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

Combining single-cell RNA sequencing and network pharmacology to explore the target of cangfu daotan decoction in the treatment of obese polycystic ovary syndrome from an immune perspective

Background

Polycystic ovary syndrome (PCOS) is a heterogeneous gynecological endocrine disorder linked to immunity. Cangfu Daotan Decoction (CFDT), a classic Chinese medicine prescription, is particularly effective in treating PCOS, specifically in patients with obesity; however, its specific mechanism remains unclear.

Methods

Part 1: Peripheral blood mononuclear cells were collected on egg retrieval day from obese and normal-weight patients with PCOS and healthy women undergoing in vitro fertilization (IVF)-embryo transfer. Next, scRNA-seq was performed to screen the key genes of bese patients with PCOS. Part 2: Active ingredients of CFDT and obesity-related PCOS targets were identified based on public databases, and the binding ability between the active ingredients and targets was analyzed. Part 3: This part was a monocentric, randomized controlled trial. The obese women with PCOS were randomized to CFDT (6 packets/day) or placebo, and the healthy women were included in the blank control group (43 cases per group). The clinical manifestations and laboratory outcomes among the three groups were compared.

Results

Based on the scRNA-seq data from Part 1, CYLD, ARPC3, CXCR4, RORA, JUN, FGL2, ZEB2, GNLY, FTL, SMAD3, IL7R, KIR2DL1, CTSD, BTG2, CCL5, HLA, RETN, CTSZ, and NCF2 were potential key genes associated with obese PCOS were identified. The proportions of T, B, and natural killer cells were higher in patients with PCOS compared to healthy women, with even higher proportions observed in obese patients with PCOS. Gene ontology and the Kyoto encyclopedia of genes and genomes analysis depicted that the differentially expressed genes were related to immune regulation pathways. Network pharmacology analysis identified that the key active components in CFDT were quercetin, carvacrol, β-sitosterol, cholesterol, and nobiletin, and TP53, AKT1, STAT3, JUN, SRC, etc. were the core targets. The core targets and their enrichment pathways overlapped with those in Part 1. Clinical trials in Part 3 found that CFDT reduced the dosage of gonadotropins use in patients with PCOS, increased the number of high-quality embryos, and improved the ongoing pregnancy rate.

Conclusion

CFDT can improve the immune microenvironment of patients to some extent, reduce their economic burden, and enhance IVF outcomes. The improvement in the immune microenvironment in obese patients with PCOS may be linked to targets such as JUN and AKT.

Analysis of protein kinase C theta inhibitors for the control of HIV-1 replication in human CD4+ T cells reveals an effect on retrotranscription in addition to viral transcription

HIV-1 infection cannot be cured due to reservoirs formed early after infection. Decreasing the massive CD4+ T cell activation that occurs at the beginning of the disease would delay reservoir seeding, providing a better prognosis for patients. CD4+ T cell activation is mediated by protein kinase C (PKC) theta (θ), which is involved in T-cell proliferation, as well as NF-κB, NF-AT, and AP-1 activation. We found that PKCθ activity increased viral replication, but also that HIV-1 induced higher activation of PKCθ in infected CD4+ T cells, creating a feedback loop. Therefore, specific inhibition of PKCθ activity could contribute to control HIV-1 replication. We tested the efficacy of seven PKCθ specific inhibitors to control HIV-1 replication in CD4+ T cells and selected two of the more potent and safer: CGX1079 and CGX0471. They reduced PKCθ phosphorylation at T538 and its translocation to the plasma membrane, which correlated with decreased HIV-1 retrotranscription through partial inhibition of SAMHD1 antiviral activity, rendering lower proviral integration. CGX1079 and CGX0471 also interfered with viral transcription, which would reduce the production of new virions, as well as the subsequent spread and infection of new targets that would increase the reservoir size. CGX1079 and CGX0471 did not completely abrogate T-cell functions such as proliferation and CD8-mediated release of IFN-γ in PBMCs from HIV-infected patients, thereby avoiding general immunosuppresion. Consequently, using PKCθ inhibitors as adjuvant of antiretroviral therapy in recently infected patients would decrease the pool of activated CD4+ T cells, thwarting proviral integration and reducing the reservoir size.

CD43 regulates the threshold for T cell activation by targeting Cbl functions

T cell (TC) activation requires the coordinated signaling of the T cell receptor (TCR) and coreceptor molecules, allowing TCs to respond to lower degrees of TCR occupancy. Coreceptor molecules set the threshold for TC activation by controlling different regulatory signaling loops. The Cbl family members prevent undesired activation of T cells by regulating TCR signals. In this report, we show that TC prestimulation by the CD43 coreceptor molecule before TCR engagement inhibits TCR-dependent c-Cbl tyrosine phosphorylation, c-Cbl interaction with the adapter molecule Crk-L and promotes Cbl-b degradation in a PKCθ-dependent manner. Consequently, the prolonged tyrosine phosphorylation and delayed degradation of ZAP-70 and of the ζ chain lead to enhanced mitogen-activated protein kinase activation and robust TC response. These data indicates that CD43-mediated signals lower the threshold for TC activation by restricting the c-Cbl and Cbl-b inhibitory effects on TCR signaling. In addition to the strength and duration of intracellular signals, our data underscore temporality with which certain molecules are engaged as yet another mechanism to fine tune TC signal quality, and ultimately immune function.

Involvement of crk adapter proteins in regulation of lymphoid cell functions

The Crk adapter proteins consist of Src homology 2 (SH2) SH2 and SH3 domains, which bind tyrosine-phosphorylated peptides and polyproline-rich motives, respectively. They are linked to multiple signaling pathways in different cell types, including lymphocytes, and because of their lack of catalytic activity, many studies on Crk were aimed at the identification of their binding partners and determination of the physiologic meaning of these interactions. Crk proteins were found to be involved in the early steps of lymphocyte activation through their SH2-mediated transient interaction with signal-transducing molecules, such as Cbl, ZAP-70, CasL, and STAT5. In addition, Crk proteins are constitutively associated with effector molecules that mediate cell adhesion and thereby regulate lymphocyte extravasation and recruitment to sites of inflammation. This article describes selected studies of Crk, performed predominantly in lymphocytes, and discusses their potential relevance to the role of Crk in the regulation of lymphocyte functions.

Selectins and anti-CD15 (Lewis x/a) antibodies transmit activation signals in Hodgkin's lymphoma-derived cell lines

OBJECTIVE

The CD15 (Lewis x) cell surface oligosaccharide moiety is expressed in a variety of normal and tumor cells and recognized by selectins. The detection of CD15 on malignant Hodgkin-Reed-Sternberg (HRS) cells serves as a diagnostic marker of Hodgkin's lymphoma (HL). Retrospective studies suggest that the expression of nonsialylated CD15 molecules on HRS cells has a positive prognostic value while presence of sialylated CD15 may correlate with a poor outcome. However, the relevance of the CD15 antigen expression to the pathobiology of the disease is not clear. In this work, we studied the contribution of CD15 to cell adhesion and the activation of signaling cascades in two HL-derived cell lines, KMH-2 and L428.

METHODS

Immobilized anti-CD15 monoclonal antibodies and recombinant E- and P-selectins were used to activate KMH-2 and L428 cells. Immunoblotting, immunoprecipitation, and the electrophoretic mobility shift assay were performed to detect tyrosine phosphorylation of c-Cbl, c-Jun nuclear translocation, and AP-1 DNA binding.

RESULTS

Treatment of cells with antibodies against the sialylated and nonsialylated forms of CD15, or with immobilized selectins, induced changes in cell morphology. Tyrosine phosphorylation of c-Cbl, together with tyrosine phosphorylation of multiple protein substrates, was also induced. In addition, binding of the CD15 molecules induced nuclear translocation of c-Jun and an increase in AP-1 DNA binding activity.

CONCLUSIONS

We suggest that CD15 has a dual physiological role, both as an adhesion molecule recognized by selectins and as a regulatory molecule upstream to specific intracellular signaling cascades with implications to the pathogenesis of HL.

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.

LEDGF, a survival factor, activates stress-related genes

LEDGF is a survival factor and it enhances survival of various cell types against stress. LEDGF is also a transcriptional activator and it binds to promoter elements of heat shock and stress-related genes to activate expression of these genes. The elevated levels of the stress-related family of proteins, such as heat shock proteins, antioxidant proteins, and detoxication enzymes might suppress apoptosis induced by stress. The protective mechanisms against stress in mammalian cells and in yeast are surprisingly similar.

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.

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.

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.

Src-catalyzed phosphorylation of c-Cbl leads to the interdependent ubiquitination of both proteins

The protooncogene c-Cbl has recently emerged as an E3 ubiquitin ligase for activated receptor tyrosine kinases. We report here that c-Cbl also mediates the ubiquitination of another protooncogene, the non-receptor tyrosine kinase c-Src, as well as of itself. The c-Cbl-dependent ubiquitination of Src and c-Cbl requires c-Cbl's RING finger, Src kinase activity, and c-Cbl's tyrosine phosphorylation, probably on Tyr-371. In vitro, c-Cbl forms a stable complex with the ubiquitin-conjugating enzyme UbcH7, but active Src destabilizes this interaction. In contrast, Src inhibition stabilizes the c-Cbl. UbcH7.Src complex. Finally, c-Cbl reduces v-Src protein levels and suppresses v-Src-induced STAT3 activation. Thus, in addition to mediating the ubiquitination of activated receptor tyrosine kinases, c-Cbl also acts as a ubiquitin ligase for the non-receptor tyrosine kinase Src, thereby down-regulating Src.

Suppression of apoptosis and granulocyte colony-stimulating factor-induced differentiation by an oncogenic form of Cbl

OBJECTIVE

The retroviral oncogene v-Cbl causes pre-B cell lymphomas and myeloid leukemias in mice, and its Drosophila homologue is oncogenic, causing enhanced receptor tyrosine kinase signaling. The human Cbl gene resides at 11q23. The aim of this study is to determine the effect of oncogenic Cbl on growth-regulating responses.

MATERIALS AND METHODS

The oncogenic mutant of Cbl (CblDelta1-357) was transfected into factor-dependent 32Dcl3 myeloid cells. Consequently, cell survival and differentiation were measured. Lyn, Syk, MAP kinase, and phosphatidylinositol 3'(PI3')-kinase activities, protein phosphorylation, Bcl-2 promoter activity, ubiquitination, and levels of Bcl-2, Bax, Bad, and Bcl-x(L) were determined. In addition, the effect of v-Cbl on TF-1 cell survival upon granulocyte-macrophage colony-stimulating factor withdrawal was studied.

RESULTS

32Dcl3 and TF-1 cells expressing v-Cbl showed resistance to apoptosis upon growth factor withdrawal, and 32Dcl3 cells completely failed to respond to granulocyte colony-stimulating factor's induction of differentiation. Basal activities of Lyn, Syk, and PI3'-kinase were elevated in the v-Cbl line. There was neither enhanced tyrosine phosphorylation of cellular protein content, Cbl, or Jak2, nor serine phosphorylation of MAP kinase or Akt. After factor withdrawal, the level of Bcl-2 was greater in v-Cbl cells than in control cells.

CONCLUSIONS

Neither increased Bcl-2 promoter activity nor decreased ubiquitination of Bcl-2 could account for increased Bcl-2 levels. v-Cbl-expressing 32Dcl3 cells were resistant to differentiation. v-Cbl suppresses apoptosis and differentiation, possibly through enhancement of Lyn, Syk, and PI3'-kinase activities and Bcl-2.

Cbl: many adaptations to regulate protein tyrosine kinases

Responses to extracellular stimuli are often transduced from cell-surface receptors to protein tyrosine kinases which, when activated, initiate the formation of protein complexes that transmit signals throughout the cell. A prominent component of these complexes is the product of the proto-oncogene c-Cbl, which specifically targets activated protein tyrosine kinases and regulates their signalling. How, then, does this multidomain protein shape the responses generated by these signalling complexes?

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.

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.

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.

Negative regulation of cytokine signaling pathways

The Janus family of protein tyrosine kinases (JAKs) and STAT transcription factors regulate cellular processes involved in cell growth, differentiation, and transformation through their association with cytokine receptors. The CIS family of proteins (also referred to as the SOCS or SSI family) has been implicated in the regulation of signal transduction by a variety of cytokines. Most of them appear to be induced after stimulation with several different cytokines, and at least three of them (CIS1, CIS3/SOCS3, and JAB/SOCS1) negatively regulate cytokine signal transduction by various means: CIS1 inhibits STAT5 activation by binding to cytokine receptors that recruit STAT5, whereas JAB/SOCS-1 and CIS3/SOCS-3 directly bind to the kinase domain of JAKs, thereby inhibiting tyrosine-kinase activity. Therefore, these CIS family members seem to function in a classical negative feedback loop of cytokine signaling. Biochemical characterization as well as gene disruption studies indicate that JAB/SOCS1/SSI-1 is an important negative regulator of interferon gamma signaling. The mechanisms by which these inhibitors of cytokine signal transduction exert their effects have been extensively studied and will provide useful information for regulating tyrosine-kinase activity.

Cloning and characterization of human Lnk, an adaptor protein with pleckstrin homology and Src homology 2 domains that can inhibit T cell activation

Lnk was originally cloned from a rat lymph node cDNA library and shown to participate in T cell signaling. Human Lnk (hLnk) was cloned by screening a Jurkat cell cDNA library. hLnk has a calculated molecular mass of 63 kDa, and its deduced amino acid sequence indicates the presence of an N-terminal proline-rich region, a pleckstrin homology domain, and a Src homology 2 domain. When expressed in COS cells, hLnk migrates with an apparent molecular mass of 75 kDa. Confocal fluorescence microscope analysis indicates that in COS cells transfected with an expression vector encoding a chimeric Lnk-green fluorescent protein, hLnk is found at the juxtanuclear compartment and also appears to be localized at the plasma membrane. Lnk is tyrosine-phosphorylated by p56lck. Following phosphorylation, p56lck binds to tyrosine-phosphorylated hLnk through its Src homology 2 domain. In COS cells cotransfected with hLnk, p56lck, and CD8-zeta, hLnk associated with tyrosine-phosphorylated TCR zeta-chain through its Src homology 2 domain. The overexpression of Lnk in Jurkat cells led to an inhibition of anti-CD3 mediated NF-AT-Luc activation. Our study reveals a potentially new mechanism of T cell-negative regulation.

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.

High Affinity IgG Receptor Activation of Src Family Kinases Is Required for Modulation of the Shc-Grb2-Sos Complex and the Downstream Activation of the Nicotinamide Adenine Dinucleotide Phosphate (Reduced) Oxidase

We used the U937 cell line to examine the modulation of adaptor protein interactions (Shc, Grb2, and Cbl) after high affinity IgG receptor (FcγRI) cross-linking, leading to the formation of the Grb2-Sos complex, the activation of Ras, and the regulation of the respiratory burst. Cross-linking of FcγRI induced the conversion of GDP-Ras to GTP-Ras reaching a maximum 5 min after stimulation. Concomitant with Ras activation, Sos underwent an electrophoretic mobility shift and the Sos-Grb2 association was increased (6-fold). The Grb2-Sos complex was present only in the membrane fraction and was augmented after FcγRI stimulation. Tyrosine-phosphorylated Shc, mainly the p52 isoform, was observed to transiently onload to the membrane Grb2-Sos complex on FcγRI stimulation. Cross-linking of FcγRI induces the tyrosine phosphorylation of Cbl, which forms a complex with Grb2 and Shc via the Cbl C terminus. Kinetic experiments confirm that Cbl-Grb2 is relatively stable, whereas Grb2-Sos, Grb2-Shc, and Cbl-Shc interactions are highly inducible. The Src family tyrosine kinase inhibitor, PP1, was shown to completely inhibit Shc tyrosine phosphorylation, the Shc-Grb2 interaction, and the FcγR-induced respiratory burst. Our results provide the first evidence that the upstream activation of Src kinases is required for the modulation of the Shc-Grb2 interaction and the myeloid NADPH oxidase response.

Ligand-induced ubiquitination of the epidermal growth factor receptor involves the interaction of the c-Cbl RING finger and UbcH7

c-Cbl plays a negative regulatory role in tyrosine kinase signaling by an as yet undefined mechanism. We demonstrate here, using the yeast two-hybrid system and an in vitro binding assay, that the c-Cbl RING finger domain interacts with UbcH7, a ubiquitin-conjugating enzyme (E2). UbcH7 interacted with the wild-type c-Cbl RING finger domain but not with a RING finger domain that lacks the amino acids that are deleted in 70Z-Cbl, an oncogenic mutant of c-Cbl. The in vitro interaction was enhanced by sequences on both the N- and C-terminal sides of the RING finger. In vivo and in vitro experiments revealed that c-Cbl and UbcH7 synergistically promote the ligand-induced ubiquitination of the epidermal growth factor receptor (EGFR). In contrast, 70Z-Cbl markedly reduced the ligand-induced, UbcH7-mediated ubiquitination of the EGFR. MG132, a proteasome inhibitor, significantly prolonged the ligand-induced phosphorylation of both the EGFR and c-Cbl. Thus, c-Cbl plays an essential role in the ligand-induced ubiquitination of the EGFR by a mechanism that involves an interaction of the RING finger domain with UbcH7. This mechanism participates in the down-regulation of tyrosine kinase receptors and loss of this function, as occurs in the naturally occurring 70Z-Cbl isoform, probably contributes to oncogenic transformation.

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.

The Cbl protooncoprotein: a negative regulator of immune receptor signal transduction

The Cbl protooncoprotein has recently emerged as a component of tyrosine kinase-mediated signal transduction in a variety of cell types. Here, we discuss evidence that supports a role for Cbl as a novel negative regulator of immune receptor signaling, and present models for its mode of function.

Role of protein kinase C in signal attenuation following T cell receptor engagement

T lymphocyte activation through stimulation of the T cell receptor complex and co-stimulatory receptors is associated with acute tyrosine phosphorylation of intracellular proteins, which in turn mediate downstream signaling events that regulate interleukin-2 expression and cell proliferation. The extent of protein tyrosine phosphorylation is rapidly attenuated after only 1-2 min of stimulation as a means of tightly controlling the initial signaling response. Here we show that this attenuation of tyrosine phosphorylation of Shc, CrkL, and the proto-oncogene Cbl is mimicked by treatment of mouse T lymphocytes or cultured Jurkat cells with phorbol 12-myristate 13-acetate. This effect is blocked by the specific protein kinase C inhibitor GF109203X, but not by PD98059, an inhibitor of MEK1/2 kinase. Activation of protein kinase C by phorbol ester also causes rapid (t(1)/(2) = 2 min) dissociation of both CrkL and p85/phosphoinositide 3-kinase from Cbl concomitant with Cbl tyrosine dephosphorylation. More important, GF109203X treatment of Jurkat cells prior to T cell receptor stimulation by anti-CD3/CD4 antibodies results in an enhanced (2-fold) peak of Cbl phosphorylation compared with that observed in control cells. Furthermore, the rate of attenuation of both Cbl tyrosine phosphorylation and its association with CrkL following stimulation with anti-CD3/CD4 antibodies is much slower in Jurkat cells treated with GF109203X. Taken together, these data provide strong evidence that one or more isoforms of phorbol ester-responsive protein kinase C play a key role in a feedback mechanism that attenuates tyrosine phosphorylation of proteins and reverses formation of signaling complexes in response to T cell receptor activation.

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.

Perturbed Regulation of ZAP-70 and Sustained Tyrosine Phosphorylation of LAT and SLP-76 in c-Cbl-Deficient Thymocytes

Recent studies indicate that c-Cbl and its oncogenic variants can modulate the activity of protein tyrosine kinases. This finding is supported by studies showing that c-Cbl interacts directly with a negative regulatory tyrosine in ZAP-70, and that the levels of tyrosine-phosphorylated ZAP-70 and numerous other proteins are increased in TCR-stimulated thymocytes from c-Cbl-deficient mice. Here, we demonstrate that this enhanced phosphorylation of ZAP-70 and that of two substrates, LAT and SLP-76, is not due to altered protein levels but is the consequence of two separate events. First, we find increased expression of tyrosine-phosphorylated TCRζ chain in c-Cbl-deficient thymocytes, which results in a higher level of ζ-chain-associated ZAP-70 that is initially accessible for activation. Thus, more ZAP-70 is activated and more of its substrates (LAT and SLP-76) become tyrosine-phosphorylated after TCR stimulation. However, an additional mechanism of ZAP-70 regulation is evident at a later time poststimulation. At this time, ZAP-70 from both normal and c-Cbl−/− thymocytes becomes hyperphosphorylated; however, only in normal thymocytes does this correlate with ZAP-70 down-regulation and a diminished ability to phosphorylate LAT and SLP-76. In contrast, c-Cbl-deficient thymocytes display altered phosphorylation kinetics, for which LAT phosphorylation is increased and SLP-76 phosphorylation is sustained. Thus, the ability to down-regulate the phosphorylation of two ZAP-70 substrates is impaired in c-Cbl−/− thymocytes. These findings provide evidence that c-Cbl is involved in the negative regulation of the phosphorylation of LAT and SLP-76 by ZAP-70.

Integration of T cell receptor-dependent signaling pathways by adapter proteins

The initiation of biochemical signal transduction following ligation of surface receptors with intrinsic cytoplasmic tyrosine kinase activity is common for many cell types. T lymphocytes also require activation of tyrosine kinases following T cell receptor (TCR) ligation for maximal stimulation. However, the TCR has no intrinsic tyrosine kinase activity. Instead, the TCR must rely on cytoplasmic tyrosine kinases that localize to the TCR complex and initiate TCR-mediated signaling events. Although much has been learned regarding how these cytosolic tyrosine kinases are activated and recruited to the TCR complex, relatively little is understood about how these initial events are translated into transcriptional activation of genes that regulate cytokine production, cell proliferation, and cell death. Recently, it has become clear that the class of intracellular molecules known collectively as adapter proteins, molecules with modular domains capable of recruiting additional proteins but that exhibit no intrinsic enzymatic activity, serve to couple proximal biochemical events initiated by TCR ligation with more distal signaling pathways.

Cbl-mediated negative regulation of platelet-derived growth factor receptor-dependent cell proliferation. A critical role for Cbl tyrosine kinase-binding domain

The Cbl proto-oncogene product has emerged as a novel negative regulator of receptor and non-receptor tyrosine kinases. Our previous observations that Cbl overexpression in NIH3T3 cells enhanced the ubiquitination and degradation of the platelet-derived growth factor receptor-alpha (PDGFRalpha) and that the expression of oncogenic Cbl mutants up-regulated the PDGFRalpha signaling machinery strongly suggested that Cbl negatively regulates PDGFRalpha signaling. Here, we show that, similar to PDGFRalpha, selective stimulation of PDGFRbeta induces Cbl phosphorylation, and its physical association with the receptor. Overexpression of wild type Cbl in NIH3T3 cells led to an enhancement of the ligand-dependent ubiquitination and subsequent degradation of the PDGFRbeta, as observed with PDGFRalpha. We show that Cbl-dependent negative regulation of PDGFRalpha and beta results in a reduction of PDGF-induced cell proliferation and protection against apoptosis. A point mutation (G306E) that inactivates the tyrosine kinase binding domain in the N-terminal transforming region of Cbl compromised the PDGF-inducible tyrosine phosphorylation of Cbl although this mutant could still associate with the PDGFR. More importantly, the G306E mutation abrogated the ability of Cbl to enhance the ligand-induced ubiquitination and degradation of the PDGFR and to inhibit the PDGF-dependent cell proliferation and protection from apoptosis. These results demonstrate that Cbl can negatively regulate PDGFR-dependent biological responses and that this function requires the conserved tyrosine kinase binding domain of Cbl.

Modulation of Integrin Function in Hematopoietic Progenitor Cells by CD43 Engagement: Possible Involvement of Protein Tyrosine Kinase and Phospholipase C-γ

Attachment of cells to extracellular matrix components is critical for the regulation of hematopoiesis. CD43 is a mucin-like transmembrane sialoglycoprotein expressed on the surface of almost all hematopoietic cells. A highly extended structure of extracellular mucin with negative charge may function as a repulsive barrier to hematopoietic cells. However, some investigators have shown that CD43 has proadhesive properties, and engagement of CD43 has been reported to upregulate integrin-mediated cell adhesion in T cells. We found that cross-linking of CD43 with monoclonal antibodies (MoAbs) enhanced integrin 4β1 (very late antigen [VLA]-4) and 5 β1 (VLA-5)-dependent adhesion of human cord blood CD34+ cells to fibronectin. CD34+ CD38hi, but not CD34+CD38−/low cells responded significantly to the stimulus, suggesting that committed, but not stem and more immature progenitors are sensitive to CD43-mediated activation of integrin. To elucidate the molecular mechanism leading to integrin activation, we used the growth factor-dependent cell line MO7e. Cross-linking of CD43 induced tyrosine phosphorylation of several intracellular molecules including the protein tyrosine kinase Syk, the proto-oncogene product Cbl, and phospholipase C (PLC)-γ2 in MO7e cells. Moreover, protein tyrosine kinase inhibitor herbimycin A and PLC inhibitor U73122 both blocked CD43-induced enhancement of adhesion to fibronectin. These results indicate that signals mediated through CD43 may increase integrin affinity to fibronectin via a pathway dependent on protein tyrosine kinase and PLC-γ activation in hematopoietic progenitors.

The proto-oncogene c-Cbl is a negative regulator of DNA synthesis initiated by both receptor and cytoplasmic tyrosine kinases

In C. elegans, genetic and biochemical data indicate that the Cbl homolog Sli-1 attenuates Let-23 (EGFR) signaling. To investigate whether c-Cbhl might have a role in mammalian growth factor-mediated mitogenic signaling, we microinjected NIH3T3 mouse fibroblasts with expression plasmids encoding wt and G306ECbl (a 'loss of function' mutant identified in C. elegans). We observed inhibition of PDGF BB- and EGF-induced DNA synthesis by wt Cbl but not the mutant. Microinjection of two different affinity purified polyclonal antisera against Cbl boosted a suboptimal PDGF-stimulated mitogenic response. The inhibition of both PDGF BB- and EGF-induced DNA synthesis by wt Cbl was reversed by co-expression with Myc but not with Fos. DNA synthesis initiated by constitutively activated Src was also blocked by Cbl expression, but curiously by the G306E mutant as well. These data are all consistent with the proposition that Cbl negatively affects mitogenic signaling in mammalian fibroblasts.

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.

A direct interaction between the adaptor protein Cbl-b and the kinase zap-70 induces a positive signal in T cells

Engagement of the T-cell receptor (TCR)-CD3 complex induces a rapid increase in the activities of Src-family and Syk/Zap-70-family kinases [1] [2]. These activated kinases then induce the tyrosine phosphorylation of multiple intracellular proteins, eventually leading to T-cell activation. One of the prominent substrates for these kinases is the adaptor protein Cbl [3] and recent studies suggest that Cbl negatively regulates upstream kinases such as Syk and Zap-70 [4] [5]. Cbl-b, a homologue of Cbl, is widely expressed in many tissues and cells including hematopoietic cells [6] [7]. Cbl-b undergoes rapid tyrosine phosphorylation upon stimulation of the TCR and cytokine receptors [8] [9]. The role of Cbl-b is unclear, however. Here, we show that overexpression of Cbl-b in T cells induced the constitutive activation of the transcription factor nuclear factor of activated T cells (NFAT). A loss-of-function mutation in Cbl-b disrupted the interaction between Cbl-b and Zap-70 and nearly completely abrogated the Cbl-b-mediated activation of NFAT. Unlike the proposed role of Cbl as a negative regulator, our results suggest that the Cbl homologue Cbl-b has a positive role in T-cell signaling, most likely via a direct interaction with the upstream kinase Zap-70.

Dual regulation of T cell receptor-mediated signaling by oncogenic Cbl mutant 70Z

We previously showed that an oncogenic Cbl mutant (70Z) is constitutively active in transcriptional activation of nuclear factor at activated T cells (NFAT). However, the mechanism underlying this effect remains unclear. Here we analyzed the effects of 70Z mutations at an amino-terminal loss of function site (Gly-306) and at carboxyl-terminal potential tyrosine or serine phosphorylation sites on association with signaling proteins and on NFAT activation. Mutation at Gly-306 of 70Z disrupted its association with Zap-70 and almost completely abolished its ability to induce NFAT activation under basal and ionomycin-stimulated conditions. However, mutations at potential tyrosine or serine phosphorylation sites had little effect. In fact, expression of 70Z with Tyr-700, Tyr-731, or Tyr-774 mutated to Phe increased NFAT activity in comparison with unmutated 70Z. These findings suggest that an amino terminus-mediated interaction of 70Z with Zap-70 plays a positive role and that a carboxyl terminus-mediated, phosphotyrosine-dependent interaction with their binding proteins plays a negative role in 70Z-mediated NFAT activation. In support of this notion are the observations that 70Z reduced T cell receptor-induced NFAT activation and that wild-type Cbl further inhibited this event, suggesting that both 70Z and wild-type Cbl employ a similar mechanism by which Cbl proteins dually regulate T cell receptor-mediated signaling.

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.

Fc receptor-mediated platelet activation is dependent on phosphatidylinositol 3-kinase activation and involves p120(Cbl)

The platelet receptor for the Fc domain of IgGs (FcgammaRIIa) triggers intracellular signaling through protein tyrosine phosphorylations leading to platelet aggregation. In this study, we focused on the adaptor protein p120(cbl) (Cbl), which became tyrosine-phosphorylated after platelet activation induced by antibodies. Cbl phosphorylation was dependent on Fc receptor engagement. An association of Cbl with the p85 subunit of phosphatidylinositol 3-kinase (PI 3-K) occurred in parallel with Cbl tyrosine phosphorylation. We showed by in vitro experiments that Cbl/p85 association was mediated by the Src homology 3 domain of p85/PI 3-K and the proline-rich region of Cbl. Inhibition of PI 3-K activity by wortmannin led to the blockade of both platelet aggregation and serotonin release mediated by FcgammaRIIa engagement, whereas it only partly inhibited those induced by thrombin. Thus, PI 3-K may play a crucial role in the initiation of platelet responses after FcgammaRIIa engagement. Our results suggest that Cbl is involved in platelet signal transduction by the recruitment of PI 3-K to the FcgammaRIIa pathway, possibly by increasing PI 3-K activity.

APS, an adaptor protein containing PH and SH2 domains, is associated with the PDGF receptor and c-Cbl and inhibits PDGF-induced mitogenesis

Previously we cloned a novel adaptor protein, APS (adaptor molecules containing PH and SH2 domains) which was tyrosine phosphorylated in response to c-kit or B cell receptor stimulation. Here we report that APS was expressed in some human osteosarcoma cell lines, markedly so in SaOS-2 cells, and was tyrosine-phosphorylated in response to several growth factors, including platelet derived growth factor (PDGF), insulin-like growth factor (IGF), and granulocyte-macrophage colony stimulating factor (GM-CSF). Ectopic expression of the wild type APS, but not C-terminal truncated APS, in NIH3T3 fibroblasts suppressed PDGF-induced MAP kinase (Erk2) activation, c-fos and c-myc induction as well as cell proliferation. In vitro binding experiments suggest that APS bound to the beta type PDGF receptor, mainly via phosphotyrosine 1021 (pY1021). Indeed, tyrosine phosphorylation of PLC-gamma, which has been demonstrated to bind to pY1021, but not that of PI3 kinase and associated proteins, was reduced in APS transformants. PDGF induced phosphorylation of the tyrosine residue of APS close to the C-terminal end. In vitro and in vivo binding experiments indicate that the tyrosine phosphorylated C-terminal region of APS bound to c-Cbl, which has been shown to be a negative regulator of tyrosine kinases. Since coexpression of c-Cbl with wild type APS, but not C-terminal truncated APS, synergistically inhibited PDGF-induced c-fos promoter activation, c-Cbl could be a mechanism of inhibitory action of APS on PDGF receptor signaling.

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.

Differential Interaction of Crkl with Cbl or C3G, Hef-1, and γ Subunit Immunoreceptor Tyrosine-Based Activation Motif in Signaling of Myeloid High Affinity Fc Receptor for IgG (FcγRI)

Cbl-Crkl and Crkl-C3G interactions have been implicated in T cell and B cell receptor signaling and in the regulation of the small GTPase, Rap1. Recent evidence suggests that Rap1 plays a prominent role in the regulation of immunoreceptor tyrosine-based activation motif (ITAM) signaling. To gain insight into the role of Crkl in myeloid ITAM signaling, we investigated Cbl-Crkl and Crkl-C3G interactions following FcγRI aggregation in U937IF cells. FcγRI cross-linking of U937IF cells results in the tyrosine phosphorylation of Cbl, Crkl, and Hef-1, an increase in the association of Crkl with Cbl via direct SH2 domain interaction and increased Crkl-Hef-1 binding. Crkl constitutively binds to the guanine nucleotide-releasing protein, C3G, via direct SH3 domain binding. Our data show that distinct Cbl-Crkl and Crkl-C3G complexes exist in myeloid cells, suggesting that these complexes may modulate distinct signaling events. Anti-Crkl immunoprecipitations demonstrate that the ITAM-containing γ subunit of FcγRI is induced to form a complex with the Crkl protein, and Crkl binds to the cytoskeletal protein, Hef-1. The induced association of Crkl with Cbl, Hef-1, and FcγRIγ after FcγRI activation and the constitutive association between C3G and Crkl provide the first evidence that a FcγRIγ-Crkl-C3G complex may link ITAM receptors to the activation of Rap1 in myeloid cells.

T-cell anergy and altered T-cell receptor signaling: effects on autoimmune disease

Immunological self-tolerance can be acquired by several mechanisms, including the induction of anergy in autoreactive T cells. In this sense, anergy is predictably advantageous for the immune system. Here, Konstantin Salojin and colleagues present an alternative view that the induction of anergy in regulatory T cells may be harmful to the host and elicit autoimmune disease.

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.

SLP-76-Cbl-Grb2-Shc Interactions in FcγRI Signaling

SLP-76 and Cbl are complex adapter proteins that have the capacity to bind to smaller adapter proteins, such as Grb2, which subsequently binds the nucleotide exchange protein Sos in the transmission of intracellular signals. SLP-76, Cbl, Shc, and Grb2 have been implicated in immunoreceptor tyrosine-based activation motif (ITAM) signaling, leading to activation of Ras. However, their mechanism of action has not been determined. To date, there have been no reports of SLP-76 involvement in FcγRI-receptor signaling and no data exist for an interaction between Cbl, Shc, and SLP-76 in vivo. We provide evidence that SLP-76, Cbl, and Shc are tyrosine phosphorylated on FcγRI-receptor stimulation and are associated with the adapter protein Grb2 in γ-interferon–differentiated U937 cells (U937IF). The interactions between SLP-76 and Cbl and SLP-76 and Grb2 are present in resting U937IF cells. However, the interaction between SLP-76 and Grb2 becomes augmented twofold on FcγRI-receptor aggregation. Our results provide the first evidence for a phosphorylation-dependent interaction between SLP-76 and Shc, induced at least 10-fold on FcγRI receptor stimulation. Our data indicate that a significant portion of a multimolecular complex containing Cbl, SLP-76, Shc, and Grb2 is distinct from a trimolecular complex containing the Ras guanine nucleotide exchanger Sos, Shc, and Grb2. FcγRI-induced tyrosine phosphorylation of SLP-76, Cbl, Shc, and the highly induced SLP-76-Shc interaction provide the first evidence that SLP-76 and Cbl are involved in FcγRI signaling and suggest a functional significance for these interactions in FcγRI signal relay in the control of Ras in myeloid cells.

© 1998 by The American Society of Hematology.

SLP-76-Cbl-Grb2-Shc Interactions in FcγRI Signaling

SLP-76 and Cbl are complex adapter proteins that have the capacity to bind to smaller adapter proteins, such as Grb2, which subsequently binds the nucleotide exchange protein Sos in the transmission of intracellular signals. SLP-76, Cbl, Shc, and Grb2 have been implicated in immunoreceptor tyrosine-based activation motif (ITAM) signaling, leading to activation of Ras. However, their mechanism of action has not been determined. To date, there have been no reports of SLP-76 involvement in FcγRI-receptor signaling and no data exist for an interaction between Cbl, Shc, and SLP-76 in vivo. We provide evidence that SLP-76, Cbl, and Shc are tyrosine phosphorylated on FcγRI-receptor stimulation and are associated with the adapter protein Grb2 in γ-interferon–differentiated U937 cells (U937IF). The interactions between SLP-76 and Cbl and SLP-76 and Grb2 are present in resting U937IF cells. However, the interaction between SLP-76 and Grb2 becomes augmented twofold on FcγRI-receptor aggregation. Our results provide the first evidence for a phosphorylation-dependent interaction between SLP-76 and Shc, induced at least 10-fold on FcγRI receptor stimulation. Our data indicate that a significant portion of a multimolecular complex containing Cbl, SLP-76, Shc, and Grb2 is distinct from a trimolecular complex containing the Ras guanine nucleotide exchanger Sos, Shc, and Grb2. FcγRI-induced tyrosine phosphorylation of SLP-76, Cbl, Shc, and the highly induced SLP-76-Shc interaction provide the first evidence that SLP-76 and Cbl are involved in FcγRI signaling and suggest a functional significance for these interactions in FcγRI signal relay in the control of Ras in myeloid cells.

© 1998 by The American Society of Hematology.

Sequences surrounding the Src-homology 3 domain of phospholipase Cgamma-1 increase the domain's association with Cbl

SH3 domains are protein modules that interact with proline-rich polypeptide fragments. Cbl is an adapter-like protein known to interact with several SH3 domains, including the PLCgamma1 SH3 domain and the Grb2 amino terminal SH3 domain. Here we explore whether sequences surrounding the PLCgamma1 SH3 domain core sequence (aa.796-851) can affect the binding to Cbl, a target used as a prototypical ligand. Consistent with previous reports, our results demonstrated a weak binding of Cbl to GST fusion proteins that strictly encompass the structural core of the PLCgamma1 SH3 domain but a high-avidity binding to the Grb2 amino-terminal SH3 domain. Inclusion of amino acids immediately flanking the PLCgamma1 SH3 core domain, however, substantially increased binding of Cbl to a level comparable to that of the Grb2 amino-terminal SH3 domain. The interaction of this extended PLCgamma1 SH3 domain fusion protein with Cbl was shown to depend entirely upon the interaction of the domain with a proline-rich motif in Cbl, ruling out the possibility that amino acids adjacent to the core SH3 domain of PLCgamma1 provide independent Cbl binding. These data suggest that sequences surrounding the SH3 domain of PLCgamma1 may contribute to or stabilize the association of the domain with the target protein, thus increasing its binding efficiency.