Cbl: complex formation and functional implications

Roles of paxillin phosphorylation in IL-3 withdrawal-induced Ba/F3 cell apoptosis

BACKGROUND

Ba/F3, a mouse pro-B cell line, is dependent on IL-3 for its survival and proliferation. IL-3 withdrawal causes cells to round, stop in G1 phase, then undergo apoptosis. Additionally, IL-3 is known to induce tyrosine phosphorylation of paxillin, a scaffold and signaling protein. We previously determined that overexpression of paxillin prohibited Ba/F3 cell apoptosis induced by IL-3 withdrawal.

OBJECTIVE

Address whether phosphorylation is essential for the anti-apoptotic effect of overexpressed paxillin.

METHODS

Mutations were introduced into paxillin cDNA at five phosphorylation sites-Y31F, Y40F, Y118F, Y181F, S273A, or S273D. After overexpression of paxillin mutants in Ba/F3 cells, the apoptotic proportions of cell populations were measured by an annexin V conjugation assay while cells were undergoing IL-3 withdrawal.

RESULTS

The anti-apoptotic effect of paxillin overexpression was abolished by site-directed mutagenesis replacing Y31, Y40, Y118, and Y181 with phenylalanine, and S273 with aspartic acid. In contrast, the mutation replacing S273 with alanine had no effect on the anti-apoptotic effect.

CONCLUSION

The above results suggest that paxillin-mediated phosphorylation at Y31, Y40, Y118, and Y181 is essential for the anti-apoptotic effect of paxillin overexpression in Ba/F3 cells and contributes to the cell survival signaling pathway triggered by IL-3. Conversely, phosphorylation at S273 is involved in the negative regulation of the anti-apoptotic action of overexpressed paxillin.

A PKC-SHP1 signaling axis desensitizes Fcγ receptor signaling by reducing the tyrosine phosphorylation of CBL and regulates FcγR mediated phagocytosis

BACKGROUND

Fcγ receptors mediate important biological signals in myeloid cells including the ingestion of microorganisms through a process of phagocytosis. It is well-known that Fcγ receptor (FcγR) crosslinking induces the tyrosine phosphorylation of CBL which is associated with FcγR mediated phagocytosis, however how signaling molecules coordinate to desensitize these receptors is unclear. An investigation of the mechanisms involved in receptor desensitization will provide new insight into potential mechanisms by which signaling molecules may downregulate tyrosine phosphorylation dependent signaling events to terminate important signaling processes.

RESULTS

Using the U937IF cell line, we observed that FcγR1 crosslinking induces the tyrosine phosphorylation of CBL, which is maximal at 5 min. followed by a kinetic pattern of dephosphorylation. An investigation of the mechanisms involved in receptor desensitization revealed that pretreatment of U937IF or J774 cells with PMA followed by Fcγ receptor crosslinking results in the reduced tyrosine phosphorylation of CBL and the abrogation of downstream signals, such as CBL-CRKL binding, Rac-GTP activation and the phagocytic response. Pretreatment of J774 cells with GF109203X, a PKC inhibitor was observed to block dephosphorylation of CBL and rescued the phagocytic response. We demonstrate that the PKC induced desensitization of FcγR/ phagocytosis is associated with the inactivation of Rac-GTP, which is deactivated in a hematopoietic specific phosphatase SHP1 dependent manner following ITAM stimulation. The effect of PKC on FcγR signaling is augmented by the transfection of catalytically active SHP1 and not by the transfection of catalytic dead SHP1 (C124S).

CONCLUSIONS

Our results suggest a functional model by which PKC interacts with SHP1 to affect the phosphorylation state of CBL, the activation state of Rac and the negative regulation of ITAM signaling i.e. Fcγ receptor mediated phagocytosis. These findings suggest a mechanism for Fcγ receptor desensitization by which a serine-threonine kinase e.g. PKC downregulates tyrosine phosphorylation dependent signaling events via the reduced tyrosine phosphorylation of the complex adapter protein, CBL.

The carboxy-terminal region of CD5 is required for c-CBL mediated TCR signaling downmodulation in thymocytes

CD5 functions as a negative regulator of TCR signaling during thymocyte development, however, the molecular mechanisms involved in this process remain elusive. A key molecule involved in the down modulation of TCR signaling is c-Cbl, an ubiquitin ligase that physically associates with CD5. Crosslinking of TCR in thymocytes leads to ubiquitylation and lysosomal/proteasomal degradation of TCR downstream signaling effectors and CD5 itself. The present report shows that co-engagement of CD3 with CD5 enhanced c-Cbl phosphorylation, which was not affected by the deletion of the pseudo-ITAM domain of CD5, the putative binding site for c-Cbl. However, amino acids present in the carboxy-terminal region of CD5, were necessary for this effect, indicating that ITAM-independent sites were involved in the interaction of c-Cbl with CD5. The carboxy-terminal region of CD5 was also required for Vav degradation, a well-known target for c-Cbl-dependent ubiquitylation. These results support the notion that the distal cytoplasmic domain of CD5, including Y463, plays a relevant role in the downmodulation of TCR signals in thymocytes via c-Cbl.

Expression and function of Cbl-b in T cells from patients with systemic lupus erythematosus, and detection of the 2126 A/G Cblb gene polymorphism in the Mexican mestizo population

Systemic lupus erythematosus (SLE) is characterized by abnormalities in the function of T and B lymphocytes and in the signaling pathways induced through their receptors. Cbl-b is an intracellular adaptor protein that plays a key role in the negative regulation of lymphocyte activity. We explored the expression and function of Cbl-b in T lymphocytes from SLE patients. In addition, the possible association of SLE and a single nucleotide polymorphism (SNP) of the Cblb gene was determined. We studied 150 SLE patients, 163 healthy individuals, and 14 patients with rheumatoid arthritis (RA). The expression of Cbl-b was analyzed in the peripheral blood mononuclear cells, and the negative regulatory function of Cbl-b was assessed by analyzing actin polymerization and the phosphorylation of JNK and c-Jun induced through CD3. Furthermore, the 2126(A/G) SNP of the Cblb gene was detected by real-time polymerase chain reaction. We found a significant small reduction in the expression of Cbl-b as well as increased levels of activation of c-Jun and actin polymerization in T lymphocytes from patients with SLE compared with healthy controls or RA patients. In addition, a significant association between the 2126(A/G) SNP and SLE was detected. Our data suggest that Cbl-b may contribute to the deregulated activation of T lymphocytes observed in SLE.

Directing traffic in neural cells: determinants of receptor tyrosine kinase localization and cellular responses

The trafficking of receptor tyrosine kinases (RTKs) to distinct subcellular locations is essential for the specificity and fidelity of signal transduction and biological responses. This is particularly important in the PNS and CNS in which RTKs mediate key events in the development and maintenance of neurons and glia through a wide range of neural processes, including survival, proliferation, differentiation, neurite outgrowth, and synaptogenesis. The mechanisms that regulate the targeting of RTKs to their subcellular destinations for appropriate signal transduction, however, are still elusive. In this review, we discuss evidence for the spatial organization of signaling machinery into distinct subcellular compartments, as well as the role for ligand specificity, receptor sorting signals, and lipid raft microdomains in RTK targeting and the resultant cellular responses in neural cells.

c-Cbl tyrosine kinase-binding domain mutant G306E abolishes the interaction of c-Cbl with CD38 and fails to promote retinoic acid-induced cell differentiation and G0 arrest

Retinoic acid (RA) causes HL-60 human myeloblastic leukemia cell myeloid differentiation that is dependent on MAPK signaling. The process is propelled by c-Cbl, which binds the CD38 receptor as part of a signaling complex generating MAPK signaling. Here we report that the capability of c-Cbl to do this is lost in the G306E tyrosine kinase-binding domain mutant. Unlike wild-type (WT) c-Cbl, the G306E mutant c-Cbl fails to propel RA-induced differentiation, and disrupts the normal association with CD38. The G306E mutant does, like WT c-Cbl, co-immunoprecipitate with Vav, Slp-76, and p38. But unlike WT c-Cbl, does not cause MAPK signaling. In contrast, the C381A Ring finger domain mutant functions like WT c-Cbl. It binds CD38 and is part of the same apparent c-Cbl/Slp-76/Vav/p38 signaling complex. The C381A mutant causes MAPK signaling and propels RA-induced differentiation. In addition to HL-60 cells and their WT or mutant c-Cbl stable transfectants, the c-Cbl/Vav/Slp-76 complex is also found in NB4 cells where c-Cbl was previously also found to bind CD38. The data are consistent with a model in which the G306E mutant c-Cbl forms a signaling complex that includes Slp-76, Vav, and p38; but does not drive MAPK signaling because it fails to bind the CD38 receptor. Without the G306E mutation the c-Cbl unites CD38 with the signaling complex and delivers a MAPK signal that drives RA-induced differentiation. The results demonstrate the importance of the Gly306 residue in the ability of c-Cbl to propel RA-induced differentiation.

Inhibitory receptor signaling via tyrosine phosphorylation of the adaptor Crk

Many cellular responses, such as autoimmunity and cytotoxicity, are controlled by receptors with cytoplasmic immunoreceptor tyrosine-based inhibition motifs (ITIMs). Here, we showed that binding of inhibitory natural killer (NK) cell receptors to human leukocyte antigen (HLA) class I on target cells induced tyrosine phosphorylation of the adaptor Crk, concomitant with dephosphorylation of the guanine exchange factor Vav1. Furthermore, Crk dissociated from the guanine exchange factor C3G and bound to the tyrosine kinase c-Abl during inhibition. Membrane targeting of a tyrosine-mutated form of Crk could overcome inhibition of NK cell cytotoxicity, providing functional evidence that Crk phosphorylation contributes to inhibition. The specific phosphorylation of Crk and its dissociation from a signaling complex, observed here with two types of inhibitory receptors, expands the signaling potential of the large ITIM-receptor family and reveals an unsuspected component of the inhibitory mechanism.

Gene expression and biological processes influenced by deletion of Stat3 in pulmonary type II epithelial cells

Background

The signal transducer and activator of transcription 3 (STAT3) mediates gene expression in response to numerous growth factors and cytokines, playing an important role in many cellular processes. To better understand the molecular mechanisms by which Stat3 influences gene expression in the lung, the effect of pulmonary epithelial cell specific deletion of Stat3 on genome wide mRNA expression profiling was assessed. Differentially expressed genes were identified from Affymetrix Murine GeneChips analysis and subjected to gene ontology classification, promoter analysis, pathway mapping and literature mining.

Results

Total of 791 mRNAs were significantly increased and 314 mRNAs were decreased in response to the deletion of Stat3^Δ/Δ in the lung. STAT is the most enriched cis-elements in the promoter regions of those differentially expressed genes. Deletion of Stat3 induced genes influencing protein metabolism, transport, chemotaxis and apoptosis and decreased the expression of genes mediating lipid synthesis and metabolism. Expression of Srebf1 and 2, genes encoding key regulators of fatty acid and steroid biosynthesis, was decreased in type II cells from the Stat3^Δ/Δ mice, consistent with the observation that lung surfactant phospholipids content was decreased. Stat3 influenced both pro- and anti-apoptotic pathways that determine cell death or survival. Akt, a potential transcriptional target of Stat3, was identified as an important participant in Stat3 mediated pathways including Jak-Stat signaling, apoptosis, Mapk signaling, cholesterol and fatty acid biosynthesis.

Conclusion

Deletion of Stat3 from type II epithelial cells altered the expression of genes regulating diverse cellular processes, including cell growth, apoptosis and lipid metabolism. Pathway analysis indicates that STAT3 regulates cellular homeostasis through a complex regulatory network that likely enhances alveolar epithelial cell survival and surfactant/lipid synthesis, necessary for the protection of the lung during injury.

Novel c-CBL and CBL-b ubiquitin ligase mutations in human acute myeloid leukemia

The CBL ubiquitin ligase targets a variety of activated tyrosine kinases (TKs) for degradation. Many TKs are mutationally or autocrine activated and/or often overexpressed at the mRNA and protein levels in acute leukemias. We hypothesized that CBL is mutated in patients with acute myeloid leukemia (AML). Four of 12 patients and the MOLM-13 cell line harbored c-CBL mutations, either RNA splicing mutations, missense mutations, or a nucleotide insertion. Additionally, 1 of the 12 patients harbored a missense mutation in the related CBL-b gene. Each c-CBL mutation involves the structurally important alpha-helix within the linker region, while the mutation in CBL-b was located in the Ub-E2 protein-binding RING finger. Short-interfering RNA knockdown of mutant c-CBL present in MOLM-13 cells was growth inhibitory. In summary, novel mutations in c-CBL and CBL-b have been identified in human AML and may represent potential targets for novel therapeutics.

Redundancy in B Cell Developmental Pathways: c-Cbl Inactivation Rescues Early B Cell Development through a B Cell Linker Protein-Independent Pathway

Deficiency in the adaptor protein B cell linker protein (BLNK) results in a substantial but incomplete block in B cell development, suggesting that alternative pathways exist for B lineage differentiation. Another adaptor protein, c-Cbl, plays a negative regulatory role in several BCR-signaling pathways. We therefore investigated the role of c-Cbl during B cell development and addressed the possibility that redundancies in pathways for B cell differentiation could be further revealed by eliminating negative effects mediated by c-Cbl. Strikingly, c-Cbl inactivation reversed a number of the critical defects in early B cell differentiation that are seen in BLNK-deficient mice. c-Cbl(-/-)BLNK(-/-) mice exhibited normalized down-regulation of pre-BCR and CD43, up-regulation of MHC class II, and augmented L chain rearrangement, resulting in a successful transition from pre-B cells to immature B cells. c-Cbl inactivation also reversed the potentially tumor-predisposing hyperproliferative response of BLNK(-/-) pre-B cells to IL-7. Pre-BCR cross-linking induced enhanced and prolonged tyrosine phosphorylation in c-Cbl(-/-)BLNK(-/-) pre-BCR(+) pre-B cells compared with c-Cbl(+/-)BLNK(-/-) cells, including elevated phosphorylation of Lyn, Syk, Btk, and phospholipase C-gamma2. Our studies suggest that some, but not all, pre-BCR-triggered developmental events can be mediated by BLNK-independent pathways that are negatively regulated by c-Cbl, and further suggest that different events during early B cell development require different strength or duration of pre-BCR signaling.

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.

TGF-beta, c-Cbl, and PDGFR-alpha the in mammary stroma

Transforming growth factor-beta (TGF-beta) is thought to regulate ductal and lobuloalveolar development as well as involution in the mammary gland. In an attempt to understand the role TGF-beta plays during normal mammary gland development, and ultimately cancer, we previously generated transgenic mice that express a dominant-negative TGF-beta type II receptor under control of the metallothionine promoter (MT-DNIIR). Upon stimulation with zinc sulfate, the transgene was expressed in the mammary stroma and resulted in an increase in ductal side branching. In this study, mammary gland transplantation experiments confirm that the increase in side branching observed was due to DNIIR activity in the stroma. Development during puberty through the end buds was also accelerated. Cbl is a multifunctional intracellular adaptor protein that regulates receptor tyrosine kinase ubiquitination and downregulation. Mice with a targeted disruption of the c-Cbl gene displayed increased side branching similar to that observed in MT-DNIIR mice; however, end bud development during puberty was normal. Transplantation experiments showed that the mammary stroma was responsible for the increased side branching observed in Cbl-null mice. Cbl expression was reduced in mammary glands from DNIIR mice compared to controls and TGF-beta stimulated expression of Cbl in cultures of primary mammary fibroblasts. In addition, both TGF-beta and Cbl regulated platelet-derived growth factor receptor-alpha (PDGFR alpha) expression in vivo and in isolated mammary fibroblasts. The hypothesis that TGF-beta mediates the levels of PDGFR alpha protein via regulation of c-Cbl was tested. We conclude that TGF-beta regulates PDGFR alpha in the mammary stroma via a c-Cbl-independent mechanism. Finally, the effects of PDGF-AA on branching were determined. Treatment in vivo with PDGF-AA did not affect branching making a functional interaction between TGF-beta and PDGF unlikely.

Regulation of integrin-mediated cellular responses through assembly of a CAS/Crk scaffold

The molecular coupling of CAS and Crk in response to integrin activation is an evolutionary conserved signaling module that controls cell proliferation, survival and migration. However, when deregulated, CAS/Crk signaling also contributes to cancer progression and developmental defects in humans. Here we highlight recent advances in our understanding of how CAS/Crk complexes assemble in cells to modulate the actin cytoskeleton, and the molecular mechanisms that regulate this process. We discuss in detail the spatiotemporal dynamics of CAS/Crk assembly and how this scaffold recruits specific effector proteins that couple integrin signaling networks to the migration machinery of cells. We also highlight the importance of CAS/Crk signaling in the dual regulation of cell migration and survival mechanisms that operate in invasive cells during development and pathological conditions associated with cancer metastasis.

Ubiquitin ligases and the immune response

Ubiquitin (Ub)-protein conjugation represents a novel means of posttranscriptional modification in a proteolysis-dependent or -independent manner. E3 Ub ligases play a key role in governing the cascade of Ub transfer reactions by recognizing and catalyzing Ub conjugation to specific protein substrates. The E3s, which can be generally classified into HECT-type and RING-type families, are involved in the regulation of many aspects of the immune system, including the development, activation, and differentiation of lymphocytes, T cell-tolerance induction, antigen presentation, immune evasion, and virus budding. E3-promoted ubiquitination affects a wide array of biological processes, such as receptor downmodulation, signal transduction, protein processing or translocation, protein-protein interaction, and gene transcription, in addition to proteasome-mediated degradation. Deficiency or mutation of some of the E3s like Cbl, Cbl-b, or Itch, causes abnormal immune responses such as autoimmunity, malignancy, and inflammation. This review discusses our current understanding of E3 Ub ligases in both innate and adaptive immunity. Such knowledge may facilitate the development of novel therapeutic approaches for immunological diseases.

Regulation of ubiquitin protein ligase activity in c-Cbl by phosphorylation-induced conformational change and constitutive activation by tyrosine to glutamate point mutations

c-Cbl down-regulates receptor tyrosine kinases by conjugating ubiquitin to them, leading to receptor internalization and degradation. The ubiquitin protein ligase activity of c-Cbl (abbreviated as E3 activity) is mediated by its RING finger domain. We show here that the E3 activity of c-Cbl is negatively regulated by other domains present in the amino-terminal half of the protein (the TKB and linker helix domains) and that this negative regulation is removed when the protein is phosphorylated on tyrosine residues. Protease digestion studies indicate that tyrosine phosphorylation alters the conformation of c-Cbl. We also show that mutation of certain conserved tyrosine residues to glutamate can constitutively activate the E3 activity of c-Cbl. In particular, a Y371E mutant shows constitutive E3 activity while retaining the ability to bind epidermal growth factor receptor (EGFR). The Y371E mutant also has altered protease sensitivity from wild type, instead resembling the proteolytic pattern seen with tyrosine-phosphorylated c-Cbl. Mutation of the homologous tyrosine residue in Cbl-b to glutamate also leads to E3 activation while retaining EGFR-binding ability. These studies argue that Tyr-371 plays a key role in activating the E3 activity of c-Cbl and that the Y371E mutant may partially mimic phosphorylation at that site. However, Tyr-371 point mutants of c-Cbl are still able to undergo phosphorylation-induced E3 activation, and we show that Tyr-368 can also be phosphorylated in addition to Tyr-371, and contributes to activation.

Reciprocal regulation of MelCAM and AKT in human melanoma

Alteration in the expression of invasion/metastasis-related melanoma cell adhesion molecule (MelCAM) is strongly associated with the acquisition of malignancy by human melanoma. However, little is known about the molecular and biochemical mechanisms that regulate the expression and function of MelCAM, or its downstream signaling transduction. In this study, we show that there is a reciprocal regulation loop between AKT and MelCAM. Pharmacological inhibition of AKT in human melanoma cell lines substantially reduced the expression of MelCAM. Overexpression of constitutively active AKT upregulated the levels of MelCAM in melanoma cell lines, whereas expression of a dominant-negative PI-3 kinase downregulated MelCAM. On the other hand, overexpression of MelCAM activated endogenous AKT and inhibited proapoptotic protein BAD in melanoma cells, leading to increased survival under stress conditions. Constitutive activation of AKT was observed in most melanoma cell lines and tumor samples of different progression stages. These data link AKT activation with MelCAM expression, and implicate that intervention of MelCAM-AKT signaling axis in melanoma is a potential therapeutical approach.

The roles of Cbl-b and c-Cbl in insulin-stimulated glucose transport

Previous studies suggest that the stimulation of glucose transport by insulin involves the tyrosine phosphorylation of c-Cbl and the translocation of the c-Cbl/CAP complex to lipid raft subdomains of the plasma membrane. We now demonstrate that Cbl-b also undergoes tyrosine phosphorylation and membrane translocation in response to insulin in 3T3-L1 adipocytes. Ectopic expression of APS facilitated insulin-stimulated phosphorylation of tyrosines 665 and 709 in Cbl-b. The phosphorylation of APS produced by insulin drove the translocation of both c-Cbl and Cbl-b to the plasma membrane. Like c-Cbl, Cbl-b associates constitutively with CAP and interacts with Crk upon insulin stimulation. Cbl proteins formed homo- and heterodimers in vivo, which required the participation of a conserved leucine zipper domain. A Cbl mutant incapable of dimerization failed to interact with APS and to undergo tyrosine phosphorylation in response to insulin, indicating an essential role of Cbl dimerization in these processes. Thus, both c-Cbl and Cbl-b can initiate a phosphatidylinositol 3-kinase/protein kinase B-independent signaling pathway critical to insulin-stimulated GLUT4 translocation.

70Z/3 Cbl induces PLC gamma 1 activation in T lymphocytes via an alternate Lat- and Slp-76-independent signaling mechanism

The oncoprotein 70Z/3 Cbl signals in an autonomous fashion or through blockade of endogenous c-Cbl, a negative regulator of signaling. The mechanism of 70Z/3 Cbl-induced signaling was investigated by comparing the molecular requirements for 70Z/3 Cbl- and TCR-induced phospholipase C gamma 1 (PLC gamma 1) activation. 70Z/3 Cbl-induced PLC gamma 1 tyrosine phosphorylation required, in addition to the PLC gamma 1 N-terminal SH2 domain, the C-terminal SH2 and SH3 domains that were dispensable for TCR-induced phosphorylation. Deletion of the leucine zipper of 70Z/3 Cbl did not eliminate 70Z/3 Cbl-induced PLC gamma 1 phosphorylation, suggesting that blockage of c-Cbl via dimerization with 70Z/3 Cbl cannot fully explain 70Z/3 Cbl activating characteristics. The complete elimination of PLC gamma 1 phosphorylation required deleting the SH3 domain-binding region of 70Z/3 Cbl, consistent with 70Z/3 Cbl binding the PLC gamma 1 SH3 domain. 70Z/3 Cbl-induced PLC gamma 1 phosphorylation required Zap-70, as for the TCR, and the tyrosine kinase binding domain of 70Z/3 Cbl, which binds Zap-70, but did not require PLC gamma 1 binding to Lat, a crucial interaction in TCR-induced PLC gamma 1 phosphorylation. Furthermore, 70Z/3 Cbl-induced activation of NFAT, a PLC gamma 1/Ca(2+)-dependent transcriptional event, required Zap-70, but was independent of Slp-76, an adapter required for TCR-induced NFAT activation. These results suggest that 70Z/3 Cbl and PLC gamma 1 form a TCR-, Lat- and Slp-76-independent complex that leads to PLC gamma 1 phosphorylation and activation.

Molecular recognition by SH2 domains

In this chapter, we have described the biophysical investigations which have dissected the mechanisms of SH2 domain function. Due to nearly a decade and a half of investigation on SH2 domains, much about their binding mechanism has been characterized. SH2 domains have been found to have a positively charged binding cavity, largely conserved between different SH2 domains, which coordinates binding of the pTyr in the target. The ionic interactions between this pocket and the pTyr, in particular, between Arg beta B5 and the phosphate, provide the majority of the binding energy stabilizing SH2 domain-target interactions. The specificity in SH2 domain-target interactions emanates most often from the interactions between the residues C-terminal to the pTyr in the target and the specificity determining residues in the C-terminal half of the SH2 domain. However, the interactions in the specificity determining region of SH2 domains are weak, and hence single SH2 domains show only a modest level of specificity for tyrosine phosphorylated targets. Greater specificity in SH2 domain-containing protein-tyrosine phosphorylated target interactions can be achieved by placing SH2 domains in tandem (as is often found) or possibly through specific localization of SH2 domain-containing proteins within the cell. Although a relatively good understanding of how SH2 domains function in isolation has been obtained, the ways in which SH2 domain binding is coupled to allosteric transmission of signals in larger SH2 domain-containing proteins are still not clear. Hence, the future should bring further investigations of the mechanisms by which SH2 domain ligation alters the enzymatic activity and cellular localization of SH2 domain-containing proteins.

The chaperone protein 14-3-3 interacts with 3BP2/SH3BP2 and regulates its adapter function

Lymphocyte stimulation by immunoreceptors is achieved through the activation of multiple signaling pathways leading to cytokine gene transcription. Adapter proteins are critical signaling components that can integrate multiple pathways by allowing the assembly of multimolecular signaling complexes. We previously showed that the cytoplasmic adapter 3BP2 (also known as SH3BP2) promotes NFAT/AP-1 transcriptional activities in T cells through the activation of Ras- and calcineurin-dependent pathways. However, the molecular mechanisms by which 3BP2/SH3BP2 regulates cell signaling and activation remain poorly documented. In this study, using a combination of yeast two-hybrid analysis and biochemical approaches, we present evidence for a physical interaction between 3BP2 and the chaperone protein 14-3-3. This interaction was direct and constitutively detected in yeast and in mammalian cells. Phorbol ester, pervanadate, and forskolin/isobutylmethylxanthine stimulations enhanced this interaction, as well as co-expression of constitutive active mutants of serine/threonine kinases, including protein kinase C. We found that dephosphorylation of 3BP2 by alkaline phosphatase disrupted its interaction with 14-3-3 and that 3BP2 was a substrate of purified protein kinase C in vitro, suggesting that the phosphorylation of 3BP2 by upstream kinases was required for 14-3-3 binding. Using deletion mutants of 3BP2, two 14-3-3 binding domains were mapped to two proline-rich (residues 201-240 and 270-310) domains of 3BP2. These domains were shown to contain two 14-3-3 consensus binding motifs. We identified residues Ser(225) and Ser(277) of 3BP2 as being essential for interaction with 14-3-3 family proteins, optimal 3BP2 serine phosphorylation, and then for 3BP2-dependent function. Indeed, a 3BP2 mutant protein incapable of binding 14-3-3 showed increased capacity to stimulate NFAT transcriptional activities, suggesting that 14-3-3 binding to 3BP2 negatively regulates 3BP2 adapter function in lymphocytes.

CIN85 participates in Cbl-b-mediated down-regulation of receptor tyrosine kinases

The Cbl family of ubiquitin ligases in mammals contains three members, Cbl, Cbl-b, and Cbl-3, that are involved in down-regulation of receptor tyrosine kinases (RTKs) by mediating receptor ubiquitination and degradation. More recently, a novel pathway has been identified whereby Cbl promotes internalization of EGF receptor via a CIN85/endophilin pathway that is functionally separable from the ubiquitin ligase activity of Cbl (1). Here we show that Cbl-b, but not Cbl-3, utilize the same mechanism to down-regulate multiple RTKs. CIN85 was shown to bind to the minimal binding domain identified in the carboxyl terminus of Cbl-b. Ligand-induced phosphorylation of Cbl-b further increased their interactions and led to a rapid and sustained recruitment of CIN85 in the complex with EGF or PDGF receptors. Inhibition of binding between CIN85 and Cbl-b was sufficient to impair Cbl-b-mediated internalization of EGF receptors, while being dispensable for Cbl-b-directed polyubiquitination of EGF receptors. Moreover, CIN85 and Cbl/Cbl-b were constitutively associated with activated PDGF, EGF, or c-Kit receptors in several tumor cell lines. Our data reveal a common pathway utilized by Cbl and Cbl-b that may have an important and redundant function in negative regulation of ligand-activated as well as oncogenically activated RTKs in vivo.

Repression of c-Cbl leads to enhanced G-CSF Jak-STAT signaling without increased cell proliferation

Engagement of the Granulocyte-Colony-Stimulating Factor (G-CSF) receptor activates non-receptor protein tyrosine kinases Lyn and Jak2. We found that Lyn-deficient DT40 cells that express the G-CSF receptor (DT40GR) do not demonstrate G-CSF-induced mitogenic signaling. Lyn associates with and phosphorylates a small set of molecules, including c-Cbl. c-Cbl is an adaptor involved in cell growth and cytoskeletal reorganization, predominantly in hematopoietic cells. Using yeast two-hybrid analysis, we found that c-Cbl directly couples Lyn to PI 3-kinase. We also found that expression of the c-CblY731F mutant, which uncouples PI 3-kinase, resulted in the inhibition of G-CSF-induced proliferative signaling in DT40GR cells. As a complementary strategy, we sought to analyse the effects of c-Cbl deficiency in DT40GR cells. We isolated, cloned and sequenced the full-length cDNA for chicken c-Cbl and constructed antisense vectors. Antisense inhibition of c-Cbl expression in DT40GR cells led to enhanced Jak-STAT activation following G-CSF stimulation. Yet, this enhancement of Jak-STAT activation was associated with decreased G-CSF-induced PI 3-kinase activity and DNA synthesis. PI 3-kinase activity correlated with DNA synthesis and physiological levels of c-Cbl. Together, these data suggest that physiologic level of c-Cbl provides a growth stimulatory pathway for G-CSF and that enhanced Jak-STAT activation is not sufficient for G-CSF-induced growth.

Inhibition of Src family kinases blocks epidermal growth factor (EGF)-induced activation of Akt, phosphorylation of c-Cbl, and ubiquitination of the EGF receptor

Stimulation of T47D cells with epidermal growth factor (EGF) results in the activation of the intrinsic tyrosine kinases of the receptor and the phosphorylation of multiple cellular proteins including the receptor, scaffold molecules such as c-Cbl, adapter molecules such as Shc, and the serine/threonine protein kinase Akt. We demonstrate that EGF stimulation of T47D cells results in the activation of the Src protein-tyrosine kinase and that the Src kinase inhibitor PP1 blocks the EGF-induced phosphorylation of c-Cbl but not the activation/phosphorylation of the EGF receptor itself. PP1 also blocks EGF-induced ubiquitination of the EGF receptor, which is presumably mediated by phosphorylated c-Cbl. Src is associated with c-Cbl, and we have previously demonstrated that the Src-like kinase Fyn can phosphorylate c-Cbl at a preferred binding site for the p85 subunit of phosphatidylinositol 3'-kinase. PP1 treatment blocks EGF-induced activation of the anti-apoptotic protein kinase Akt suggesting that Src may regulate activation of Akt, perhaps by a Src --> c-Cbl --> phosphatidylinositol 3'-kinase --> Akt pathway.

Protein kinase C(theta) in T cell activation

The novel protein kinase C (PKC) isoform, PKC theta, is selectively expressed in T lymphocytes and is a sine qua non for T cell antigen receptor (TCR)-triggered activation of mature T cells. Productive engagement of T cells by antigen-presenting cells (APCs) results in recruitment of PKC theta to the T cell-APC contact area--the immunological synapse--where it interacts with several signaling molecules to induce activation signals essential for productive T cell activation and IL-2 production. The transcription factors NF-kappa B and AP-1 are the primary physiological targets of PKC theta, and efficient activation of these transcription factors by PKC theta requires integration of TCR and CD28 costimulatory signals. PKC theta cooperates with the protein Ser/Thr phosphatase, calcineurin, in transducing signals leading to activation of JNK, NFAT, and the IL-2 gene. PKC theta also promotes T cell cycle progression and regulates programmed T cell death. The exact mode of regulation and immediate downstream substrates of PKC theta are still largely unknown. Identification of these molecules and determination of their mode of operation with respect to the function of PKC theta will provide essential information on the mechanism of T cell activation. The selective expression of PKC theta in T cells and its essential role in mature T cell activation establish it as an attractive drug target for immunosuppression in transplantation and autoimmune diseases.

APS facilitates c-Cbl tyrosine phosphorylation and GLUT4 translocation in response to insulin in 3T3-L1 adipocytes

APS is a Cbl-binding protein that is tyrosine phosphorylated by the insulin receptor kinase. Insulin-stimulated phosphorylation of tyrosine 618 in APS is necessary for its association with c-Cbl and the subsequent tyrosine phosphorylation of Cbl by the insulin receptor in both 3T3-L1 adipocytes and CHO-IR cells. When overexpressed in these cells, wild-type APS but not an APS/Y(618)F mutant facilitated the tyrosine phosphorylation of coexpressed Cbl and its association with Crk upon insulin stimulation. APS-facilitated phosphorylation occurred on tyrosines 371, 700, and 774 in the Cbl protein. APS also interacted directly with the c-Cbl-associated protein (CAP) and colocalized with the protein in cells. The association was dependent on the SH3 domains of CAP and was independent of insulin treatment. Overexpression of the APS/Y(618)F mutant in 3T3-L1 adipocytes blocked the insulin-stimulated tyrosine phosphorylation of endogenous Cbl and binding to Crk. Moreover, the translocation of GLUT4 from intracellular vesicles to the plasma membrane was also inhibited by overexpression of the APS/Y(618)F mutant. These data suggest that APS serves as an adapter protein linking the CAP/Cbl pathway to the insulin receptor and, further, that APS-facilitated Cbl tyrosine phosphorylation catalyzed by the insulin receptor is a crucial event in the stimulation of glucose transport by insulin.

Isolation and characterization of a novel, transforming allele of the c-Cbl proto-oncogene from a murine macrophage cell line

The c-Cbl proto-oncogene acts as an E3 ubiquitin ligase via its RING finger domain to negatively regulate activated cellular signal transduction pathways. We have identified an aberrant Cbl-protein of approximately 95 kDa, which we have called p95Cbl, from the murine reticulum sarcoma cell-line, J-774. Cloning of the p95Cbl cDNA revealed that it contains a deletion resulting in the loss of 111 amino acids, eliminating two critical tyrosine residues in the linker region as well as the entire RING finger domain. p95Cbl displays a propensity for its interaction with the Src-family kinase Hck over cellular Cbl expressed in the same cells. Like its wildtype counterpart, p95Cbl is inducibly tyrosine phosphorylated in response to Fcgamma receptor engagement on hematopoietic cells, however this phosphorylation is sustained beyond that of cellular Cbl. NIH3T3 fibroblasts stably expressing p95Cbl acquire the typical refractile morphology associated with cellular transformation and form colonies in a focus-formation assay. The exogenously expressed mutant protein is constitutively phosphorylated in fibroblasts and partitions into the particulate fraction of cells, while cellular Cbl is exclusively cytoplasmic. p95Cbl is a novel, oncogenic mutant of the c-Cbl proto-oncogene, which might act in a dominant negative fashion to prolong normal cellular signaling responses by interfering with the down-regulation of activated signaling complexes through c-Cbl.

Membrane-anchored Cbl suppresses Hck protein-tyrosine kinase mediated cellular transformation

The mammalian proto-oncogene Cbl and its cellular homologues in Caenorhabditis elegans (Sli-1) and Drosophila (D-Cbl) are negative regulators of some growth factor receptor signaling pathways. Herein we show that Cbl can negatively regulate another signaling molecule, namely theSrc-family kinase Hck by targeting it for degradation. Hck-mediated cellular transformation of murine fibroblasts is reverted by ectopic expression of a membrane-anchored allele of Cbl as assessed by the cellular morphology, suppression of anchorage independent growth, and an overall reduction in the total tyrosine phosphorylation levels within the cells. The expression of Cbl at the plasma membrane targets both Hck and itself for ubiquitination and degradation, requiring an intact RING finger. Pharmacological inhibition of the proteasome prevents the degradation of Hck correlating with an increase in the phosphotyrosine levels within the cells. Activated Hck and membrane-anchored Cbl are present in similar subcellular localizations and co-immunoprecipitate, suggesting that their interaction is required for subsequent ubiquitination and degradation. Interestingly, both constitutively active and kinase-inactive Hck interact with and are targeted for degradation by Cbl. This work illustrates alternate means to regulate Src-family kinases, and suggests that Cbl may be able to suppress many signaling pathways that are activated in various proliferative syndromes including cancer.

The Lck SH3 domain negatively regulates localization to lipid rafts through an interaction with c-Cbl

Lck is a member of the Src family of protein-tyrosine kinases and is essential for T cell development and function. Lck is localized to the inner surface of the plasma membrane and partitions into lipid rafts via dual acylation on its N terminus. We have tested the role of Lck binding domains in regulating Lck localization to lipid rafts. A form of Lck containing a point mutation inactivating the SH3 domain (W97ALck) was preferentially localized to lipid rafts compared with wild type or SH2 domain-inactive (R154K) Lck when expressed in Lck-deficient J.CaM1 cells. W97ALck incorporated more of the radioiodinated version of palmitic acid, 16-[(125)I]iodohexadecanoic acid. Overexpression of c-Cbl, a ligand of the Lck SH3 domain, depleted Lck from lipid rafts in Jurkat cells. Additionally, Lck localization to lipid rafts was enhanced in c-Cbl-deficient T cells. The association of Lck with c-Cbl in vivo required a functional SH3 domain. These results suggest a model whereby the SH3 domain negatively regulates basal localization of Lck to lipid rafts via association with c-Cbl.

Tyrosine 315 determines optimal recruitment of ZAP-70 to the T cell antigen receptor

Recruitment of ZAP-70 protein tyrosine kinase to the T cell antigen receptor (TCR) is mediated by the binding of the SH2 domains of this enzyme to phosphorylated ITAM motifs in the CD3 and TCRzeta subunits. We have previously shown that the efficiency of both positive and negative thymocyte selection was decreased in knock-in mice expressing ZAP-70 mutated at Tyr315 (ZAP-70-Y315F), a residue laying in the interdomain B of this protein. Surprisingly, in these cells the amount of phosphorylated TCRzeta chain co-precipitating with ZAP-70-Y315F was significantly reduced compared to control mice. We report now that the binding affinity of ZAP-70-Y315F to phosphorylated ITAM is reduced as compared to the wild-type protein, whereas the intrinsic catalytic activity is untouched. Consequently, phosphorylated ITAM appear to be more accessible to protein tyrosine phosphatases (PTP) and can be readily dephosphorylated. We provide evidence suggesting that the defective ITAM binding induced by Tyr315 mutation is independent of the putative role of this residue as a binding site for Vav-1. Finally, we found that the extracellular signal-regulated kinase pathway is impaired in ZAP-70-Y315F-expressing mice. Collectively, these results demonstrate that Tyr315 has an unsuspected structural role in ZAP-70 and may allosterically regulate the function of the nearby SH2 domains.

Molecular complexes that contain both c-Cbl and c-Src associate with Golgi membranes

Cbl is an adaptor protein that is phosphorylated and recruited to several receptor and non-receptor tyrosine kinases upon their activation. After binding to the activated receptor, Cbl plays a key role as a kinase inhibitor and as an E3 ubiquitin ligase, thereby contributing to receptor down-regulation and internalization. In addition, Cbl translocates to intracellular vesicular compartments following receptor activation. We report here that Cbl also associates with Golgi membranes. Confocal immunofluorescence staining of Cbl in a variety of unstimulated cells, including CHO cells, revealed a prominent perinuclear colocalization of Cbl and a Golgi marker. Both the prominent Cbl staining and the Golgi marker were dispersed by brefeldin A. Subcellular fractionation of CHO cells demonstrated that about 10% of Cbl is stably associated with membranes, and that Golgi-enriched membrane fractions produced by isopycnic density centrifugation and free-flow electrophoresis are also enriched in Cbl, relative to other membrane fractions. The membrane-bound Cbl was hyperphosphorylated and it co-immunoprecipitated with endogenous Src. By immunofluorescence, some Src colocalized with Cbl and Golgi markers, and Src, like Cbl, was present in the Golgi-enriched fraction prepared by sequential density centrifugation and free-flow electrophoresis. Transfection of an activated form of Src, but not wild-type Src, increased the amount of Src that co-immunoprecipitated with Cbl, and increased the intensity of Cbl staining on the Golgi. This result, together with the increased tyrosine phosphorylation of the membrane-associated Cbl, suggests that Golgi-associated Cbl could be part of a molecular complex that contains activated Src. The localization and interaction of Src and Cbl at the Golgi and the regulation of the interaction of Cbl with Golgi membrane suggest that this complex may contribute to the regulation of Golgi function.

T cell activation induces direct binding of the Crk adapter protein to the regulatory subunit of phosphatidylinositol 3-kinase (p85) via a complex mechanism involving the Cbl protein

The Crk adapter proteins are assumed to play a role in T lymphocyte activation because of their induced association with tyrosine-phosphorylated proteins, such as ZAP-70 and Cbl, and with the phosphatidylinositol 3kinase regulatory subunit, p85, following engagement of the T cell antigen receptor. Although the exact mechanism of interaction between these molecules has not been fully defined, it has been generally accepted that Crk, ZAP-70, and p85 interact with tyrosine-phosphorylated Cbl, which serves as a major scaffold protein in activated T lymphocytes. Our present results demonstrate a cell activation-dependent reciprocal co-immunoprecipitation of CrkII and p85 from lysates of Jurkat T cells and a direct binding of CrkII to p85 in an overlay assay. The use of bead-immobilized GST fusion proteins indicated a complex mechanism of interaction between CrkII and p85 involving two distinct and mutually independent regions in each molecule. A relatively high affinity binding of the CrkII-SH3(N) domain to p85 and the p85-proline-B cell receptor-proline (PBP) region to CrkII was observed in lysates of either resting or activated T cells. Direct physical interaction between the CrkII-SH3(N) and the p85-PBP domain was demonstrated using recombinant fusion proteins and was further substantiated by binding competition studies. In addition, immobilized fusion proteins possessing the CrkII-SH2 and p85-SH3 domains were found to pull down p85 and CrkII, respectively, but only from lysates of activated T cells. Nevertheless, the GST-CrkII-SH2 fusion protein was unable to mediate direct association with p85 from lysates of either resting or activated T cells. Our results support a model in which T cell activation dependent conformational changes in CrkII and/or p85 promote an initial direct or indirect low affinity interaction between the two molecules, which is then stabilized by a secondary high affinity interaction mediated by direct binding of the CrkII-SH3(N) to the p85-PBP domain.

Signal transduction via the growth hormone receptor

Rapid progress has been made recently in the definition of growth hormone (GH) receptor signal transduction pathways. It is now apparent that many cytokines, including GH, share identical or similar signalling components to exert their cellular effects. This review provides a brief discourse on the signal transduction pathways, which have been demonstrated to be utilized by GH. The identification of such pathways provides a basis for understanding the pleiotropic actions of GH. The mechanisms by which the specific cellular effects of GH are achieved remain to be elucidated.

Potentiation of insulin-related signal transduction by a novel protein-tyrosine phosphatase inhibitor, Et-3,4-dephostatin, on cultured 3T3-L1 adipocytes

We previously isolated dephostatin from Streptomyces as a novel inhibitor of CD45-associated protein-tyrosine phosphatase. We prepared Et-3,4-dephostatin as a stable analogue and found it to inhibit PTP-1B and SHPTP-1 protein-tyrosine phosphatases selectively but not to inhibit CD45 and leukocyte common antigen-related phosphatase ones effectively. Et-3,4-dephostatin increased the tyrosine phosphorylation of the insulin receptor and insulin receptor substrate-1 with or without insulin in differentiated 3T3-L1 mouse adipocytes. The increase of tyrosine phosphorylation by Et-3,4-dephostatin was more prominent in 6-h than in 30-min incubation. It also increased phosphorylation and activation of Akt with or without insulin. Et-3,4-dephostatin also enhanced translocation of glucose transporter 4 from the cytoplasm to the membrane and 2-deoxy-glucose transport. Et-3,4-dephostatin-induced glucose uptake was inhibited by SB203580, a p38 inhibitor, but not by PD98059, a MEK inhibitor, or by cycloheximide as insulin-induced uptake. Interestingly, although LY294002, a phosphatidylinositol 3-kinase inhibitor, inhibited the insulin-induced glucose uptake completely, it only partially inhibited the Et-3,4-dephostatin-induced uptake. It also blocked insulin-induced glucose transporter 4 translocation but not the Et-3,4-dephostatin-induced one. The increase in c-Cbl tyrosine phosphorylation caused by Et-3,4-dephostatin was stronger than that in insulin receptor phosphorylation. These observations indicate that a phosphatidylinositol 3-kinase-independent pathway involving c-Cbl is more important in Et-3,4-dephostatin-induced glucose uptake than in insulin-induced uptake. Et-3,4-dephostatin showed an in vivo antidiabetic effect in terms of reducing the high blood glucose level in KK-A(y) mice after oral administration. Thus, Et-3,4-dephostatin potentiated insulin-related signal transductions in cultured mouse adipocytes and showed an antidiabetic effect in mice.

Essential role for Gab2 in the allergic response

Dos/Gab family scaffolding adapters (Dos, Gab1, Gab2) bind several signal relay molecules, including the protein-tyrosine phosphatase Shp-2 and phosphatidylinositol-3-OH kinase (PI(3)K); they are also implicated in growth factor, cytokine and antigen receptor signal transduction. Mice lacking Gab1 die during embryogenesis and show defective responses to several stimuli. Here we report that Gab2-/- mice are viable and generally healthy; however, the response (for example, degranulation and cytokine gene expression) of Gab2-/- mast cells to stimulation of the high affinity immunoglobulin-epsilon (IgE) receptor Fc(epsilon)RI is defective. Accordingly, allergic reactions such as passive cutaneous and systemic anaphylaxis are markedly impaired in Gab2-/- mice. Biochemical analyses reveal that signalling pathways dependent on PI(3)K, a critical component of Fc(epsilon)RI signalling, are defective in Gab2-/- mast cells. Our data identify Gab2 as the principal activator of PI(3)K in response to Fc(epsilon)RI activation, thereby providing genetic evidence that Dos/Gab family scaffolds regulate the PI(3)K pathway in vivo. Gab2 and/or its associated signalling molecules may be new targets for developing drugs to treat allergy.

Vav2 activates c-fos serum response element and CD69 expression but negatively regulates nuclear factor of activated T cells and interleukin-2 gene activation in T lymphocyte

Vav1 and Vav2 are members of the Dbl family of guanine nucleotide exchange factors for the Rho family of small GTPases. Although the role of Vav1 during lymphocyte development and activation is well characterized, the function of Vav2 is still unclear. In this study, we compared the signaling pathways regulated by Vav1 and Vav2 following engagement of the T cell receptor (TCR). We show that Vav2 is tyrosine-phosphorylated upon TCR stimulation and by co-expressed Src and Syk family kinases. Using glutathione S-transferase fusion proteins, we observed that the Src homology 2 domain of Vav2 binds tyrosine-phosphorylated proteins from TCR-stimulated Jurkat T cell lysates, including c-Cbl and SLP-76. Like Vav1, Vav2 cooperated with TCR stimulation to increase extracellular signal-regulated kinase activation and to promote c-fos serum response element transcriptional activity. Moreover, both proteins displayed a similar action in increasing the expression of the early activation marker CD69 in Jurkat T cells. However, in contrast to Vav1, Vav2 dramatically suppressed TCR signals leading to nuclear factor of activated T cells (NF-AT)-dependent transcription and induction of the interleukin-2 promoter. Vav2 appears to act upstream of the phosphatase calcineurin because a constitutively active form of calcineurin rescued the effect of Vav2 by restoring TCR-induced NF-AT activation. Interestingly, the Dbl homology and Src homology 2 domains of Vav2 were necessary for its inhibitory effect on NF-AT activation and for induction of serum response element transcriptional activity. Taken together, our results indicate that Vav1 and Vav2 exert overlapping but nonidentical functions in T cells. The negative regulatory pathway elicited by Vav2 might play an important role in regulating lymphocyte activation processes.

Identification of the mechanisms regulating the differential activation of the mapk cascade by epidermal growth factor and nerve growth factor in PC12 cells

In PC12 cells, epidermal growth factor (EGF) transiently stimulates the mitogen-activated protein (MAP) kinases, ERK1 and ERK2, and provokes cellular proliferation. In contrast, nerve growth factor (NGF) stimulation leads to the sustained activation of the MAPKs and subsequently to neuronal differentiation. It has been shown that both the magnitude and longevity of MAPK activation governs the nature of the cellular response. The activations of MAPKs are dependent upon two distinct small G-proteins, Ras and Rap1, that link the growth factor receptors to the MAPK cascade by activating c-Raf and B-Raf, respectively. We found that Ras was transiently stimulated upon both EGF and NGF treatment of PC12 cells. However, EGF transiently activated Rap1, whereas NGF stimulated prolonged Rap1 activation. The activation of the ERKs was due almost exclusively (>90%) to the action of B-Raf. The transient activation of the MAPKs by EGF was a consequence of the formation of a short lived complex assembling on the EGF receptor itself, composed of Crk, C3G, Rap1, and B-Raf. In contrast, NGF stimulation of the cells resulted in the phosphorylation of FRS2. FRS2 scaffolded the assembly of a stable complex of Crk, C3G, Rap1, and B-Raf resulting in the prolonged activation of the MAPKs. Together, these data provide a signaling link between growth factor receptors and MAPK activation and a mechanistic explanation of the differential MAPK kinetics exhibited by these growth factors.

Cholesterol depletion disrupts caveolae and insulin receptor signaling for metabolic control via insulin receptor substrate-1, but not for mitogen-activated protein kinase control

Insulin exerts its cellular control through receptor binding in caveolae in plasmalemma of target cells (Gustavsson, J., Parpal, S., Karlsson, M., Ramsing, C., Thorn, H., Borg, M., Lindroth, M., Peterson, K. H., Magnusson, K.-E., and Strålfors, P. (1999) FASEB. J. 13, 1961-1971). We now report that a progressive cholesterol depletion of 3T3-L1 adipocytes with beta-cyclodextrin gradually destroyed caveolae structures and concomitantly attenuated insulin stimulation of glucose transport, in effect making cells insulin-resistant. Insulin access to or affinity for the insulin receptor on rat adipocytes was not affected as determined by (125)I-insulin binding. By immunoblotting of plasma membranes, total amount of insulin receptor and of caveolin remained unchanged. Receptor autophosphorylation in response to insulin was not affected by cholesterol depletion. Insulin treatment of isolated caveolae preparations increased autophosphorylation of receptor before and following cholesterol depletion. Insulin-increased tyrosine phosphorylation of an immediate downstream signal transducer, insulin receptor substrate-1, and activation of the further downstream protein kinase B were inhibited. In contrast, insulin signaling to mitogenic control as determined by control of the extracellular signal-related kinases 1/2, mitogen-activated protein kinase pathway was not affected. Insulin did not control Shc phosphorylation, and Shc did not control extracellular signal-related kinases 1/2, whereas cholesterol depletion constitutively phosphorylated Shc. In conclusion, caveolae are critical for propagating the insulin receptor signal to downstream targets and have the potential for sorting signal transduction for metabolic and mitogenic effects.

c-Cbl facilitates fibronectin matrix production by v-Abl-transformed NIH3T3 cells via activation of small GTPases

The protooncogenic protein c-Cbl has been shown to act as a multivalent adaptor and a negative regulator of protein tyrosine kinase-mediated signaling. Recent studies have implicated it in the regulation of cell adhesion-related events. We have previously shown that c-Cbl facilitates adhesion and spreading of v-Abl-transformed fibroblasts, and that these effects are dependent on its tyrosine phosphorylation. However, the mechanisms mediating effects of c-Cbl on fibroblast adhesion remain poorly understood. In this study we demonstrate that the tyrosine phosphorylation-dependent effect of c-Cbl on adhesion of v-Abl-transformed fibroblasts is primarily mediated by an increase in fibronectin matrix deposition by these cells. This increase in fibronectin matrix deposition and, hence, in cell adhesion is dependent on cytoskeletal rearrangements induced by RhoA, Rac1 and, possibly, Rap1 activation caused by c-Cbl. The observed activation of these GTPases is mediated by the recruitment of phosphatidylinositol-3' kinase, CrkL and Vav2 to the C-terminal tyrosine residues of c-Cbl. The results of this study also demonstrate that ubiquitination is essential for the observed effects of c-Cbl on fibronectin matrix production and cell adhesion.

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.

Evidence for direct interaction between Sprouty and Cbl

Sprouty (SPRY) was first identified in a genetic screen in Drosophila as an antagonist of fibroblast and epidermal growth factor receptors and Sevenless signaling, seemingly by inhibiting the receptor tyrosine kinase (RTK)/Ras/MAPK pathway. To date, four mammalian Sprouty genes have been identified; the primary sequences of the gene products share a well conserved cysteine-rich C-terminal domain with their Drosophila counterpart. The N-terminal regions do not, however, exhibit a large degree of homology. This study was aimed at identifying proteins with which human SPRY2 (hSPRY2) interacts in an attempt to understand the mechanism by which Sprouty proteins exert their down-regulatory effects. Here, we demonstrate that hSPRY2 associates directly with c-Cbl, a known down-regulator of RTK signaling. A short sequence in the N terminus of hSPRY2 was found to bind directly to the Ring finger domain of c-Cbl. Parallel binding was apparent between the Drosophila homologs of Sprouty and Cbl, with cross-species associations occurring at least in vitro. Coexpression of hSPRY2 abrogated an increase in the rate of epidermal growth factor receptor internalization induced by c-Cbl, whereas a mutant hSPRY2 protein unable to bind c-Cbl showed no such effect. Our results suggest that one function of hSPRY2 in signaling processes downstream of RTKs may be to modulate c-Cbl physiological function such as that seen with receptor-mediated endocytosis.

RING finger mutations that abolish c-Cbl-directed polyubiquitination and downregulation of the EGF receptor are insufficient for cell transformation

The c-Cbl protooncogene can function as a negative regulator of receptor protein tyrosine kinases (RPTKs) by targeting activated receptors for polyubiquitination and downregulation. This function requires its tyrosine kinase binding (TKB) domain for targeting RPTKs and RING finger domain to recruit E2 ubiquitin-conjugating enzymes. It has therefore been proposed that oncogenic Cbl proteins act in a dominant-negative manner to block this c-Cbl activity. In testing this hypothesis, we found that although mutations spanning the RING finger abolish c-Cbl-directed polyubiquitination and downregulation of RPTKs, they do not induce transformation. In contrast, it is mutations within a highly conserved alpha-helical structure linking the SH2 and RING finger domains that render Cbl proteins oncogenic. Thus, Cbl transformation involves effects additional to polyubiquitination of RPTKs that are independent of the RING finger and its ability to recruit E2-conjugating enzymes.

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.

Characterization of the CIN85 adaptor protein and identification of components involved in CIN85 complexes

CIN85 is an 85-kDa adaptor protein whose functions in signaling pathways are presently unknown. Using the yeast two-hybrid screen, the B cell linker protein (BLNK) was identified as a binding partner of CIN85. Coimmunoprecipitation experiments using mammalian cells revealed that CIN85 directly bound to BLNK through its SH3 domains. Immunostaining analysis showed that CIN85 and BLNK were colocalized in the cytoplasm. These results indicate a potential role of CIN85 in the B cell receptor-mediated signaling pathway. It was also found that Crk-I, Crk-II, p130(Cas), p85-PI3K, Grb2, and Sos1 were components of CIN85 complexes. CIN85 interacted with itself through its coiled-coil region, resulting in formation of a tetramer. Both the coiled-coil region and SH3 domains of CIN85 were responsible for its subcellular localization. Our data suggest that CIN85 may serve for regulation of various signaling events through formation of its diverse complexes.

Surfactant modulates intracellular signaling of the adhesion receptor L-selectin

Intraalveolar leukocyte accumulation is one of the hallmarks during respiratory distress. In the intraalveolar space, leukocyte activation is mediated by pathogens, cytokines, and different ligands binding to adhesion molecules. Leukocyte stimulation via the adhesion molecule L-selectin is specifically induced by ligands expressed on leukocytes, platelets, endothelial cells, or lipopolysaccharide. Recently, we have demonstrated that leukocyte activation by L-selectin transmits several intracellular signaling cascades resulting in capping and cytoskeletal changes, the activation of kinases and neutral sphingomyelinase, the recruitment of adaptor proteins to the cell membrane, the activation of the small G-proteins Ras and Rac, and the release of oxygen. In the present study, we examined the effects of surfactant on L-selectin-induced signal transduction in leukocytes. Using fluorescence microscopy, we provide evidence that preincubation of leukocytes with surfactant significantly inhibits receptor capping; 28+/-7% of cells show capping after L-selectin stimulation versus 8+/-5% and 3+/-1% of cells after preincubation with Exosurf and Curosurf, respectively (p < 0.05). The activity of the neutral sphingomyelinase in cell lysates is also modulated by surfactant. In addition, we show that the activation of the tyrosine kinase p56lck is diminished by approximately 50% after surfactant treatment. This results in inhibition in tyrosine phosphorylation of certain intracellular proteins. The interaction of the L-selectin molecule with its antibody was not influenced by surfactant as shown by flow cytometry. Surfactant inhibits intracellular signaling events of the L-selectin receptor in leukocytes and might therefore contribute to the modulatory effects of surfactant on immune function.

The EGF receptor: a nexus for trafficking and signaling

Ligand binding to the EGF receptor initiates both the activation of mitogenic signal transduction pathways plus trafficking events that relocalize the receptor on the cell surface and within intracellular compartments. The trafficking compartments include caveolae, clathrin-coated pits, and various endosome populations prior to receptor degradation in lysosomes. Evidence is presented that distinct signaling pathways are initiated from these different compartments. These include the Ras/MAP kinase cascade and the PLC-dependent hydrolysis of PI-4,5 P(2). Multiple tyrosine kinase substrates that facilitate EGF receptor trafficking between these various compartments, as well as the participation of phosphoinositides and Ras-like G proteins in the trafficking pathway are also described.

Cutting edge: CD46, a new costimulatory molecule for T cells, that induces p120CBL and LAT phosphorylation

The widely expressed transmembrane molecule CD46 is the complement regulatory receptor for C3b as well as the receptor for several pathogens. Beside its binding functions, CD46 is also able to transduce signals. We showed that CD46 aggregation on human T cells induces p120CBL and linker for activation of T cells (LAT) phosphorylation. These two proteins are adaptor proteins known to regulate TCR signaling. p120CBL is a complex adaptor protein involved in negatively regulating signaling events, whereas LAT is a transmembrane adaptor protein found in glycolipid-enriched microdomains essential for T cell activation. Therefore, we investigated if a CD46/TCR costimulation would affect T cell activation. Indeed, CD46/CD3 costimulation strongly promotes T cell proliferation. Therefore, we propose that CD46 acts as a potent costimulatory molecule for human T cells.