Crk interacts with tyrosine-phosphorylated p116 upon T cell activation

The tyrosine phosphatase SHP-1 promotes T cell adhesion by activating the adaptor protein CrkII in the immunological synapse

The adaptor protein CrkII regulates T cell adhesion by recruiting the guanine nucleotide exchange factor C3G, an activator of Rap1. Subsequently, Rap1 stimulates the integrin LFA-1, which leads to T cell adhesion and interaction with antigen-presenting cells (APCs). The adhesion of T cells to APCs is critical for their proper function and education. The interface between the T cell and the APC is known as the immunological synapse. It is characterized by the specific organization of proteins that can be divided into central supramolecular activation clusters (c-SMACs) and peripheral SMACs (p-SMACs). Through total internal reflection fluorescence (TIRF) microscopy and experiments with supported lipid bilayers, we determined that activated Rap1 was recruited to the immunological synapse and localized to the p-SMAC. C3G and the active (dephosphorylated) form of CrkII also localized to the same compartment. In contrast, inactive (phosphorylated) CrkII was confined to the c-SMAC. Activation of CrkII and its subsequent movement from the c-SMAC to the p-SMAC depended on the phosphatase SHP-1, which acted downstream of the T cell receptor. In the p-SMAC, CrkII recruited C3G, which led to Rap1 activation and LFA-1-mediated adhesion of T cells to APCs. Functionally, SHP-1 was necessary for both the adhesion and migration of T cells. Together, these data highlight a signaling pathway in which SHP-1 acts through CrkII to reshape the pattern of Rap1 activation in the immunological synapse.

TCR crosslinking promotes Crk adaptor protein binding to tyrosine-phosphorylated CD3ζ chain

T cell antigen receptor (TCR) binding of a peptide antigen presented by antigen-presenting cells (APCs) in the context of surface MHC molecules initiates signaling events that regulate T cell activation, proliferation and differentiation. A key event in the activation process is the phosphorylation of the conserved tyrosine residues within the CD3 chain immunoreceptor tyrosine-based activation motifs (ITAMs), which operate as docking sites for SH2 domain-containing effector proteins. Phosphorylation of the CD3ζ ITAMs renders the CD3 chain capable of binding the ζ-chain associated protein 70 kDa (ZAP70), a protein tyrosine kinase that is essential for T cell activation. We found that TCR/CD3 crosslinking in Jurkat T cells promotes the association of Crk adaptor proteins with the transiently phosphorylated CD3ζ chain. Pull down assays using bead-immobilized GST fusion proteins revealed that the Crk-SH2 domain mediates binding of phospho-CD3ζ. Phospho-CD3ζ binding is selective and is mediated by the three types of Crk, including CrkI, CrkII, and CrkL, but not by other SH2 domain-containing adaptor proteins, such as Grb2, GRAP and Nck. Crk interaction with phospho-CD3ζ is rapid and transient, peaking 1 min post TCR/CD3 crosslinking. The results suggest the involvement of Crk adaptor proteins in the early stages of T cell activation in which Crk might help recruiting effector proteins to the vicinity of the phospho-CD3ζ and contribute to the fine-tuning of the TCR/CD3-coupled signal transduction pathways.

Structural Determinants of the Gain-of-Function Phenotype of Human Leukemia-associated Mutant CBL Oncogene

Mutations of the tyrosine kinase-directed ubiquitin ligase CBL cause myeloid leukemias, but the molecular determinants of the dominant leukemogenic activity of mutant CBL oncogenes are unclear. Here, we first define a gain-of-function attribute of the most common leukemia-associated CBL mutant, Y371H, by demonstrating its ability to increase proliferation of hematopoietic stem/progenitor cells (HSPCs) derived from CBL-null and CBL/CBL-B-null mice. Next, we express second-site point/deletion mutants of CBL-Y371H in CBL/CBL-B-null HSPCs or the cytokine-dependent human leukemic cell line TF-1 to show that individual or combined Tyr → Phe mutations of established phosphotyrosine residues (Tyr-700, Tyr-731, and Tyr-774) had little impact on the activity of the CBL-Y371H mutant in HSPCs, and the triple Tyr → Phe mutant was only modestly impaired in TF-1 cells. In contrast, intact tyrosine kinase-binding (TKB) domain and proline-rich region (PRR) were critical in both cell models. PRR deletion reduced the stem cell factor (SCF)-induced hyper-phosphorylation of the CBL-Y371H mutant and the c-KIT receptor and eliminated the sustained p-ERK1/2 and p-AKT induction by SCF. GST fusion protein pulldowns followed by phospho-specific antibody array analysis identified distinct CBL TKB domains or PRR-binding proteins that are phosphorylated in CBL-Y371H-expressing TF-1 cells. Our results support a model of mutant CBL gain-of-function in which mutant CBL proteins effectively compete with the remaining wild type CBL-B and juxtapose TKB domain-associated PTKs with PRR-associated signaling proteins to hyper-activate signaling downstream of hematopoietic growth factor receptors. Elucidation of mutant CBL domains required for leukemogenesis should facilitate targeted therapy approaches for patients with mutant CBL-driven leukemias.

The Role of Crk Adaptor Proteins in T-Cell Adhesion and Migration

Crk adaptor proteins are key players in signal transduction from a variety of cell surface receptors. They are involved in early steps of lymphocyte activation through their SH2-mediated transient interaction with signal transducing effector molecules, such as Cbl, ZAP-70, CasL, and STAT5. In addition, they constitutively associate, via their SH3 domain, with effector molecules, such as C3G, that mediate cell adhesion and regulate lymphocyte extravasation and recruitment to sites of inflammation. Recent studies demonstrated that the conformation and function of CrkII is subjected to a regulation by immunophilins, which also affect CrkII-dependent T-cell adhesion to fibronectin and migration toward chemokines. This article addresses mechanisms that regulate CrkII conformation and function, in general, and emphasizes the role of Crk proteins in receptor-coupled signaling pathways that control T-lymphocyte adhesion and migration to inflammatory sites.

The adaptor protein Crk in immune response

The adaptor proteins Crk (CT10 (chicken tumor virus number 10) regulator of kinase), including CrkI, CrkII and Crk-like, are important signal molecules that regulate a variety of cellular processes. Considerable progress has been made in understanding the roles of the Crk family proteins in signal transduction, with a focus on cellular transformation and differentiation. However, since Crk was identified in 1988, very few studies have addressed how Crk regulates the immune response. Recent work demonstrates that Crk proteins function as critical signal molecules in regulating immune cell functions. Emerging data on the roles of Crk in activation and inhibitory immunoreceptor signaling suggest that Crk proteins are potential immunotherapeutic targets in cancer and infectious diseases. The aim of this review is to summarize recent key findings regarding the role of Crk in immune responses mediated by T, B and natural killer (NK) cells. In particular, the roles of Crk in NK cell functions are discussed.

Src, p130Cas, and Mechanotransduction in Cancer Cells

Anchorage-independent growth is the most significant hallmark of cell transformation, which has an intimate relevance to cancer. Anchorage or adhesion physically links cells to the extracellular matrix and allows the transmission of external mechanical cues to intracellular signaling machineries. Transformation involves acquiring the ability to proliferate without requiring mechanically initiated signal transduction, known as mechanotransduction. A number of signaling and cytoskeletal molecules are located at focal adhesions. Src and its related proteins, including p130Cas, localize to adhesion sites, where their functions can be mechanically regulated. In addition, the aberrant activation and expression of Src and p130Cas are linked to transformation and malignancy both in vitro and in vivo. These findings shed light on the importance of mechanotransduction in tumorigenesis and the regulation of cancer progression and also provide insights into the mechanical aspects of cancer signaling.

CrkL is a co-activator of estrogen receptor alpha that enhances tumorigenic potential in cancer

Signaling via estrogen receptor (ER) occurs by interacting with many proteins. Nuclear interactome analysis of ERα in an embryo implantation model revealed the association of chicken tumor virus no. 10 regulator of kinase like (CrkL) with ERα, which was further validated by mammalian two-hybrid assay as well as coimmunoprecipitation and colocalization. Mutation in LPALL motif of CrkL disrupts the ERα-CrkL interaction and its transactivation potential, thereby suggesting that the interaction is mediated via its single ER binding motif, Leu-Pro-Ala-Leu-Leu (LXXLL) motif in the sarcoma homology (SH)2 domain. CrkL deletion constructs of SH2 domain target to the nucleus due to presence of nuclear localization signal. Interestingly, the SH2-SH3 (N terminal) construct shows an increased transactivation potential like CrkI. Weak interaction capability of mutated ERα-Y538F with CrkL validates that CrkL interacts with ERα via its YDLL motif at Tyr 541. In an attempt to understand the physiological relevance of this association, we investigated the impact on cell proliferation using a cancer model, because events associated in the process of pregnancy and cancer are analogous. Also, overexpression of CrkL is frequently associated with tumorigenesis. However, its significance in hormone-regulated cancers still remains obscure. Here, we demonstrate that association of ERα and CrkL directly enhances the tumorigenic potential of CrkL, thus pointing to its role in cell proliferation. In human endometrial cancers, we observed a strong association between ERα and CrkL levels. Thus, the molecular signaling set off by ERα and CrkL association may have a central role in pregnancy and cancer, two events which share parallels in growth, invasion, and immune tolerance.

Crk and CrkL adaptor proteins: networks for physiological and pathological signaling

The Crk adaptor proteins (Crk and CrkL) constitute an integral part of a network of essential signal transduction pathways in humans and other organisms that act as major convergence points in tyrosine kinase signaling. Crk proteins integrate signals from a wide variety of sources, including growth factors, extracellular matrix molecules, bacterial pathogens, and apoptotic cells. Mounting evidence indicates that dysregulation of Crk proteins is associated with human diseases, including cancer and susceptibility to pathogen infections. Recent structural work has identified new and unusual insights into the regulation of Crk proteins, providing a rationale for how Crk can sense diverse signals and produce a myriad of biological responses.

A new twist to adaptor proteins contributes to regulation of lymphocyte cell signaling

Cell growth and differentiation are highly controlled processes mediated by effector molecules, which are regulated by posttranslational chemical modifications. Adaptor molecules are critical players in these mechanisms because of their ability to simultaneously interact with multiple effector molecules and orchestrate the assembly of signaling complexes downstream of activated surface receptors. One family of adaptor molecules includes the CrkII/CrkL proteins that are also involved in the regulation of lymphocyte function. Although Crk proteins are amenable to regulation by protein tyrosine kinases, recent data suggest that peptidyl-prolyl cis-trans isomerases (PPIases) can alter their conformation and hence their ability to associate with binding partners. This emerging new function of PPIases is the subject of the current review.

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.

The lectin jacalin induces phosphorylation of ERK and JNK in CD4+ T cells

The CD4 molecule plays an essential role in mediating the transduction of intracellular signals by functioning as a coreceptor for the complex T cell receptor/CD3 and also acts as the primary receptor for human immunodeficiency virus (HIV). Several authors have shown evidence that jacalin, a plant lectin, binds to CD4 and inhibits in vitro HIV infection. We analyzed jacalin-induced intracellular signaling events in CD4(+) T cells and have shown that cell activation resulted in tyrosine phosphorylation of intracellular substrates p56(lck), p59(fyn), ZAP-70, p95 (vav), phospholipase C-gamma1, and ras activation, as assessed by conversion of ras guanosine 5'-diphosphate to ras guanosine 5'-triphosphate. We further examined extracellular regulated kinase (ERK) and c-jun NH(2)-terminal kinase (JNK) phosphorylation following stimulation with jacalin. The data indicate that the kinetics of JNK phosphorylation is delayed. Optimum phosphorylation of ERK2 was observed by 10 min, and that of JNK was observed by 30 min. Pretreatment with gp120 followed by stimulation with jacalin resulted in marked inhibition of all of the aforementioned intracellular events. The data presented here provide insight into the intracellular signaling events associated with the CD4 molecule-jacalin-gp120 interactions and HIV-induced CD4(+) T cell anergy. Jacalin may be used as a possible tool for the study of CD4-mediated signal transduction and HIV-impaired CD4(+) T cell activation.

DOCK2 mediates T cell receptor-induced activation of Rac2 and IL-2 transcription

DOCK2, a CDM family protein exclusively found in hematopoietic cells, has been shown to play a role in lymphocyte migration by the regulation of actin cytoskeleton. Although DOCK2 has been shown to induce the activation of Rac1, the regulatory mechanism of Rac2, which is a hematopoietic cell-specific small GTPase, is still unknown. In this study, we examined the role of DOCK2 in the activation of Rac2 in hematopoietic cells. DOCK2 was found to associate with the zeta subunit of the CD3 complex of T cell receptors in Jurkat cells and to activate forced expressed Rac2 in 293T cells. In addition, the stable expression of DOCK2 in Jurkat cells exhibited the elevated activity of endogenous Rac2. Furthermore, the transcriptional activity of interleukin-2 (IL-2) was enhanced in DOCK2-expressing Jurkat cells and the dominant negative form of Rac2 suppressed its elevated IL-2 promoter activity. These results suggest that DOCK2 mediates TCR-dependent activation of Rac2, leading to the regulation of IL-2 promoter activity in T cells.

Crk family adaptors-signalling complex formation and biological roles

Crk family adaptors are widely expressed and mediate the timely formation of signal transduction protein complexes upon a variety of extracellular stimuli, including various growth and differentiation factors. Selective formation of multi-protein complexes by the Crk and Crk-like (CRKL) proteins depends on specific motifs recognized by their SH2 and SH3 domains. In the case of the first SH3 domains [SH3(1)] a P-x-x-P-x-K motif is crucial for highly selective binding, while the SH2 domains prefer motifs which conform to the consensus pY-x-x-P. Crk family proteins are involved in the relocalization and activation of several different effector proteins which include guanine nucleotide releasing proteins like C3G, protein kinases of the Abl- and GCK-families and small GTPases like Rap1 and Rac. Crk-type proteins have been found not only in vertebrates but also in flies and nematodes. Major insight into the function of Crk within organisms came from the genetic model organism C. elegans, where the Crk-homologue CED-2 regulates cell engulfment and phagocytosis. Other biological outcomes of the Crk-activated signal transduction cascades include the modulation of cell adhesion, cell migration and immune cell responses. Crk family adaptors also appear to play a role in mediating the action of human oncogenes like the leukaemia-inducing Bcr-Abl protein. This review summarizes some key findings and highlights recent insights and open questions.

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.

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.

The involvement of the proto-oncogene p120 c-Cbl and ZAP-70 in CD2-mediated T cell activation

The CD2 co-receptor expressed on the surface of T lymphocytes is able to stimulate T cell activation, proliferation and cytokine production in the absence of direct engagement of the antigen-specific TCR. Engagement of human CD2 by mitogenic pairs of anti-CD2 mAb induces tyrosine phosphorylation of a number of intracellular proteins including a 120 kDa phosphoprotein that we identify as the proto-oncogene c-Cbl. Rapidly tyrosine phosphorylated following stimulation of a number of cell surface receptors, c-Cbl is an adaptor protein that has been shown to associate with a complex of intracellular signaling molecules, and to mediate both positive and negative regulatory effects. Here we show that, like TCR-CD3 stimulation, stimulation of CD2 enhanced the association of c-Cbl with both Crk(L) and the p85 subunit of phosphatidylinositol-3 kinase. Overexpression of wild-type c-Cbl protein inhibited both CD2and CD3-induced NF-AT transcriptional activity, suggesting that CD2 signaling is also negatively regulated by c-Cbl. The inhibitory effect of c-Cbl depended upon its N-terminal phosphotyrosine-binding domain, the domain that has been shown to be required for inhibition of the Syk/ZAP-70 family kinases. In Syk(-) Jurkat T cells stably expressing wild-type ZAP-70, CD2 stimulation induced only a minimal increase in ZAP-70 tyrosine phosphorylation. Nevertheless, ZAP-70 kinase was required for CD2-mediated NF-AT transcriptional activity. Thus, CD2-mediated NF-AT transcriptional activity appears to depend upon ZAP-70/Syk kinases and to be negatively regulated by c-Cbl.

Engagement of the CrkL adapter in interleukin-5 signaling in eosinophils

Interleukin-5 (IL-5) drives the terminal differentiation of myeloid progenitors to the eosinophil lineage; blocks eosinophil apoptosis; and primes eosinophils for enhanced functional activities in allergic, parasitic, and other eosinophil-associated diseases. Here we describe a novel signaling pathway activated by the IL-5 receptor in eosinophils involving the CrkL adapter protein. We determined whether IL-5 induces activation of CrkL and STAT5 in eosinophils using both the human eosinophil-differentiated AML14.3D10 cell line and purified peripheral blood eosinophils from normal donors. Stimulation of AML14.3D10 cells or blood eosinophils with IL-5 induced rapid tyrosine phosphorylation of the CrkL adapter and STAT5 and the association of CrkL and STAT5 in vivo as evidenced by the detection of STAT5 in anti-CrkL immunoprecipitates. The resulting CrkL.STAT5 complexes translocated to the nucleus and bound STAT5 consensus DNA-binding sites present in the promoters of IL-5-regulated genes, as shown in gel mobility and antibody supershift assays. IL-5 also induced marked activity of an 8X-GAS (interferon gamma-activated site)-luciferase reporter construct in transient transfections of AML14.3D10 eosinophils, demonstrating that these complexes play a functional role in IL-5 signaling. CrkL was also found to interact, via its N-terminal SH3 domain, with C3G, a guanine exchange factor for the small G-protein Rap1, which was also rapidly activated in an IL-5-dependent manner in these cells, establishing that CrkL mediates downstream activation of at least two signaling cascades in IL-5-stimulated eosinophils. Thus, the CrkL adapter plays an important role in IL-5 signaling in the eosinophil, acting as a nuclear adapter for STAT5 and as an upstream regulator of the C3G-Rap1 signaling pathway.

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.

IFN-gamma activates the C3G/Rap1 signaling pathway

IFN-gamma transduces signals by activating the IFN-gamma receptor-associated Jak-1 and Jak-2 kinases and by inducing tyrosine phosphorylation and activation of the Stat-1 transcriptional activator. We report that IFN-gamma activates a distinct signaling cascade involving the c-cbl protooncogene product, CrkL adapter, and small G protein Rap1. During treatment of NB-4 human cells with IFN-gamma, c-cbl protooncogene product is rapidly phosphorylated on tyrosine and provides a docking site for the src homology 2 domain of CrkL, which also undergoes IFN-gamma-dependent tyrosine phosphorylation. CrkL then regulates activation of the guanine exchange factor C3G, with which it interacts constitutively via its N terminus src homology 3 domain. This results in the IFN-gamma-dependent activation of Rap1, a protein known to exhibit tumor suppressor activity and mediate growth inhibitory responses. In a similar manner, Rap1 is also activated in response to treatment of cells with type I IFNs (IFN-alpha, IFN-beta), which also engage CrkL in their signaling pathways. On the other hand, IFN-gamma does not induce formation of nuclear CrkL-Stat5 DNA-binding complexes, which are induced by IFN-alpha and IFN-beta, indicating that pathways downstream of CrkL are differentially regulated by different IFN subtypes. Taken altogether, our data demonstrate that, in addition to activating the Stat pathway, IFN-gamma activates a distinct signaling cascade that may play an important role in the generation of its growth inhibitory effects on target cells.

CrkL and CrkII participate in the generation of the growth inhibitory effects of interferons on primary hematopoietic progenitors

Interferons are potent regulators of normal and malignant hematopoietic cell proliferation in vitro and in vivo, but the signaling mechanisms by which they exhibit their growth inhibitory effects are unknown. We have recently shown that CrkL is engaged in Type I IFN signaling, as shown by its rapid tyrosine phosphorylation during engagement of the Type I IFN receptor. In the present study, we provide evidence that the related CrkII protein is also rapidly phosphorylated on tyrosine during treatment of U-266 and Daudi cells with IFNalpha or IFNbeta. We also show that both members of the Crk-family, CrkL and CrkII, are phosphorylated in an interferon-dependent manner in primary hematopoietic progenitors. Furthermore, inhibition of CrkL or CrkII protein expression by antisense oligonucleotides, reverses the inhibitory effects of IFNalpha or IFNgamma on the proliferation of normal bone marrow progenitor cells (colony forming units-granulocytic/monocytic [CFU-GM] and burst-forming units-erythroid [BFU-E]). Thus, both CrkL and CrkII are engaged in a signaling pathway (s) that mediates interferon-regulated inhibition of hematopoietic cell proliferation.

T cell activation stimulates the association of enzymatically active tyrosine-phosphorylated ZAP-70 with the Crk adapter proteins

Engagement of the T cell antigen receptor initiates signal transduction involving tyrosine phosphorylation of multiple effector molecules and the formation of multimolecular complexes at the receptor site. Adapter proteins that possess SH2 and SH3 protein-protein interaction domains are implicated in the assembly of cell activation-induced signaling complexes. We found that Crk adapter proteins undergo activation-induced interaction with the zeta-chain associated protein (ZAP-70) tyrosine kinase in the human T cell line, Jurkat. Incubation of various glutathione S-transferase fusion proteins with a lysate of activated Jurkat cells resulted in selective association of ZAP-70 with Crk, but not Grb2 or Nck, adapter proteins. In addition, tyrosine-phosphorylated ZAP-70 co-immunoprecipitated with Crk from a lysate of activated Jurkat cells, and ZAP-70 association with GST-Crk was observed in a lysate of activated human peripheral blood T cells. Association between the two molecules was mediated by direct physical interaction and involved the Crk-SH2 domain and phosphotyrosyl-containing sequences on ZAP-70. The association required intact Lck, considered to be an upstream regulator of ZAP-70, because it could not take place in activated JCaM1 cells, which express normal levels of ZAP-70 but are devoid of Lck. Finally, glutathione S-transferase-Crk fusion proteins were found to interact predominantly with membrane-residing tyrosine-phosphorylated ZAP-70 that exhibited autophosphorylation activity as well as phosphorylation of an exogenous substrate, CFB3. These findings suggest that Crk adapter proteins play a role in the early activation events of T lymphocytes, apparently, by direct interaction with, and regulation of, the membrane-residing ZAP-70 protein tyrosine kinase.

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.

Protein Kinase C Activation Inhibits Tyrosine Phosphorylation of Cbl and Its Recruitment of Src Homology 2 Domain-Containing Proteins

One of the major proteins that is rapidly tyrosine phosphorylated upon stimulation of the TCR/CD3 complex is the 120-kDa product of the c-cbl protooncogene (Cbl). Upon activation, tyrosine-phosphorylated Cbl interacts with the Src homology 2 (SH2) domains of several signaling proteins, e.g., phosphatidylinositol 3-kinase (PI3-K) and CrkL. In the present study, we report that pretreatment of Jurkat T cells with PMA reduced the anti-CD3-induced tyrosine phosphorylation of Cbl and, consequently, its activation-dependent association with PI3-K and CrkL. A specific protein kinase C (PKC) inhibitor (GF-109203X) reversed the effect of PMA on tyrosine phosphorylation of Cbl and restored the activation-dependent association of Cbl with PI3-K and CrkL. We also provide evidence that PKCα and PKCθ can physically associate with Cbl and are able to phosphorylate it in vitro and in vivo. Furthermore, a serine-rich motif at the C terminus of Cbl, which is critical for PMA-induced 14-3-3 binding, is also phosphorylated by PKCα and PKCθ in vitro. These results suggest that, by regulating tyrosine and serine phosphorylation of Cbl, PKC is able to control the association of Cbl with signaling intermediates, such as SH2 domain-containing proteins and 14-3-3 proteins, which may consequently result in the modulation of its function.

Involvement of the adapter protein CRKL in integrin-mediated adhesion

CRKL, an SH2-SH3-SH3 adapter protein, is one of the major tyrosine phosphoproteins detected in primary leukemic neutrophils from patients with CML. CRKL binds directly to BCR/ABL through its N-terminal SH3 domain, suggesting it may be involved in BCR/ABL signal transduction. However, the biological function of CRKL in either normal or leukemic cells is still largely unknown. In this study, we have examined the effects of overexpressing full length or deletion mutants of CRKL in hematopoietic cell lines. Full length, SH2- and SH3(N)-domain deletion mutants of CRKL were transfected into an interleukin-3-dependent hematopoietic cell line, Ba/F3, and 3-5 individual sublines which stably overexpressed each transgene were obtained [Ba/F-CRKL, Ba/F-CRKL deltaSH2, and Ba/F-CRKL deltaSH3(N)]. The growth properties of these transfected cells in the presence or absence of IL-3 were not different from mock transfected or untransfected Ba/F3 cells. However, Ba/F3 cells overexpressing full length CRKL, but not deletion mutants of CRKL, were found to have an increase in their ability to bind to fibronectin-coated surfaces. Further, expression of full length, but not deltaSH2- or deltaSH3-CRKL deletion mutants, was found to alter cell morphology on fibronectin-coated plates, an effect which was further enhanced by certain kinds of stress stimuli, such as ionizing radiation. Similar results were obtained when CRKL was transiently overexpressed in Ba/F3 cells, and were also obtained in a second IL-3 dependent hematopoietic cell line, 32Dcl3. Adhesion to fibronectin was blocked by anti-beta1 integrin monoclonal antibody, but overexpression of CRKL did not affect surface expression of beta1 integrins, nor did it spontaneously induce expression of the beta1 integrin 'activation' epitope recognized by the 9EG7 monoclonal antibody. These data suggest a role for CRKL in signaling pathways which regulate adhesion to fibronectin.

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 a CrkL-stat5 signaling complex by type I interferons

Type I interferons (IFNalpha and IFNbeta) transduce signals by inducing tyrosine phosphorylation of Jaks and Stats, as well as the CrkL adapter, an SH2/SH3-containing protein which provides a link to downstream pathways that mediate growth inhibition. We report that Stat5 interacts constitutively with the IFN receptor-associated Tyk-2 kinase, and during IFNalpha stimulation its tyrosine-phosphorylated form acts as a docking site for the SH2 domain of CrkL. CrkL and Stat5 then form a complex that translocates to the nucleus. This IFN-inducible CrkL-Stat5 complex binds in vitro to the TTCTAGGAA palindromic element found in the promoters of a subset of IFN-stimulated genes. Thus, during activation of the Type I IFN receptor, CrkL functions as a nuclear adapter protein and, in association with Stat5, regulates gene transcription through DNA binding.

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.

Regulation of Tyrosine Phosphorylation in Isolated T Cell Membrane by Inhibition of Protein Tyrosine Phosphatases

Jurkat T cells activated by the phosphotyrosine phosphatase inhibitors H2O2 or vanadate were found to have a similar pattern of tyrosine phosphorylation when compared with T cells stimulated by anti-CD3 Ab cross-linking, suggesting that protein tyrosine phosphatase (PTP) inhibitors affect the early steps of TCR signaling. To study the role of PTPs in the most proximal membrane events of tyrosine phosphorylation, subcellular fractions of T cells were treated with the PTP inhibitors in the presence of ATP. In the membrane fraction, tyrosine phosphorylation of Lck, Fyn, and CD3ζ can be induced by PTP inhibitors, but not by anti-CD3. Detailed characterization of this cell-free system showed that the pattern and the order of induced tyrosine phosphorylation is similar to that induced in intact cells. Upon removal of the PTP inhibitor, the tyrosine-phosphorylated proteins, including Lck, Fyn, Syk, Zap70, and CD3ζ, are rapidly dephosphorylated. Preliminary characterizations indicate that a PTP distinct from CD45, SHP1, and SHP2 is present in T cell membranes and the inhibition of this yet unidentified PTP is most likely responsible for the Lck-dependent tyrosine phosphorylation triggered by PTP inhibitors.

Interplay of the proto-oncogene proteins CrkL and CrkII in insulin-like growth factor-I receptor-mediated signal transduction

The closely related proto-oncogene proteins CrkII and CrkL consist of one SH2 and two SH3 domains and share 60% overall homology with the highest identity within their functional domains. In this study we show that CrkL and CrkII may play overlapping but different roles in insulin-like growth factor (IGF)-I receptor-mediated signal transduction. While both proteins are substrates involved in IGF-I receptor signaling, they apparently demonstrate important different properties and different biological responses. Evidence supporting this hypothesis includes (a) the oncogenic potential of CrkL versus the absence of this potential in CrkII overexpressing cell lines, (b) the inhibition of IGF-I-dependent cell cycle progression by overexpression of CrkII, and (c) the differential regulation of the phosphorylation status of selective proteins in CrkII and CrkL overexpressing cell lines. In addition we demonstrate the specific association of CrkL and CrkII with the newly characterized IRS-4 protein, again in a differential manner. Whereas CrkL strongly interacts with IRS-4 via its SH2 and N-terminal SH3 domains, CrkII interacts only via its SH2 domain, possibly explaining the unstable nature of IRS-4-CrkII association. The results obtained allow us to propose a unique mechanism of CrkL and CrkII tyrosine phosphorylation in response to IGF-I stimulation. Thus these highly homologous proteins apparently possess structural features that allow for the differential association of each protein with different effector molecules, thereby activating different signaling pathways and resulting in unique biological roles of these proteins.

T cell activation through the CD43 molecule leads to Vav tyrosine phosphorylation and mitogen-activated protein kinase pathway activation

CD43, the most abundant membrane protein of T lymphocytes, is able to initiate signals that lead to Ca2+ mobilization and interleukin-2 production, yet the molecular events involved in signal transduction pathway of the CD43 molecule are only beginning to be understood. We have shown recently that cross-linking CD43 on the cell surface of human T lymphocytes with the anti-CD43 monoclonal antibody L10 leads to CD43-Fyn kinase interactions and to Fyn phosphorylation on tyrosine residues. This interaction seems to be mediated by the SH3 domain of Fyn and a proline-rich sequence located in the cytoplasmic domain of CD43. Here we show that CD43-specific activation of human T lymphocytes induced tyrosine phosphorylation of the adaptor protein Shc and of the guanine exchange factor Vav, as well as the formation of a macromolecular complex that comprises Shc, GRB2, and Vav. CD43 ligation resulted in enhanced formation of Vav.SLP-76 complexes and in the activation and nuclear translocation of ERK2. Cross-linking of the CD43 molecule in 3T3-CD43(+) cells induced luciferase activity from a construct under the control of the Fos serum responsive element. Altogether, these data suggest that the mitogen-activated protein kinase pathway is involved in CD43-dependent interleukin-2 gene expression.

Cbl: complex formation and functional implications

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

Coordinated regulation of the tyrosine phosphorylation of Cbl by Fyn and Syk tyrosine kinases

Cross-linking of the T cell antigen receptor (TCR)-CD3 complex induces rapid tyrosine phosphorylation and activation of Src (Lck and Fyn) and Syk (Syk and Zap-70) family protein tyrosine kinases (PTKs) which, in turn, phosphorylate multiple intracellular substrates. Cbl is a prominent PTK substrate suggesting a pivotal role for it in early signal transduction events. However, the regulation of Cbl function and tyrosine phosphorylation in T cells by upstream PTKs remains poorly understood. In the present study, we used genetic and biochemical approaches to demonstrate that Cbl directly interacts with Syk and Fyn via its N-terminal and C-terminal regions, respectively. Tyr-316 of Syk was required for the interaction with Cbl as well as for the maximal tyrosine phosphorylation of Cbl. However, both wild-type Syk and Y316F-mutated Syk phosphorylated equally well the C-terminal fragment of Cbl in vivo, suggesting the existence of an alternative, N terminus-independent mechanism for the Syk-induced tyrosine phosphorylation of Cbl. This mechanism appears to involve Fyn, since, in addition to its association with the C-terminal region of Cbl, Fyn also associated with Syk and enhanced the Syk-induced tyrosine phosphorylation of Cbl. These findings implicate Fyn as an adaptor protein that facilitates the interaction between Syk and Cbl, and suggest that Src and Syk family PTKs coordinately regulate the tyrosine phosphorylation of Cbl.

Tyrosine kinase chimeras for antigen-selective T-body therapy

Protein tyrosine kinases (PTKs) transmit activation signals in almost every cell type, including immune effector cells. The aberrant or constitutive activation of PTKs can often cause neoplastic transformation. The use of chimeric receptors based on PTKs may enable us to elucidate the signaling pathways of normal immune cells and other cell types, and the abnormal events that can lead to malignant transformation. In this review, we focus on antigen specific chimeric PTKs in which antibody-derived scFv are joined to the Syk family of PTKs. These chimeric receptors yielded reagents that can selectively redirect immune effector cells and specifically activate them to produce cytokines or lyse their target. The advantages of using such PTK-based chimeras to redirect lymphocytes to tumor targets and their potential as an immunotherapeutic approach to malignant disease is discussed.

Constitutive association of EGF receptor with the CrkII-23 mutant that inhibits transformation of NRK cells by EGF and TGF-beta

Crk belongs to the adapter proteins that participate in many signalling pathways from cell surface receptors. We have characterised the CrkII-23 mutant that inhibits the transformation of NRK cells induced by epidermal growth factor (EGF) and transforming growth factor (TGF)-beta. To study the biochemical difference, cDNAs of the wild-type CrkII and the CrkII-23 mutant were introduced stably into NIH 3T3 cells expressing EGF receptor (EGFR). Both CrkII and CrkII-23 were phosphorylated on tyrosine upon EGF simulation with similar time course and dose dependency. Whereas the wild-type CrkII bound to EGFR only after EGF stimulation, CrkII-23 bound to EGFR from before stimulation. Mutation in the Src homology (SH) 2 or amino-terminal SH3 domain did not abolish the binding of CrkII-23 to EGFR in the quiescent cells, suggesting that the binding is mediated by a novel mechanism. These CrkII-23-derived mutants, however, did not suppress transformation of NRK cells by EGF and TGF-beta. Hence, both the SH2 and amino-terminal SH3 domains are required to inhibit transformation of NRK cells. These results suggest that persistent signalling from CrkII-23 bound to EGFR suppresses transformation by EGF and TGF-beta in NRK23 cells.

Interaction in vitro of the product of the c-Crk-II proto-oncogene with the insulin-like growth factor I receptor

The Crk proto-oncogene product is an SH2 and SH3 domain-containing adaptor protein. We have previously demonstrated that Crk-II becomes rapidly tyrosine-phosphorylated in response to stimulation with insulin-like growth factor I (IGF-I) and might be involved in the IGF-I receptor signalling pathway. To determine whether this involvement includes the direct interaction of Crk-II with the cytoplasmic region of the receptor, studies were performed in vitro with glutathione S-transferase (GST) fusion proteins containing various domains of Crk-II. The kinase assay in vitro showed that activated IGF-I receptors efficiently phosphorylated the GST-Crk-II fusion protein. This phosphorylation was dependent on the presence of the SH2 domain and Tyr-221 located in the spacer region between the two SH3 domains. Mutation of Tyr-221 not only prevented phosphorylation of GST-Crk in vitro, but also significantly increased the ability of GST-Crk proteins to co-precipitate activated IGF-I receptors from total cell lysates. Additional binding experiments in vitro showed that Crk-II might interact with the phosphorylated IGF-I receptor through its SH2 domain. To elucidate which region of the IGF-I receptor interacts with Crk-II, a peptide association assay was used in vitro. Different domains of the IGF-I receptor were expressed as (His)6-tagged fusion peptides, phosphorylated with activated wheat germ agglutinin-purified IGF-I receptors and tested for association with GST-Crk-II fusion proteins. Using wild-type as well as mutated peptides, we showed that the SH2 domain of Crk-II preferentially binds the peptide encoding the juxtamembrane region of the IGF-I receptor. Phosphorylation of Tyr-950 and Tyr-943 of the receptor is important for this interaction. These findings allow us to propose a model of direct interaction of Crk-II and the IGF-I receptor in vivo. On activation of the IGF-I receptor, Crk-II binds to phosphorylated tyrosine residues, especially in the juxtamembrane region. As a result of this binding, the IGF-I receptor kinase phosphorylates Tyr-221 of Crk-II, resulting in a change in intramolecular folding and binding of the SH2 domain to the phosphorylated Tyr-221, which causes rapid disassociation of the Crk-II-IGF-I receptor complex.

Crystal-induced neutrophil activation VI. Involvment of FcgammaRIIIB (CD16) and CD11b in response to inflammatory microcrystals

The inflammatory reaction associated with the deposition of monosodium urate (MSU) crystals in synovial spaces is known to be due to interactions with polymorphonuclear neutrophils mediated by presently unidentified surface structures. In this study, we have observed that antibodies directed against CD16 (VIFcRIII) and CD11b (VIM12) selectively and potently inhibit the activation of neutrophils by MSU crystals. The responses affected include the stimulation of tyrosine phosphorylation, activation of the tyrosine kinase syk, tyrosine phosphorylation of the proto-oncogene Cbl, mobilization of calcium, and stimulation of the activity of phospholipase D and of the production of superoxide anions. Tyrosine phosphorylation responses to MSU crystals develop during the Me2SO4-induced differentiation of HL-60 cells in parallel with the surface expression of CD16. These data strongly support the hypothesis that inflammatory microcrystals interact opportunistically with CD16 initially, and that the signal transduction pathways activated thereby depend on CD11b. An examination of the relevance of the hypothesis that an uncontrolled activation of CD16/CD11b may play a role in inflammatory reactions associated with a dysregulation of neutrophil function (other than crystal arthropathies) appears warranted on the basis of the present results.

Harnessing Syk Family Tyrosine Kinases as Signaling Domains for Chimeric Single Chain of the Variable Domain Receptors: Optimal Design for T Cell Activation

T cells of tumor bearers often show defective TCR-mediated signaling events and, therefore, exhibit impaired immune responses. As such, patients with heavy tumor burden are often not amenable to adoptive T cell therapy. To overcome this limitation, we have developed a chimeric receptor that joins an extracellular single chain Fv (scFv) of a specific Ab for Ag recognition to an intracellular protein tyrosine kinase (PTK) for signal propagation. Stimulation through the scFv-PTK receptor should bypass defective TCR-proximal events and directly access the T cell’s effector mechanisms. In this study we describe the optimization of a scFv-PTK configuration, leading to complete T cell activation. The cytosolic PTK Syk is superior to its family member, Zap-70, for intracellular signaling. As a transmembrane (TM) domain, CD4 performs better than CD8 when plastic-immobilized Ag serves as a stimulator. However, when APC are used to trigger chimeric receptors, the need for a flexible spacer between the scFv and TM domains becomes apparent. The CD8α-derived hinge successfully performs this task in chimeric scFv-Syk receptors regardless of its cysteine content. A cytotoxic T cell hybridoma expressing chimeric receptor genes composed of scFv-CD8hinge-CD8TM-Syk or scFv-CD8hinge-CD4TM-Syk is efficiently stimulated to produce IL-2 upon interaction with APC and specifically lyses appropriate target cells in a non-MHC-restricted manner.

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

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

The type I interferon receptor mediates tyrosine phosphorylation of the CrkL adaptor protein

Interferon (IFN) alpha induces rapid and transient tyrosine phosphorylation of the Src homology 2/Src homology 3 (SH2/SH3)-containing CrkL adaptor protein in a time- and dose-dependent manner. Such phosphorylation is most likely regulated by the Type I interferon receptor (IFNR)-associated Tyk-2 kinase, as suggested by the detection of Type I IFN-dependent tyrosine kinase activity in anti-CrkL immunoprecipitates and the IFNalpha-dependent association of CrkL with Tyk-2 in intact cells. Two other Type I IFNs, IFNbeta and IFNomega, also induce tyrosine phosphorylation of CrkL, suggesting that the protein is involved in the signaling pathways of several different Type I IFNs. In the IFNalpha-sensitive U-266 and Daudi cell lines, CrkL interacts via its N terminus SH3 domain with the guanine exchange factor C3G that regulates activation of Rap-1, a small G-protein that exhibits tumor suppressor activity. Thus, tyrosine phosphorylation of CrkL links the functional Type I IFNR complex to the C3G-Rap-1 signaling cascade that mediates growth inhibitory responses.

In vivo regulation of CrkII and CrkL proto-oncogenes in the uterus by insulin-like growth factor-I. Differential effects on tyrosine phosphorylation and association with paxillin

Changes in CrkII and CrkL phosphorylation are associated with insulin-like growth factor receptor activation in cultured cells. We examined whether similar changes also occur following administration of recombinant human insulin-like growth factor-I to the intact animal. In female rats starved overnight, CrkL phosphorylation was significantly increased 12 min after insulin-like growth factor-I administration. Tyrosine phosphorylation of CrkII was not detectable in either control or treated animals. Paxillin, a 65-70-kDa phosphoprotein containing high affinity binding sites common for the Src homology 2 (SH2) domains of CrkII and CrkL, was observed in both CrkII and CrkL immunoprecipitates. Insulin-like growth factor-I treatment stimulated the association of CrkII with paxillin. In contrast, the same treatment resulted in the dissociation of the CrkL-paxillin complex. Similar effects of insulin-like growth factor-I treatment on the association of CrkL with tyrosine phosphorylated paxillin were observed in fibroblasts overexpressing CrkL. This study demonstrates that the activation of the insulin-like growth factor-I receptor induces changes in the tyrosine phosphorylation and protein-protein interactions of the Crk proteins in vivo. The different responses of CrkL and CrkII to insulin-like growth factor-I receptor activation suggest distinct roles for these two adapter proteins in signal transduction.

CD4 Cross-Linking (CD4XL) Induces RAS Activation and Tumor Necrosis Factor-α Secretion in CD4+ T Cells

CD4 molecules are the primary receptors for human immunodeficiency virus (HIV) and bind the envelope glycoprotein gp120 of HIV with high-affinity. We have previously shown that cross-linking of CD4 molecules (CD4XL) in normal peripheral blood mononuclear cells (PBMC) results in secretion of cytokines tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ), but not of interleukin-2 (IL-2) or IL-4. To investigate the intracellular signaling events associated with CD4-gp120 interaction, we incubated CD4+ T cells from peripheral blood of HIV-negative healthy donors with HIV envelope protein gp160 alone or performed CD4XL with gp160 and anti-gp160 antibody. This procedure resulted in tyrosine phosphorylation of intracellular substrates p59fyn, zap 70, and p95vav and also led to ras activation, as assessed by conversion of rasGDP to rasGTP. The role of ras in CD4 signaling was further investigated using CD4+ Jurkat cells transfected with a dominant negative ras mutant. CD4+ T cells expressing dn-ras secreted significantly reduced levels of TNF-α in response to CD4XL. These studies indicate that interaction of HIV gp160 with CD4 molecules activates the ras pathway in T cells, which may result in the cells becoming unresponsive to subsequent stimulation.

CD4 Cross-Linking (CD4XL) Induces RAS Activation and Tumor Necrosis Factor-α Secretion in CD4+ T Cells

CD4 molecules are the primary receptors for human immunodeficiency virus (HIV) and bind the envelope glycoprotein gp120 of HIV with high-affinity. We have previously shown that cross-linking of CD4 molecules (CD4XL) in normal peripheral blood mononuclear cells (PBMC) results in secretion of cytokines tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ), but not of interleukin-2 (IL-2) or IL-4. To investigate the intracellular signaling events associated with CD4-gp120 interaction, we incubated CD4+ T cells from peripheral blood of HIV-negative healthy donors with HIV envelope protein gp160 alone or performed CD4XL with gp160 and anti-gp160 antibody. This procedure resulted in tyrosine phosphorylation of intracellular substrates p59fyn, zap 70, and p95vav and also led to ras activation, as assessed by conversion of rasGDP to rasGTP. The role of ras in CD4 signaling was further investigated using CD4+ Jurkat cells transfected with a dominant negative ras mutant. CD4+ T cells expressing dn-ras secreted significantly reduced levels of TNF-α in response to CD4XL. These studies indicate that interaction of HIV gp160 with CD4 molecules activates the ras pathway in T cells, which may result in the cells becoming unresponsive to subsequent stimulation.

Characterization of Lnk. An adaptor protein expressed in lymphocytes

Stimulation of the T cell antigen receptor (TCR) activates a set of non-receptor protein tyrosine kinases that assist in delivering signals to the cell interior. Among the presumed substrates for these kinases, adaptor proteins, which juxtapose effector enzyme systems with the antigen receptor complex, figure prominently. Previous studies suggested that Lnk, a 38-kDa protein consisting of a single SH2 domain and a region containing potential tyrosine phosphorylation sites, might serve to join Grb2, phospholipase C-gamma1, and phosphatidylinositol 3-kinase to the TCR. To elucidate the physiological roles of Lnk in T cell signal transduction, we isolated the mouse Lnk cDNA, characterized the structure of the mouse Lnk gene, and generated transgenic mice that overproduce Lnk in thymocytes. Here we report that although Lnk becomes phosphorylated during T cell activation, it plays no limiting role in the TCR signaling process. Moreover, we have distinguished p38(Lnk) from the more prominent 36-kDa tyrosine phosphoproteins that appear in activated T cells. Together these studies suggest that Lnk participates in signaling from receptors other than antigen receptors in lymphocytes.

Erythropoietin and Interleukin-3 Activate Tyrosine Phosphorylation of CBL and Association With CRK Adaptor Proteins

Transformation of hematopoietic cells by the Bcr-abl oncoprotein leads to constitutive tyrosine phosphorylation of a number of cellular polypeptides that function in normal growth factor-dependent cell proliferation. Recent studies have shown that the CrkL adaptor protein and the Cbl protooncoprotein are constitutively tyrosine phosphorylated and form a preformed complex in cells expressing Bcr-abl. In the current study, we have examined cytokine-dependent tyrosine phosphorylation of Cbl and its association with Crk proteins. Erythropoietin (EPO) and interleukin-3 induced a dose and time-dependent tyrosine phosphorylation of Cbl in both EPO-dependent Ba/F3 and DA-3 transfectants, and the erythroid cell line HCD-57. Furthermore, once phosphorylated, Cbl associated with Crk adaptor proteins. Of the three Crk isoforms expressed in hematopoietic cells (CrkL, CrkII, and CrkI), tyrosine phosphorylated Cbl binds preferentially to CrkL and CrkII. The amount of Cbl associated with CrkL and CrkII exceeded the fraction of Cbl associated with Grb2 indicating that unlike other receptor systems, the Cbl-Crk association represents the dominant complex of Cbl in growth factor-stimulated hematopoietic cells. In factor-dependent hematopoietic cell lines, CrkL constitutively associated with the guanine nucleotide release factor, C3G, which is known to interact via Crk src-homology 3 (SH3) domains. Our data suggest that the inducible Cbl-Crk association is a proximal component of a signaling pathway downstream of multiple cytokine receptors.

Divergence in signal transduction pathways of platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) receptors. Involvement of sphingosine 1-phosphate in PDGF but not EGF signaling

Platelet-derived growth factor (PDGF) and serum, but not epidermal growth factor (EGF), stimulated sphingosine kinase activity in Swiss 3T3 fibroblasts and increased intracellular concentrations of sphingosine 1-phosphate (SPP), a sphingolipid second messenger (Olivera, A., and Spiegel, S. (1993) Nature 365, 557-560). We report herein that DL-threo-dihydrosphingosine (DHS), a competitive inhibitor of sphingosine kinase that prevents PDGF-induced SPP formation, specifically inhibited the activation of two cyclin-dependent kinases (p34(cdc2) kinase and Cdk2 kinase) induced by PDGF, but not by EGF. SPP reversed the inhibitory effects of DHS on PDGF-stimulated cyclin-dependent kinases and DNA synthesis, demonstrating that the DHS effects were mediated via inhibition of sphingosine kinase. DHS also markedly reduced PDGF-stimulated but not EGF-stimulated mitogen-activated protein kinase activity and DNA binding activity of activator protein-1. Examination of the early signaling events of PDGF action revealed that DHS did not affect PDGF-induced autophosphorylation of the growth factor receptor or phosphorylation of the SH2/SH3 adaptor protein Shc and its association with Grb2. This sphingosine kinase inhibitor did not abrogate activation of phosphatidylinositol 3-kinase by PDGF. In agreement, treatment with SPP had no effect on these responses but did, however, potently stimulate phosphorylation of Crk, another SH2/SH3 adaptor protein. Moreover, DHS inhibited PDGF-stimulated, but not EGF-stimulated, Crk phosphorylation. Thus, regulation of sphingosine kinase activity defines divergence in signal transduction pathways of PDGF and EGF receptors leading to mitogen-activated protein kinase activation.

Serine phosphorylation of Cbl induced by phorbol ester enhances its association with 14-3-3 proteins in T cells via a novel serine-rich 14-3-3-binding motif

Stimulation of the T cell antigen receptor (TCR).CD3 complex induces rapid tyrosine phosphorylation of Cbl, a protooncogene product which has been implicated in intracellular signaling pathways via its interaction with several signaling molecules. We found recently that Cbl associates directly with a member of the 14-3-3 protein family (14-3-3tau) in T cells and that the association is increased as a consequence of anti-CD3-mediated T cell activation. We report here that phorbol 12-myristate 13-acetate stimulation of T cells also enhanced the interaction between Cbl and two 14-3-3 isoforms (tau and zeta). Tyrosine phosphorylation of Cbl was not sufficient or required for this increased interaction. Thus, cotransfection of COS cells with Cbl plus Lck and/or Syk family protein-tyrosine kinases caused a marked increase in the phosphotyrosine content of Cbl without a concomitant enhancement of its association with 14-3-3. Phorbol 12-myristate 13-acetate stimulation induced serine phosphorylation of Cbl, and dephosphorylation of immunoprecipitated Cbl by a Ser/Thr phosphatase disrupted its interaction with 14-3-3. By using successive carboxyl-terminal deletion mutants of Cbl, the 14-3-3-binding domain was mapped to a serine-rich 30-amino acid region (residues 615-644) of Cbl. Mutation of serine residues in this region further defined a binding motif distinct from the consensus sequence RSXSXP, which was recently identified as a 14-3-3-binding motif. These results suggest that TCR stimulation induces both tyrosine and serine phosphorylation of Cbl. These phosphorylation events allow Cbl to recruit distinct signaling elements that participate in TCR-mediated signal transduction pathways.

Direct T cell activation by chimeric single chain Fv-Syk promotes Syk-Cbl association and Cbl phosphorylation

The protein tyrosine kinase Syk is activated upon engagement of immune recognition receptors. We have focused on the identification of signaling elements immediately downstream to Syk in the pathway leading to T cell activation. To circumvent T cell receptor (TCR). CD3 activation of Src family kinases, we constructed a signaling molecule with an extracellular single chain Fv of an anti-TNP antibody, attached via a transmembrane region to Syk (scFv-Syk). In a murine T cell hybridoma, direct aggregation of chimeric Syk with antigen culminates in interleukin-2 production and target cell lysis. Initially, it causes an increase in the association between scFv-Syk and the cytosolic protein Cbl and subsequently promotes tyrosine phosphorylation of Cbl. Interestingly, although both Cbl and phospholipase C-gamma (PLC-gamma) are phosphorylated in this hybridoma upon TCR.CD3 cross-linking, these two events are uncoupled in scFv-Syk-transfected cells, in which we were unable to detect antigen-driven PLC-gamma phosphorylation. These results support a model in which Syk can initiate and directly activate the T cell's signaling machinery and position Cbl as a primary tyrosine kinase substrate in this pathway. Furthermore, for efficient PLC-gamma phosphorylation to occur in these cells, the combined actions of different tyrosine kinase families may be required.