of this process.

FRS2 alpha attenuates FGF receptor signaling by Grb2-mediated recruitment of the ubiquitin ligase Cbl

Attenuation of growth factor signaling is essential for the regulation of developmental processes and tissue homeostasis in most organisms. The product of Cbl protooncogene is one such regulator, which functions as an ubiquitin ligase that ubiquitinates and promotes the degradation of a variety of cell signaling proteins. Here, we demonstrate that Grb2 bound to tyrosine-phosphorylated FRS2 alpha forms a ternary complex with Cbl by means of its Src homology 3 domains resulting in the ubiquitination of fibroblast growth factor (FGF) receptor and FRS2 alpha in response to FGF stimulation. These observations highlight the importance of FRS2 alpha in the assembly of both positive (i.e., Sos, phosphatidylinositol 3-kinase) and negative (i.e., Cbl) signaling proteins to mediate a balanced FGF signal transduction. However, the partial inhibition of FGF receptor down-regulation in FRS2 alpha-/- cells indicates that the attenuation of signaling by FGF receptor is regulated by redundant or multiple mechanisms.

A mutant EGF-receptor defective in ubiquitylation and endocytosis unveils a role for Grb2 in negative signaling

Ligand-induced desensitization of the epidermal growth factor receptor (EGFR) is controlled by c-Cbl, a ubiquitin ligase that binds multiple signaling proteins, including the Grb2 adaptor. Consistent with a negative role for c-Cbl, here we report that defective Tyr1045 of EGFR, an inducible c-Cbl docking site, enhances the mitogenic response to EGF. Signaling potentiation is due to accelerated recycling of the mutant receptor and a concomitant defect in ligand-induced ubiquitylation and endocytosis of EGFR. Kinetic as well as morphological analyses of the internalization-defective mutant receptor imply that c-Cbl-mediated ubiquitylation sorts EGFR to endocytosis and to subsequent degradation in lysosomes. Unexpectedly, however, the mutant receptor displayed significant residual ligand-induced ubiquitylation, especially in the presence of an overexpressed c-Cbl. The underlying mechanism seems to involve recruitment of a Grb2 c-Cbl complex to Grb2-specific docking sites of EGFR, and concurrent acceleration of receptor ubiquitylation and desensitization. Thus, in addition to its well-characterized role in mediating positive signals, Grb2 can terminate signal transduction by accelerating c-Cbl-dependent sorting of active tyrosine kinases to destruction.

Role for adapter proteins in costimulatory signals of CD2 and IL-2 on NK cell activation

Natural killer (NK) cells participate in both innate and adaptive immunity through the prompt secretion of cytokines and ability to lyse virally infected cells or tumor cells. Triggering of NK cells requires aggregation of surface receptors such as CD2 and CD16, and NK cell activity can be augmented in vitro by stimulation with IL-2. In this study, we examined the role of adapter proteins in the increased NK activation following CD2 crosslinking and IL-2 stimulation of NK3.3 cells. NK3.3 cells lysed NK-sensitive K562 cells in a CD2-dependent manner, and IL-2 markedly enhanced lytic activity in a 4h cytotoxic assay. IL-2 also enhanced spontaneous and CD2-mediated granule exocytosis from NK3.3 cells. CD2 crosslinking markedly induced tyrosine phosphorylation of Cbl associated with Grb2 or CrkL, Shc and LAT, compared with IL-2 stimulation. However, costimulation of IL-2 with CD2 crosslinking remarkably enhanced associations of Grb2-Shc and CrkL-Cbl, compared to IL-2 stimulation or CD2 crosslinking alone. In vitro binding studies using GST-fusion proteins revealed that interactions of Grb2-Shc and CrkL-Cbl were mediated through each SH2 domain in tyrosine phosphorylation-dependent manner. Furthermore, CD2 crosslinking, but not IL-2 stimulation, markedly induced tyrosine phosphorylation of LAT. Thus, tyrosine phosphorylation of different adapter proteins and consequent interactions between signaling molecules described here may explain the molecular mechanisms of the additive effects of IL-2 stimulation and CD2 crosslinking on NK cell activation.

Beyond the RING: CBL proteins as multivalent adapters

Following discovery of c-Cbl, a cellular form of the transforming retroviral protein v-Cbl, multiple Cbl-related proteins have been identified in vertebrate and invertebrate organisms. c-Cbl and its homologues are capable of interacting with numerous proteins involved in cell signaling, including various molecular adapters and protein tyrosine kinases. It appears that Cbl proteins play several functional roles, acting both as multivalent adapters and inhibitors of various protein tyrosine kinases. The latter function is linked, to a substantial extent, to the E3 ubiquitin-ligase activity of Cbl proteins. Experimental evidence for these functions, interrelations between them, and their biological significance are addressed in this review, with the main accent placed on the adapter functions of Cbl proteins.

Suppression of apoptosis induced by growth factor withdrawal by an oncogenic form of c-Cbl

The v-Cbl oncogene induces myeloid and B-cell leukemia; however, the mechanism by which transformation occurs is not understood. An oncogenic form of c-Cbl (Cbl-DeltaY371) was expressed in the interleukin-3 (IL-3)-dependent cell line 32Dcl3 to determine whether it was able to induce growth factor-independent proliferation. We were unable to isolate clones of transfected 32Dcl3 cells expressing Cbl-DeltaY371 that proliferated in the absence of IL-3. In contrast, 32Dcl3/Cbl-DeltaY371 cells did not undergo apoptosis like parental 32Dcl3 cells when cultured in the absence of IL-3. Both 32Dcl3 and 32D/CblDeltaY371 cells arrested in G(1) when cultured in the absence of IL-3. Approximately 18% of the 32Dcl3 cells cultured in the absence of IL-3 for 24 h were present in a sub-G(1) fraction, while only 4% of the 32D/Cbl-DeltaY371 and 2% of the 32D/Bcl-2 cells were found in a sub-G(1) fraction. There was no difference in the pattern of tyrosine-phosphorylated proteins observed following stimulation of either cell type with IL-3. The phosphorylation of JAK2, STAT5, and endogenous c-Cbl was identical in both cell types. No differences were detected in the activation of Akt, ERK1, or ERK2 in unstimulated or IL-3-stimulated 32D/Cbl-DeltaY371 cells compared with parental 32Dcl3 cells. Likewise, there was no difference in the pattern of phosphorylation of JAK2, STAT5, ERK1, ERK2, or Akt when 32Dcl3 and 32D/CblDY371 cells were withdrawn from medium containing IL-3. The protein levels of various Bcl-2 family members were examined in cells grown in the absence or presence of IL-3. We observed a consistent increased amount of Bcl-2 protein in five different clones of 32D/Cbl-DeltaY317 cells. These data suggest that the Cbl-DeltaY371 mutant may suppress apoptosis by a mechanism that involves the overexpression of Bcl-2. Consistent with this result, activation of caspase-3 was suppressed in 32D/Cbl-DeltaY371 cells cultured in the absence of IL-3 compared with 32Dcl3 cells cultured under the same conditions.

L-selectin tyrosine phosphorylates cbl and induces association of tyrosine-phosphorylated cbl with crkl and grb2

L-Selectin-mediated rolling of leukocytes on endothelial cells is an important step for lymphocyte homing and an early event in the immune response to pathogens or inflammatory stimuli. We have previously elucidated intracellular signaling cascades upon L-selectin engagement resulting in activation of Ras, Rac and JNK as well as cytoskeletal changes, oxygen release, ceramide synthesis and receptor capping. Activation of the src-tyrosine kinase p56lck is followed by phosphorylation of the L-selectin molecule and MAP-K. Here we show a tyrosine kinase dependent phosphorylation of the Cbl adapter protein after L-selectin engagement in lymphocytes. Phosphorylation of Cbl was absent in Jurkat cells that are pharmacologically treated with tyrosine kinase inhibitors and in lck-deficient JCaM cells. There is an activation induced association of tyrosine phosphorylated Cbl with Grb2 and CrkL, respectively, but not CrkII. Therefore, the adapter protein Cbl plays a role in L-selectin signaling and might modulate immune function by the specific recruitment of signaling molecules to multiprotein complexes.

Cbl-b, a RING-type E3 ubiquitin ligase, targets phosphatidylinositol 3-kinase for ubiquitination in T cells

Cbl-b is implicated in setting the threshold of T lymphocyte activation. In Cbl-b-deficient T cells, the activation of Vav, a guanine nucleotide exchange factor, is significantly enhanced. The molecular mechanism underlying Cbl-b-regulated Vav activation was unclear. Here it is shown that Cbl-b interacts with and induces ubiquitin conjugation to the p85 regulatory subunit of phosphatidylinositol 3-kinase, an upstream regulator of Vav. A functional RING finger of Cbl-b was essential for p85 ubiquitination. However, a loss of function mutation at the well-conserved amino-terminal variant src homology (SH) 2 domain of Cbl-b did not affect its ligase activity. A distal carboxyl-terminal proline-rich region in Cbl-b was mapped to contain the primary binding sequences for the SH3 domain of p85. Deletion of either the distal proline-rich region in Cbl-b or the SH3 domain of p85 severely reduced ubiquitin conjugation to p85. The data suggest a molecular link for Cbl-b-mediated negative regulation of Vav, with phosphatidylinositol 3-kinase as a direct target for Cbl-b E3 ubiquitin ligase.

Regulation of Cbl molecular interactions by the co-receptor molecule CD43 in human T cells

CD43, one of the most abundant glycoproteins on the T cell surface, has been implicated in selection and maturation of thymocytes and migration, adhesion, and activation of mature T cells. The adapter molecule Cbl has been shown to be a negative regulator of Ras. Furthermore, it may also regulate intracellular signaling through the formation of several multi-molecular complexes. Here we investigated the role of Cbl in the CD43-mediated signaling pathway in human T cells. Unlike T cell receptor signaling, the interaction of the adapter protein Cbl with Vav and phosphatidylinositol 3-kinase, resulting from CD43-specific signals, is independent of Cbl tyrosine phosphorylation, suggesting an alternative mechanism of interaction. CD43 signals induced a Cbl serine phosphorylation-dependent interaction with the tau-isoform of 14-3-3. protein. Protein kinase C-mediated Cbl serine phosphorylation was required for this interaction, because the PKC inhibitor RO-31-8220 prevented it, as well as 14-3-3 dimerization. Moreover, mutation of Cbl serine residues 619, 623, 639, and 642 abolished the interaction between Cbl and 14-3-3. Overexpression of Cbl in Jurkat cells inhibited the CD43-dependent activation of the mitogen-activated protein kinase (MAPK) pathway and AP-1 transcriptional activity, confirming nevertheless a negative role for Cbl in T cell signaling. However, under normal conditions, PKC activation resulting from CD43 engagement was required to activate the MAPK pathway, suggesting that phosphorylation of Cbl on serine residues by PKC and its association with 14-3-3 molecules may play a role in preventing the Cbl inhibitory effect on the Ras-MAPK pathway. These data suggest that by inducing its phosphorylation on serine residues, CD43-mediated signals may regulate the molecular associations and functions of the Cbl adapter protein.

Differential interaction of Cbl with Grb2 and CrkL in CD2-mediated NK cell activation

Natural killer (NK) cells participate in both innate and adoptive immunity by their prompt secretion of cytokines and by their ability to lyse virally infected cells or tumor cells. CD2 is surface glycoprotein receptors and crucial for NK cell activation. However, molecular events involved in CD2-mediated NK cell activation have not been fully elucidated. Cbl-Grb2 and Cbl-CrkL interactions have been implicated in T cell and B cell receptor, and cytokine receptor signaling. Here we analyzed tyrosine phosphorylation and interactions of Cbl with adapter proteins, Grb2 and CrkL, in NK3.3 cells. CD2 crosslinking results in the marked tyrosine phosphorylation of Cbl in an antibody concentration- and time-dependent manner. Immunodepletion studies reveal that Grb2-associated tyrosine phosphorylated p120 kDa protein is Cbl. In vitro binding studies using GST-fusion proteins demonstrate that Cbl constitutively associates with the SH3 domains of Grb2, with a preference for the amino-terminal domain. In addition, we demonstrate that CrkL associates with a large portion of tyrosine phosphorylated Cbl after CD2 stimulation of NK3.3 cells. In contrast to constitutive Cbl association with Grb2, tyrosine phosphorylated Cbl interacts with CrkL via its SH2 domain only after CD2 stimulation. Although the precise roles of interactions of Cbl with Grb2 and CrkL in NK cell activation remains to be elucidated, their tyrosine phosphorylation, in addition to the multiple protein interactions described here, strongly suggest that interactions of Cbl with Grb2 and CrkL may play pivotal roles in CD2-mediated NK cell activation.

Requirements of multiple domains of SLI-1, a Caenorhabditis elegans homologue of c-Cbl, and an inhibitory tyrosine in LET-23 in regulating vulval differentiation

SLI-1, a Caenorhabditis elegans homologue of the proto-oncogene product c-Cbl, is a negative regulator of LET-23-mediated vulval differentiation. Lack of SLI-1 activity can compensate for decreased function of the LET-23 epidermal growth factor receptor, the SEM-5 adaptor, but not the LET-60 RAS, suggesting that SLI-1 acts before RAS activation. SLI-1 and c-Cbl comprise an N-terminal region (termed SLI-1:N/Cbl-N, containing a four-helix bundle, an EF hand calcium-binding domain, and a divergent SH2 domain) followed by a RING finger domain and a proline-rich C-terminus. In a transgenic functional assay, the proline-rich C-terminal domain is not essential for sli-1(+) function. A protein lacking the SH2 and RING finger domains has no activity, but a chimeric protein with the SH2 and RING finger domains of SLI-1 replaced by the equivalent domains of c-Cbl has activity. The RING finger domain of c-Cbl has been shown recently to enhance ubiquitination of active RTKs by acting as an E3 ubiquitin-protein ligase. We find that the RING finger domain of SLI-1 is partially dispensable. Further, we identify an inhibitory tyrosine of LET-23 requiring sli-1(+) for its effects: removal of this tyrosine closely mimics the loss of sli-1 but not of another negative regulator, ark-1. Thus, we suggest that this inhibitory tyrosine mediates its effects through SLI-1, which in turn inhibits signaling upstream of LET-60 RAS in a manner not wholly dependent on the ubiquitin-ligase domain.

Differential effects of Cbl and 70Z/3 Cbl on T cell receptor-induced phospholipase Cgamma-1 activity

We demonstrate that the differential effects Cbl and oncogenic 70Z/3 Cbl have on Ca(2+)/Ras-sensitive NF-AT reporters is partially due to their opposing ability to regulate phospholipase Cgamma1 (PLCgamma1) activation as demonstrated by analysis of the activation of an NF-AT reporter construct and PLCgamma1-mediated inositol phospholipid (PI) hydrolysis. Cbl over-expression resulted in reduced T cell receptor-induced PI hydrolysis, in the absence of any effect on PLCgamma1 tyrosine phosphorylation. In contrast, expression of 70Z/3 Cbl led to an increase in basal and OKT3-induced PLCgamma1 phosphorylation and PI hydrolysis. These data indicate that Cbl and 70Z/3 Cbl differentially regulate PLCgamma1 phosphorylation and activation. The implications of these data on the mechanism of Cbl-mediated signaling regulation are discussed.

Cloning and characterization of a novel adaptor protein, CIN85, that interacts with c-Cbl

The c-Cbl protooncogene product is a prominent substrate of protein tyrosine kinases and is rapidly tyrosine-phosphorylated upon stimulation of a wide variety of cell-surface receptors. We have identified a novel c-Cbl-interacting protein termed CIN85 with a molecular mass of 85 kDa which shows similarity to adaptor proteins, CMS and CD2AP. CIN85 mRNA is expressed ubiquitously in normal human tissues and cancer cell lines analyzed. CIN85 was basally associated with c-Cbl. For interaction of CIN85 with c-Cbl, the second SH3 domain of CIN85 was shown to serve as a central player. The CIN85-c-Cbl association was enhanced shortly after stimulation of 293 cells with epidermal growth factor (EGF) and gradually diminished to a basal level, which correlated with a tyrosine phosphorylation level of c-Cbl. Our results suggest that CIN85 may play a specific role in the EGF receptor-mediated signaling cascade via its interaction with c-Cbl.

c-Cbl localizes to actin lamellae and regulates lamellipodia formation and cell morphology

Adhesive and locomotive properties of cells have key roles in normal physiology and disease. Cell motility and adhesion require the assembly and organization of actin microfilaments into stress fibers, lamellipodia and filopodia, and the formation of these structures is mediated by signalling through Rho; GTPases. Here we identify c-Cbl (a multi-adaptor proto-oncogene product involved in protein tyrosine kinase signalling) as an important regulator of the actin cytoskeleton. By immunofluorescence microscopy we have determined that c-Cbl co-localizes with the adaptor protein Crk to submembranous actin lamellae in NIH 3T3 fibroblasts and that c-Cbl's actin localization requires specific SH3-binding sequences. Further, we have found that truncation of this SH3-binding domain in c-Cbl profoundly alters the morphology of NIH 3T3 fibroblasts by inhibiting the formation of actin lamellae, lamellipodia and membrane ruffles. The induction of lamellipodia and membrane ruffles are also inhibited during cell spreading and migration, conditions when these structures are normally most prominent. The inhibitory effect of truncated c-Cbl expression on lamellipodia formation can be reversed by mutational inactivation of its divergent SH2 domain, by the co-expression of constitutively active Rac or by the overexpression of c-Cbl. This study therefore identifies a cytoskeletal role for c-Cbl which may involve the regulation of Crk and Rac, and which is dependent on targeting of c-Cbl to actin lamellae and the ability to recruit signalling protein(s) associated with its divergent SH2 domain.

The oncogenic 70Z Cbl mutation blocks the phosphotyrosine binding domain-dependent negative regulation of ZAP-70 by c-Cbl in Jurkat T cells

T-cell receptor (TCR) engagement results in the activation of Src family (Lck and Fyn) and ZAP-70 protein tyrosine kinases, leading to tyrosine phosphorylation of multiple cellular substrates including the complex adapter protein c-Cbl. Moreover, Cbl is tyrosine phosphorylated upon engagement of growth factor receptors, cytokine receptors, and immunoreceptors and functions as a negative regulator of tyrosine kinase signalling pathways. Cbl associates via its phosphotyrosine binding (PTB) domain to the ZAP-70 pY292 negative regulatory phosphotyrosine. We recently demonstrated that the oncogenic Cbl mutant, 70Z Cbl, requires its PTB domain to upregulate NFAT in unstimulated Jurkat T cells. Here, we demonstrate that kinase-dead but not wild-type forms of Fyn, Lck, and ZAP-70 block 70Z Cbl-mediated NFAT activation. Moreover, 70Z Cbl does not upregulate NFAT in the ZAP-70-deficient P116 Jurkat T-cell line. The requirement for Fyn, Lck, and ZAP-70 is not due to tyrosine phosphorylation of 70Z Cbl, as mutation of all tyrosines in, or deletion of, the C-terminal region of 70Z Cbl (amino acids 655 to 906) blocks 70Z Cbl tyrosine phosphorylation but enhances 70Z Cbl-mediated NFAT activation. Further, 70Z Cbl does not cooperate with ZAP-70 Y292F to upregulate NFAT, indicating that 70Z Cbl and ZAP-70 do not activate parallel signalling pathways. Finally, the upregulation of NFAT observed upon ZAP-70 overexpression is blocked by Cbl in a PTB domain-dependent manner. We conclude that oncogenic 70Z Cbl acts as a dominant negative to block the PTB domain-dependent negative regulatory role of endogenous Cbl on ZAP-70, leading to constitutive ZAP-70 signalling and activation of transcription factors.

Evidence that phospholipase C-gamma2 interacts with SLP-76, Syk, Lyn, LAT and the Fc receptor gamma-chain after stimulation of the collagen receptor glycoprotein VI in human platelets

Platelet activation by collagen is mediated by the sequential tyrosine phosphorylation of the Fc receptor gamma-chain (FcR gamma-chain), which is part of the collagen receptor glycoprotein VI, the tyrosine kinase Syk and phospholipase C-gamma2 (PLC-gamma2). In this study tyrosine-phosphorylated proteins that associate with PLC-gamma2 after stimulation by a collagen-related peptide (CRP) were characterized using glutathione S-transferase fusion proteins of PLC-gamma2 Src homology (SH) domains and by immunoprecipitation of endogenous PLC-gamma2. The majority of the tyrosine-phosphorylated proteins that associate with PLC-gamma2 bind to its C-terminal SH2 domain. These were found to include PLC-gamma2, Syk, SH2-domain-containing leucocyte protein of 76 kDa (SLP-76), Lyn, linker for activation of T cells (LAT) and the FcR gamma-chain. Direct association was detected between PLC-gamma2 and SLP-76, and between PLC-gamma2 and LAT upon CRP stimulation of platelets by far-Western blotting. FcR gamma-chain and Lyn were found to co-immunoprecipitate with PLC-gamma2 as well as with unidentified 110-kDa and 75-kDa phosphoproteins. The absence of an in vivo association between Syk and PLC-gamma2 in platelets is in contrast with that for PLC-gamma1 and Syk in B cells. The in vivo function of PLC-gamma2 SH2 domains was examined through measurement of Ca2+ increases in mouse megakaryocytes that had been microinjected with recombinant proteins. This revealed that the C-terminal SH2 domain is involved in the regulation of PLC-gamma2. These data indicate that the C-terminal SH2 domain of PLC-gamma2 is important for PLC-gamma2 regulation through possible interactions with SLP-76, Syk, Lyn, LAT and the FcR gamma-chain.

Membrane-targeting of signalling molecules by SH2/SH3 domain-containing adaptor proteins

SH2/SH3 domain-containing adaptor proteins play a critical role in regulating tyrosine kinase signalling pathways. The major function of these adaptors, such as Grb2, Nck, and Crk, is to recruit proline-rich effector molecules to tyrosine-phosphorylated kinases or their substrates. In recent years dozens of novel proteins have emerged that are capable of associating with the SH2 and the SH3 domains of adaptors. In this review, the author attempts to summarise these novel binding partners of Grb2, Nck, and Crk, and to discuss current controversies regarding function and regulation of protein multicomplexes held together by SH2/SH3 adaptor molecules at the plasma membrane.

GrpL, a Grb2-related adaptor protein, interacts with SLP-76 to regulate nuclear factor of activated T cell activation

Propagation of signals from the T cell antigen receptor (TCR) involves a number of adaptor molecules. SH2 domain-containing protein 76 (SLP-76) interacts with the guanine nucleotide exchange factor Vav to activate the nuclear factor of activated cells (NF-AT), and its expression is required for normal T cell development. We report the cloning and characterization of a novel Grb2-like adaptor molecule designated as Grb2-related protein of the lymphoid system (GrpL). Expression of GrpL is restricted to hematopoietic tissues, and it is distinguished from Grb2 by having a proline-rich region. GrpL can be coimmunoprecipitated with SLP-76 but not with Sos1 or Sos2 from Jurkat cell lysates. In contrast, Grb2 can be coimmunoprecipitated with Sos1 and Sos2 but not with SLP-76. Moreover, tyrosine-phosphorylated LAT/pp36/38 in detergent lysates prepared from anti-CD3 stimulated T cells associated with Grb2 but not GrpL. These data reveal the presence of distinct complexes involving GrpL and Grb2 in T cells. A functional role of the GrpL-SLP-76 complex is suggested by the ability of GrpL to act alone or in concert with SLP-76 to augment NF-AT activation in Jurkat T cells.

Association of Nck with tyrosine-phosphorylated SLP-76 in activated T lymphocytes

The Nck adaptor protein links tyrosine kinases or their substrates to proteins containing proline-rich motifs. Here we show that in activated T cells two tyrosine phosphoproteins of 75 and 120 kDa are co-immunoprecipitated with polyclonal antibodies against Nck. Analysis of Nck immunoprecipitates with various candidate antibodies revealed that the 75-kDa tyrosine phosphoprotein is the SH2 domain-containing leukocyte protein referred to as SLP-76. In vitro experiments show that the interaction between Nck and SLP-76 is mediated via the Nck SH2 domain. Using specific phosphopeptides corresponding to the major tyrosine phosphorylation sites of SLP-76, it was found that Y113 and Y128 phosphopeptides could compete binding of SLP-76 to the SH2 domain of Nck. In addition, the 120-kDa tyrosine phosphoprotein was recognized by an antibody raised against Cbl, a proto-oncogene product that has been previously found to be associated with Nck. These results suggest that the Nck adaptor protein interacts with key signaling molecules and may play an important role in activation of T lymphocytes.

Activation of nuclear factor of activated T cells-(NFAT) and activating protein 1 (AP-1) by oncogenic 70Z Cbl requires an intact phosphotyrosine binding domain but not Crk(L) or p85 phosphatidylinositol 3-kinase association

The Cbl proto-oncogene product is a complex adapter protein that functions as a negative regulator of protein tyrosine kinases. It is rapidly tyrosine-phosphorylated and associates with Crk(L) and p85 phosphatidylinositol 3-kinase (PI3K) upon engagement of numerous receptors linked to tyrosine kinases. Elucidation of the mechanism(s) underlying Cbl deregulation is therefore of considerable interest. The 70Z Cbl oncoprotein shows increased baseline tyrosine phosphorylation in fibroblasts and enhances nuclear factor of activated T cells (NFAT) activity in Jurkat T cells. Its transforming ability has been proposed to relate to its increased phosphotyrosine content. We demonstrate that 70Z Cbl shows increased basal and activation-induced tyrosine phosphorylation and association with Crk(L) and p85 PI3K in Jurkat T cells. 70Z Cbl, however, retains the ability to enhance NFAT and activating protein 1 (AP1) activity in the absence of Crk(L)/p85 PI3K association. In contrast, the G306E mutation, which inactivates the phosphotyrosine binding domain of Cbl, blocks NFAT/AP1 activation by 70Z Cbl. We conclude that 70Z Cbl-induced NFAT/AP1 activation requires the phosphotyrosine binding domain but not Crk(L)/p85 PI3K association. We hypothesize that 70Z Cbl acts as a dominant negative by blocking the negative regulatory function of the Cbl phosphotyrosine binding domain on protein-tyrosine kinases.

Tyrosine phosphorylation and complex formation of Cbl-b upon T cell receptor stimulation

Cbl-b, a mammalian homolog of Cbl, consists of an N-terminal region (Cbl-b-N) highly homologous to oncogenic v-Cbl, a Ring finger, and a C-terminal region containing multiple proline-rich stretches and potential tyrosine phosphorylation sites. In the present study, we demonstrate that upon engagement of the T cell receptor (TCR), endogenous Cbl-b becomes rapidly tyrosine-phosphorylated. In heterogeneous COS-1 cells, Cbl-b was phosphorylated on tyrosine residues by both Syk- (Syk/Zap-70) and Src- (Fyn/Lck) family kinases, with Syk kinase inducing the most prominent effect. Syk associates and phosphorylates Cbl-b in Jurkat T cells. A Tyr-316 Cbl-binding site in Syk was required for the association with and for the maximal tyrosine phosphorylation of Cbl-b. Mutation at a loss-of-function site (Gly-298) in Cbl-b-N disrupts its interaction with Syk. Cbl-b constitutively binds Grb2 and becomes associated with Crk-L upon TCR stimulation. The Grb2- and the Crk-L-binding regions were mapped to the C-terminus of Cbl-b. The Crk-L-binding sites were further determined to be Y655DVP and Y709KIP, with the latter being the primary binding site. Taken together, these results implicate that Cbl-b is involved in TCR-mediated intracellular signaling pathways.

Characterization of Cbl-Nck and Nck-Pak1 interactions in myeloid FcgammaRII signaling

Fc receptors modulate inflammatory processes, including phagocytosis, serotonin and histamine release, superoxide production, and secretion of cytokines. Aggregation of FcgammaRIIa, the low-affinity receptor for monomeric IgG, activates nonreceptor protein tyrosine kinases such as Lyn, Hck, and Syk, potentially driving the phosphorylation of the downstream adaptor proteins, including Cbl and/or Nck. Previous work from our laboratory using interferon-gamma-differentiated U937 (U937IF) myeloid cells investigated mechanisms which regulate Fcgamma receptor-induced assembly of adaptor complexes. Herein we report that FcgammaRII receptor signaling in U937IF and HEL cells involves Cbl and Nck, suggesting that Cbl-Nck interactions may link FcgammaRII to downstream activation of Pak kinase. FcgammaRII crosslinking induced the phosphorylation of Cbl and Nck on tyrosine. The alphaCbl immunoprecipitations revealed constitutive binding of Nck and Grb2 to Cbl and FcgammaRII-inducible binding of CrkL to Cbl. The interactions of Cbl with Nck and CrkL were phosphorylation dependent since dephosphorylation of cellular proteins with potato acid phosphatase abrogated binding. GST-Nck fusion protein pulldown experiments show that Cbl and Pak1 bind to the second SH3 domain of Nck. A specific Src inhibitor, PP1, was shown to completely abrogate the FcgammaR-induced superoxide response, correlating with a decrease in Cbl and Nck tyrosine phosphorylation. Our results provide the first evidence that Src is required for FcgammaR activation of the respiratory burst in myeloid cells and suggest that Cbl-Nck, Cbl-Pak1, and Nck-Pak1 interactions may regulate this response.

c-Cbl: a regulator of T cell receptor-mediated signalling

The 120-kDa protein product of the c-Cbl proto-oncogene is a ubiquitously expressed cytoplasmic protein that is especially abundant in the thymus, indicating an important role for Cbl in thymic signalling. c-Cbl possesses a highly conserved N-terminal phosphotyrosine binding domain, a C3HC4 RING finger motif, multiple proline-rich motifs, and a number of potential tyrosine phosphorylation sites. Cbl is an early and prominent substrate of protein tyrosine kinases following stimulation of a variety of cell surface receptors, and forms constitutive and inducible associations with a wide range of signalling intermediates. Genetic studies of the Cbl homologue Sli-1 in Caenorhabitis elegans predicted a role for Cbl as a negative regulator of protein tyrosine kinase-mediated signalling pathways. Numerous studies have now shown that expression of Cbl and its oncogenic variants can indeed modulate signalling from activated protein tyrosine kinases. The present review highlights some of the recent developments in our understanding of Cbl function, with particular reference to its participation and possible roles in TCR-mediated signalling.

Grb2 forms an inducible protein complex with CD28 through a Src homology 3 domain-proline interaction

CD28 provides a costimulatory signal that results in optimal activation of T cells. The signal transduction pathways necessary for CD28-mediated costimulation are presently unknown. Engagement of CD28 leads to its tyrosine phosphorylation and subsequent binding to Src homology 2 (SH2)-containing proteins including the p85 subunit of phosphatidylinositol 3'-kinase (PI3K); however, the contribution of PI3K to CD28-dependent costimulation remains controversial. Here we show that CD28 is capable of binding the Src homology 3 (SH3) domains of several proteins, including Grb2. The interaction between Grb2 and CD28 is mediated by the binding of Grb2-SH3 domains to the C-terminal diproline motif present in the cytoplasmic domain of CD28. While the affinity of the C-terminal SH3 domain of Grb2 for CD28 is greater than that of the N-terminal SH3 domain, optimal binding requires both SH3 domains. Ligation of CD28, but not tyrosine-phosphorylation, is required for the SH3-mediated binding of Grb2 to CD28. We propose a model whereby the association of Grb2 with CD28 occurs via an inducible SH3-mediated interaction and leads to the recruitment of tyrosine-phosphorylated proteins such as p52(shc) bound to the SH2 domain of Grb2. The inducible interaction of Grb2 to the C-terminal region of CD28 may form the basis for PI3K-independent signaling through CD28.

Cbl: complex formation and functional implications

Chbl, a 120-kDa proto-oncogene product, whose gene was first identified as part of a transforming gene of a murine retrovirus and whose expression is predominant in haematopoietic cells, consists of an amino-terminal transforming region, a zinc Ring finger, multiple proline-rich stretches, and several potential phosphotyrosine-containing motifs. Cbl is rapidly tyrosine-phosphorylated in response to stimulation of a variety of cell-surface receptors and becomes associated with a number of intracellular signalling molecules such as protein tyrosine kinases, phosphatidylinositol 3-kinase, Crk, and 14-3-3 proteins through different protein-interacting modules, leading to the formation of multimolecular signalling complexes. Cbl and its transforming mutants have been shown to display both negative and positive regulatory activities in protein tyrosine kinase- and Ras-mediated signalling pathways. Nevertheless, the exact biological function of this adaptor protein remains largely unknown. The present review summarizes recent progress in our understanding of the structure, regulation and biological function of Chl and defines open questions for future research.

CD16-mediated activation of phosphatidylinositol-3 kinase (PI-3K) in human NK cells involves tyrosine phosphorylation of Cbl and its association with Grb2, Shc, pp36 and p85 PI-3K subunit

Phosphatidylinositol 3-kinase (Pl-3K) plays a key role in several cellular processes, including mitogenesis, apoptosis, actin reorganization and vesicular trafficking. The molecular events involved in its activation have not been fully elucidated and several reports indicate that a key event for enzyme activation is the interaction of the SH2 domains of the p85 regulatory subunit of Pl-3K with tyrosine-phosphorylated proteins. In this study, we investigated the involvement of the product of the proto-oncogene c-Cbl in the activation of Pl-3K triggered by CD16 in human NK cells and the possible mechanisms leading to Pl-3K recruitment to the plasma membrane. Our results indicate that stimulation of NK cells through CD16 results in a rapid tyrosine phosphorylation of Cbl, which is constitutively associated with Grb2 and forms an activation-dependent complex with the p85 subunit of Pl-3K. In addition, we detected the presence of the Grb2-associated tyrosine-phosphorylated p36 and Shc proteins in anti-Cbl and anti-p85 immunoprecipitates from CD16-stimulated NK cell lysates. Upon CD16 stimulation, Pl-3K activity was found associated with Cbl and to a lesser extent with Grb2 and Shc as well as with the zeta chain of the CD16 receptor complex. Overall these results suggest that the formation of a complex containing either Shc or pp36 associated with Grb2, Cbl and the p85 subunit of Pl-3K is one of the major mechanisms which might couple CD16 to the Pl-3K pathway in NK cells.

Cbl-mediated regulation of T cell receptor-induced AP1 activation. Implications for activation via the Ras signaling pathway

The functional role of Cbl in regulating T cell receptor (TCR)-mediated signal transduction pathways is unknown. This study uses Cbl overexpression in conjunction with a Ras-sensitive AP1 reporter construct to examine its role in regulating TCR-mediated activation of the Ras pathway. Cbl overexpression in Jurkat T cells inhibited AP1 activity after TCR ligation. However, AP1 induction by 4beta-phorbol 12-myristate 13-acetate, which up-regulates Ras activity in a protein kinase C-dependent, TCR/tyrosine kinase-independent manner, was not affected by Cbl overexpression. Cbl overexpression also did not affect AP1 induction by an activated Ras protein or a membrane-bound form of the guanine nucleotide exchange factor Sos. In addition, activation of the mitogen-activated protein kinase Erk2 was decreased by Cbl overexpression. Therefore, Cbl regulates events that are required for full TCR-mediated Ras activation, and data are presented to support a model whereby Cbl regulates events required for Ras activation via its association with Grb2.

A c-Cbl yeast two hybrid screen reveals interactions with 14-3-3 isoforms and cytoskeletal components

The protein product of c-cbl proto-oncogene is known to interact with several proteins, including Grb2, Crk and PI3 kinase, and is thought to regulate signalling by many cell surface receptors. The precise function of c-Cbl in these pathways is not clear, although a genetic analysis in Caenorhabditis elegans suggests that c-Cbl is a negative regulator of the epidermal growth factor receptor. Here we describe a yeast two hybrid screen performed with c-Cbl in an attempt to further elucidate its role in signal transduction. The screen identified interactions involving c-Cbl and two 14-3-3 isoforms, cytokeratin 18, human unconventional myosin IC, and a recently identified SH3 domain containing protein, SH3 P17. We have used the yeast two hybrid assay to localise regions of c-Cbl required for its interaction with each of the proteins. Interaction with 14-3-3 is demonstrated in mammalian cell extracts.