Mediation by the protein-tyrosine kinase Tec of signaling between the B cell antigen receptor and Dok-1

Non-Receptor Tyrosine Kinases: Their Structure and Mechanistic Role in Tumor Progression and Resistance

Protein tyrosine kinases (PTKs) function as key molecules in the signaling pathways in addition to their impact as a therapeutic target for the treatment of many human diseases, including cancer. PTKs are characterized by their ability to phosphorylate serine, threonine, or tyrosine residues and can thereby rapidly and reversibly alter the function of their protein substrates in the form of significant changes in protein confirmation and affinity for their interaction with protein partners to drive cellular functions under normal and pathological conditions. PTKs are classified into two groups: one of which represents tyrosine kinases, while the other one includes the members of the serine/threonine kinases. The group of tyrosine kinases is subdivided into subgroups: one of them includes the member of receptor tyrosine kinases (RTKs), while the other subgroup includes the member of non-receptor tyrosine kinases (NRTKs). Both these kinase groups function as an "on" or "off" switch in many cellular functions. NRTKs are enzymes which are overexpressed and activated in many cancer types and regulate variable cellular functions in response to extracellular signaling-dependent mechanisms. NRTK-mediated different cellular functions are regulated by kinase-dependent and kinase-independent mechanisms either in the cytoplasm or in the nucleus. Thus, targeting NRTKs is of great interest to improve the treatment strategy of different tumor types. This review deals with the structure and mechanistic role of NRTKs in tumor progression and resistance and their importance as therapeutic targets in tumor therapy.

Subcellular compartmentalization of docking protein-1 contributes to progression in colorectal cancer

Full-length (FL) docking protein-1 (DOK1) is an adapter protein which inhibits growth factor and immune response pathways in normal tissues, but is frequently lost in human cancers. Small DOK1 variants remain in cells of solid tumors and leukemias, albeit, their functions are elusive. To assess the so far unknown role of DOK1 in colorectal cancer (CRC), we generated DOK1 mutants which mimic the domain structure and subcellular distribution of DOK1 protein variants in leukemia patients. We found that cytoplasmic DOK1 activated peroxisome-proliferator-activated-receptor-gamma (PPARγ) resulting in inhibition of the c-FOS promoter and cell proliferation, whereas nuclear DOK1 was inactive. PPARγ-agonist increased expression of endogenous DOK1 and interaction with PPARγ. Forward translation of this cell-based signaling model predicted compartmentalization of DOK1 in patients. In a large series of CRC patients, loss of DOK1 protein was associated with poor prognosis at early tumor stages (*p = 0.001; n = 1492). In tumors with cytoplasmic expression of DOK1, survival was improved, whereas nuclear localization of DOK1 correlated with poor outcome, indicating that compartmentalization of DOK1 is critical for CRC progression. Thus, DOK1 was identified as a prognostic factor for non-metastatic CRC, and, via its drugability by PPARγ-agonist, may constitute a potential target for future cancer treatments.

•

Forward translation of a cell-based signaling model predicted clinical relevance for DOK1 in colorectal cancer (CRC).

•

DOK1 is an independent prognostic factor in CRC patients, and its loss associated with poor survival.

•

Cancer cell growth inhibition by DOK1 was increased (“drugable”) by PPARγ-agonist.

Poor survival due to failure to respond to clinical therapies prevents effective treatment of cancer. Thus, there is a high medical need for novel drug targets and biomarkers. DOK1 blocks pro-cancer signaling in the healthy body, but is often lost in tumors. We show that colorectal cancer patients who are positive for DOK1 have a better survival outcome than patients who are negative. Anti-diabetic drugs up-regulated DOK1 and promoted its protective actions against tumor cells. Our study therefore suggests DOK1 as a marker for good prognosis and as a potential drug target for therapy of colorectal cancer.

The roles of Dok family adapters in immunoreceptor signaling

The mammalian Dok protein family has seven members (Dok-1-Dok-7). The Dok proteins share structural similarities characterized by the NH2-terminal pleckstrin homology and phosphotyrosine-binding domains followed by SH2 target motifs in the COOH-terminal moiety, indicating an adapter function. Indeed, Dok-1 was originally identified as a 62 kDa protein that binds with p120 rasGAP, a potent inhibitor of Ras, upon tyrosine phosphorylation by a variety of protein tyrosine kinases. Among the Dok family, only Dok-1, Dok-2, and Dok-3 are preferentially expressed in hematopoietic/immune cells. Dok-1 and its closest relative Dok-2 act as negative regulators of the Ras-Erk pathway downstream of many immunoreceptor-mediated signaling systems, and it is believed that recruitment of p120 rasGAP by Dok-1 and Dok-2 is critical to their negative regulation. By contrast, Dok-3 does not bind with p120 rasGAP. However, accumulating evidence has demonstrated that Dok-3 is a negative regulator of the activation of JNK and mobilization of Ca2+ in B-cell receptor-mediated signaling, where the interaction of Dok-3 with SHIP-1 and Grb2 appears to be important. Here, we review the physiological roles and underlying mechanisms of Dok family proteins.

The insulin-like growth factor (IGF) receptor type 1 (IGF1R) as an essential component of the signalling network regulating neurogenesis

The insulin-like growth factor receptor type 1 (IGF1R) signalling pathway is activated in the mammalian nervous system from early developmental stages. Its major effect on developing neural cells is to promote their growth and survival. This pathway can integrate its action with signalling pathways of growth and morphogenetic factors that induce cell fate specification and selective expansion of specified neural cell subsets. This suggests that during developmental and adult neurogenesis cellular responses to many signalling factors, including ligands of Notch, sonic hedgehog, fibroblast growth factor family members, ligands of the epidermal growth factor receptor, bone morphogenetic proteins and Wingless and Int-1, may be modified by co-activation of the IGF1R. Modulation of cell migration is another possible role that IGF1R activation may play in neurogenesis. Here, I briefly overview neurogenesis and discuss a role for IGF1R-mediated signalling in the developing and mature nervous system with emphasis on crosstalk between the signalling pathways of the IGF1R and other factors regulating neural cell development and migration. Studies on neural as well as on non-neural cells are highlighted because it may be interesting to test in neurogenic paradigms some of the models based on the information obtained in studies on non-neural cell types.

Leupaxin negatively regulates B cell receptor signaling

The role of the paxillin superfamily of adaptor proteins in B cell antigen receptor (BCR) signaling has not been studied previously. We show here that leupaxin (LPXN), a member of this family, was tyrosine-phosphorylated and recruited to the plasma membrane of human BJAB lymphoma cells upon BCR stimulation and that it interacted with Lyn (a critical Src family tyrosine kinase in BCR signaling) in a BCR-induced manner. LPXN contains four leucine-rich sequences termed LD motifs, and serial truncation and specific domain deletion of LPXN indicated that its LD3 domain is involved in the binding of Lyn. Of a total of 11 tyrosine sites in LPXN, we mutated Tyr(22), Tyr(72), Tyr(198), and Tyr(257) to phenylalanine and demonstrated that LPXN was phosphorylated by Lyn only at Tyr(72) and that this tyrosine site is proximal to the LD3 domain. The overexpression of LPXN in mouse A20 B lymphoma cells led to the suppression of BCR-induced activation of JNK, p38 MAPK, and, to a lesser extent, Akt, but not ERK and NFkappaB, suggesting that LPXN can selectively repress BCR signaling. We further show that LPXN suppressed the secretion of interleukin-2 by BCR-activated A20 B cells and that this inhibition was abrogated in the Y72F LPXN mutant, indicating that the phosphorylation of Tyr(72) is critical for the biological function of LPXN. Thus, LPXN plays an inhibitory role in BCR signaling and B cell function.

Tec kinases in T cell and mast cell signaling

The Tec family of tyrosine kinases consists of five members (Itk, Rlk, Tec, Btk, and Bmx) that are expressed predominantly in hematopoietic cells. The exceptions, Tec and Bmx, are also found in endothelial cells. Tec kinases constitute the second largest family of cytoplasmic protein tyrosine kinases. While B cells express Btk and Tec, and T cells express Itk, Rlk, and Tec, all four of these kinases (Btk, Itk, Rlk, and Tec) can be detected in mast cells. This chapter will focus on the biochemical and cell biological data that have been accumulated regarding Itk, Rlk, Btk, and Tec. In particular, distinctions between the different Tec kinase family members will be highlighted, with a goal of providing insight into the unique functions of each kinase. The known functions of Tec kinases in T cell and mast cell signaling will then be described, with a particular focus on T cell receptor and mast cell Fc epsilon RI signaling pathways.

TEC FAMILY KINASES IN T LYMPHOCYTE DEVELOPMENT AND FUNCTION

The Tec family tyrosine kinases are now recognized as important mediators of antigen receptor signaling in lymphocytes. Three members of this family, Itk, Rlk, and Tec, are expressed in T cells and activated in response to T cell receptor (TCR) engagement. Although initial studies demonstrated a role for these proteins in TCR-mediated activation of phospholipase C-gamma, recent data indicate that Tec family kinases also regulate actin cytoskeletal reorganization and cellular adhesion following TCR stimulation. In addition, Tec family kinases are activated downstream of G protein-coupled chemokine receptors, where they play parallel roles in the regulation of Rho GTPases, cell polarization, adhesion, and migration. In all these systems, however, Tec family kinases are not essential signaling components, but instead function to modulate or amplify signaling pathways. Although they quantitatively reduce proximal signaling, mutations that eliminate Tec family kinases in T cells nonetheless qualitatively alter T cell development and differentiation.

Dok-1 tyrosine residues at 336 and 340 are essential for the negative regulation of Ras-Erk signalling, but dispensable for rasGAP-binding

Dok-1 is a common substrate of many protein tyrosine kinases (PTKs). It recruits rasGAP and other SH2-containing proteins and negatively regulates Ras-Erk signalling downstream of PTKs. However, the mechanisms of its inhibitory effect are yet unclear. Here, a series of C-terminal deletion mutants of Dok-1 delineated the core domain for the inhibition of Erk from 334 to 346 amino acid, which contains two SH2-binding motifs having Tyr-336 or Tyr-340. The Dok-1 mutants having tyrosine-to-phenylalanine (YF) substitution(s) at Tyr-336 and/or Tyr-340 lost their inhibitory effect on Ras and Erk downstream of Src-like PTK, Lyn or Fyn, whereas the rasGAP-binding of each mutant remained intact. However, the Dok-1 mutant having YF substitutions at the rasGAP-binding sites (Tyr-295 and Tyr-361) also showed incapability of Ras and Erk inhibition. Moreover, the Dok-1 mutant having YF substitutions at Tyr-336 and Tyr-340 showed an impaired inhibitory effect on v-Abl-induced transformation of NIH-3T3 cells. These results demonstrate that Tyr-336 and Tyr-340 of Dok-1 are dispensable for rasGAP-binding but essential for inhibition of Ras-Erk signalling and cellular transformation downstream of PTKs. Thus, Dok-1 probably recruits as yet unidentified molecule(s), which, in concert with rasGAP, negatively regulate Ras-Erk signalling.

SHIP Family Inositol Phosphatases Interact with and Negatively Regulate the Tec Tyrosine Kinase

The Tec family of protein-tyrosine kinases (PTKs), that includes Tec, Itk, Btk, Bmx, and Txk, plays an essential role in phospholipase Cgamma (PLCgamma) activation following antigen receptor stimulation. This function requires activation of phosphatidylinositol 3-kinase (PI 3-kinase), which promotes Tec membrane localization through phosphatidylinositol 3,4,5-trisphosphate (PtdIns 3,4,5-P(3)) generation. The mechanism of negative regulation of Tec family PTKs is poorly understood. In this study, we show that the inositol 5' phosphatases SHIP1 and SHIP2 interact preferentially with Tec, compared with other Tec family members. Four lines of evidence suggest that SHIP phosphatases are negative regulators of Tec. First, SHIP1 and SHIP2 are potent inhibitors of Tec activity. Second, inactivation of the Tec SH3 domain, which is necessary and sufficient for SHIP binding, generates a hyperactive form of Tec. Third, SHIP1 inhibits Tec membrane localization. Finally, constitutively targeting Tec to the membrane relieves SHIP1-mediated inhibition. These data suggest that SHIP phosphatases can interact with and functionally inactivate Tec by de-phosphorylation of local PtdIns 3,4,5-P(3) and inhibition of Tec membrane localization.

IkappaB kinase beta phosphorylates Dok1 serines in response to TNF, IL-1, or gamma radiation

Dok1 is an abundant Ras-GTPase-activating protein-associated tyrosine kinase substrate that negatively regulates cell growth and promotes migration. We now find that IkappaB kinase beta (IKKbeta) associated with and phosphorylated Dok1 in human epithelial cells and B lymphocytes. IKKbeta phosphorylation of Dok1 depended on Dok1 S(439), S(443), S(446), and S(450). Recombinant IKKbeta also phosphorylated Dok1 or Dok1 amino acids 430-481 in vitro. TNF-alpha, IL-1, gamma radiation, or IKKbeta overexpression phosphorylated Dok1 S(443), S(446), and S(450) in vivo, as detected with Dok1 phospho-S site-specific antisera. Moreover, Dok1 with S(439), S(443), S(446), and S(450) mutated to A was not phosphorylated by IKKbeta in vivo. Surprisingly, mutant Dok1 A(439), A(443), A(446), and A(450) differed from wild-type Dok1 in not inhibiting platelet-derived growth factor-induced extracellular signal-regulated kinase 1/2 phosphorylation or cell growth. Mutant Dok1 A(439), A(443), A(446), and A(450) also did not promote cell motility, whereas wild-type Dok1 promoted cell motility, and Dok1 E(439), E(443), E(446), and E(450) further enhanced cell motility. These data indicate that IKKbeta phosphorylates Dok1 S(439)S(443) and S(446)S(450) after TNF-alpha, IL-1, or gamma-radiation and implicate the critical Dok1 serines in Dok1 effects after tyrosine kinase activation.

Mutual regulation of protein-tyrosine phosphatase 20 and protein-tyrosine kinase Tec activities by tyrosine phosphorylation and dephosphorylation

PTP20, also known as HSCF/protein-tyrosine phosphatase K1/fetal liver phosphatase 1/brain-derived phosphatase 1, is a cytosolic protein-tyrosine phosphatase with currently unknown biological relevance. We have identified that the nonreceptor protein-tyrosine kinase Tec-phosphorylated PTP20 on tyrosines and co-immunoprecipitated with the phosphatase in a phosphotyrosine-dependent manner. The interaction between the two proteins involved the Tec SH2 domain and the C-terminal tyrosine residues Tyr-281, Tyr-303, Tyr-354, and Tyr-381 of PTP20, which were also necessary for tyrosine phosphorylation/dephosphorylation. Association between endogenous PTP20 and Tec was also tyrosine phosphorylation-dependent in the immature B cell line Ramos. Finally, the Tyr-281 residue of PTP20 was shown to be critical for deactivating Tec in Ramos cells upon B cell receptor ligation as well as dephosphorylation and deactivation of Tec and PTP20 itself in transfected COS7 cells. Taken together, PTP20 appears to play a negative role in Tec-mediated signaling, and Tec-PTP20 interaction might represent a negative feedback mechanism.

Functional interaction of RasGAP-binding proteins Dok-1 and Dok-2 with the Tec protein tyrosine kinase

The Dok adaptor family of proteins binding to RasGAP, consisting of Dok-1 and Dok-2, are critical regulators in cell proliferation. These molecules are partners and/or substrates of different protein tyrosine kinases considered as oncoproteins. Here, we show that Dok-1 and Dok-2 are the major tyrosine-phosphorylated proteins associated to Tec, a protein tyrosine kinase expressed in T cells. Furthermore, we evaluate the effect of Dok-1 or Dok-2 on Tec-mediated signalling pathways in T cells. Here, we provide evidence that Dok-1 and Dok-2 proteins are involved in a negative feedback regulation of Tec via a downregulation of its tyrosine phosphorylation and downstream signalling pathways including the Ras pathway. Either Dok-1 or Dok-2 therefore represents a mean of potent retrograde control for protein tyrosine kinase signalling, and then possibly of tumor development.

Galpha 12 activates Rho GTPase through tyrosine-phosphorylated leukemia-associated RhoGEF

Heterotrimeric G proteins, G12 and G13, have been shown to transduce signals from G protein-coupled receptors to activate Rho GTPase in cells. Recently, we identified p115RhoGEF, one of the guanine nucleotide exchange factors (GEFs) for Rho, as a direct link between Galpha13 and Rho [Kozasa, T., et al. (1998) Science 280, 2109-2111; Hart, M. J., et al. (1998) Science 280, 2112-2114]. Activated Galpha13 stimulated the RhoGEF activity of p115 through interaction with the N-terminal RGS domain. However, Galpha12 could not activate Rho through p115, although it interacted with the RGS domain of p115. The biochemical mechanism from Galpha12 to Rho activation remained unknown. In this study, we analyzed the interaction of leukemia-associated RhoGEF (LARG), which also contains RGS domain, with Galpha12 and Galpha13. RGS domain of LARG demonstrated Galpha12- and Galpha13-specific GAP activity. LARG synergistically stimulated SRF activation by Galpha12 and Galpha13 in HeLa cells, and the SRF activation by Galpha12-LARG was further stimulated by coexpression of Tec tyrosine kinase. It was also found that LARG is phosphorylated on tyrosine by Tec. In reconstitution assays, the RhoGEF activity of nonphosphorylated LARG was stimulated by Galpha13 but not Galpha12. However, when LARG was phosphorylated by Tec, Galpha12 effectively stimulated the RhoGEF activity of LARG. These results demonstrate the biochemical mechanism of Rho activation through Galpha12 and that the regulation of RhoGEFs by heterotrimeric G proteins G1213 is further modulated by tyrosine phosphorylation.

Role of Dok1 in cell signaling mediated by RET tyrosine kinase

Using a yeast two-hybrid screen, we identified Dok1 as a docking protein for RET tyrosine kinase. Dok1 bound more strongly to RET with a multiple endocrine neoplasia (MEN) 2B mutation than RET with a MEN2A mutation and was highly phosphorylated in the cells expressing the former mutant protein. Analysis by site-directed mutagenesis revealed that tyrosine 361 in mouse Dok1 represents a binding site for the Nck adaptor protein and tyrosines 295, 314, 361, 376, 397, and 408 for the Ras-GTPase-activating protein. We replaced tyrosine 361 or these six tyrosines with phenylalanine (designated Y361F or 6F) in Dok1 and introduced the mutant Dok1 genes into the cells expressing the wild-type RET or RET-MEN2B protein. Overexpression of Dok1 or Dok1-Y361F, but not Dok1-6F, suppressed the Ras/Erk activation induced by glial cell line-derived neurotrophic factor or RET-MEN2B, implying that this inhibitory effect requires the Ras-GTPase-activating protein binding to Dok1. In contrast, overexpression of Dok1, but not Dok1-Y361F or Dok1-6F, enhanced the c-Jun amino-terminal kinase (JNK) and c-Jun activation. This suggested that the association of Nck to tyrosine 361 in Dok1 is necessary for the JNK and c-Jun activation by glial cell line-derived neurotrophic factor or RET-MEN2B. Because a high level of the JNK phosphorylation was observed in the cells expressing RET-MEN2B, its strong activation via Nck binding to Dok1 may be responsible for aggressive properties of medullary thyroid carcinoma developed in MEN 2B.

Chemotactic factor-induced recruitment and activation of Tec family kinases in human neutrophils. Implication of phosphatidynositol 3-kinases

The importance of the tyrosine phosphorylation cascades in the initiation and regulation of the functional responsiveness of human neutrophils is well established. On the other hand, the link between the G protein-coupled receptors (to which the receptors for chemotactic factors belong) and the activation of tyrosine kinases is very poorly characterized. Based on previous observations indicating that the stimulation of tyrosine phosphorylation was sensitive to inhibition by the phosphatidylinositol 3-kinase inhibitor wortmannin and the recent description of pleckstrin homology domain-containing tyrosine kinases (the Tec family), we have examined the potential implication of the latter in the responses of human neutrophils to chemotactic factors. The results obtained indicate firstly that several members of the Tec family of tyrosine kinases are expressed in human neutrophils, including Tec, Btk, and Bmx. Stimulation of the cells with fMet-Leu-Phe led to a rapid activation of Tec as indicated by its translocation to a membrane fraction and to increases in its in situ level of tyrosine phosphorylation and its capacity to tyrosine phosphorylate itself or an exogenous substrate (SAM68-GST) in in vitro kinase assays. The activation of Tec was inhibited by pertussis toxin as well as by wortmannin. The results of this study provide direct evidence for the implication of Tec family kinases in the responses of human neutrophils to chemotactic factors. They also suggest that one of the links between G protein-coupled receptors and tyrosine kinases depends on the activation of phosphatidylinositol 3-kinase and the generation of phosphatidylinositol 3,4,5-trisphosphate.

Negative regulation of immunoreceptor signaling

Immune cells are activated as a result of productive interactions between ligands and various receptors known as immunoreceptors. These receptors function by recruiting cytoplasmic protein tyrosine kinases, which trigger a unique phosphorylation signal leading to cell activation. In the recent past, there has been increasing interest in elucidating the processes involved in the negative regulation of immunoreceptor-mediated signal transduction. Evidence is accumulating that immunoreceptor signaling is inhibited by complex and highly regulated mechanisms that involve receptors, protein tyrosine kinases, protein tyrosine phosphatases, lipid phosphatases, ubiquitin ligases, and inhibitory adaptor molecules. Genetic evidence indicates that this inhibitory machinery is crucial for normal immune cell homeostasis.

Phosphatidylinositol 3-kinase and Src family kinases are required for phosphorylation and membrane recruitment of Dok-1 in c-Kit signaling

Dok-1 is an adaptor protein that is a substrate for Bcr-Abl and other tyrosine protein kinases. The presence of pleckstrin homology and phosphotyrosine binding domains as well as multiple tyrosine phosphorylation sites suggests that Dok-1 is involved in protein-protein and/or protein-lipid interactions. Here we show that stimulation of Mo7 hematopoietic cells with c-Kit ligand (KL) induces phosphatidylinositol (PI) 3-kinase-dependent tyrosine phosphorylation and membrane recruitment of Dok-1. Addition of the K-Ras membrane-targeting motif to Dok-1 generated a constitutively membrane-bound Dok-1 protein whose tyrosine phosphorylation was independent of PI 3-kinase. Membrane localization of Dok-1 was required for its ability to function as a negative regulator of cell proliferation. Additional experiments revealed that Dok-1 associated with the juxtamembrane region and C-terminal tail of c-Kit. Lyn promoted phosphorylation of c-Kit and association of c-Kit and Dok-1. Both Lyn and Tec were capable of phosphorylating Dok-1. However, the use of primary bone marrow mast cells from normal and Lyn-deficient mice demonstrated that Lyn is required for KL-dependent Dok-1 tyrosine phosphorylation. Taken together, these data indicate that activation of PI 3-kinase by KL promotes binding of the Dok pleckstrin homology domain and Dok-1 recruitment to the plasma membrane where Dok-1 is phosphorylated by Src and/or Tec family kinases.

T cell regulation of p62(dok) (Dok1) association with Crk-L

In addition to engagement of the T cell receptor-CD3 complex, T lymphocytes can be activated by a variety of cell surface molecules including the approximately 50-kDa surface receptor CD2. While the majority of biochemical signaling elements are triggered by either CD2 or TcR-CD3 receptors, a small number of proteins are engaged by only one receptor. Recently, p62(dok) (Dok1), a member of the Dok family of adapter molecules, has been reported to be activated by CD2 and not by CD3 engagement. Here we have examined the role of p62(dok) in CD2-dependent signaling in Jurkat T cells. As previously reported, we find that ligation of the CD2 molecule by mitogenic pairs of anti-CD2 mAbs led to phosphorylation of p62(dok). While CD2-induced p62(dok) tyrosine phosphorylation was independent of both the p36/38 membrane adapter protein linker of activated T cells (LAT) and the ZAP70/Syk family of kinases, it was dependent upon the Src family of kinases including Lck and Fyn. We find further that CD2 engagement induced the association of tyrosine-phosphorylated p62(dok) to Crk-L. The CD2-dependent association of p62(dok) to Crk-L was independent of expression of the ZAP70/Syk family of kinases. Of note, while T cell receptor-CD3 engagement did not induce either p62(dok) phosphorylation or Crk-L association in Jurkat T cells, it did inhibit CD2-dependent p62(dok)-Crk-L complexes; this TcR-CD3-mediated regulation was dependent upon ZAP70 kinase activity. Our data suggest that phosphorylation of p62(dok) and its interaction with other signaling proteins may depend upon integrated signals emanating from the CD2 receptor, utilizing a ZAP70/LAT-independent pathway, and the TcR-CD3 receptor, which is ZAP70/Syk-dependent.

Insulin receptor-mediated p62dok tyrosine phosphorylation at residues 362 and 398 plays distinct roles for binding GTPase-activating protein and Nck and is essential for inhibiting insulin-stimulated activation of Ras and Akt

A GTPase-activating protein (GAP)-associated 60-kDa protein has been found to undergo rapid tyrosine phosphorylation in response to insulin stimulation. However, whether this protein is a direct in vivo substrate for the insulin receptor (IR) tyrosine kinase and whether the tyrosine phosphorylation plays a role in insulin signaling remain to be established. Here we show that the insulin-stimulated tyrosine phosphorylation of the GAP-associated protein, now identified as p62(dok), is inhibited by Grb10, an adaptor protein that binds directly to the kinase domain of the IR, both in vitro and in cells. Replacing Tyr(362) and Tyr(398) with phenylalanine greatly decreased the IR-catalyzed p62(dok) tyrosine phosphorylation in vitro, suggesting that these two residues are the major IR-mediated phosphorylation sites. However, mutations at Tyr(362) and Tyr(398) only partially blocked insulin-stimulated p62(dok) tyrosine phosphorylation in cells, indicating that p62(dok) is also a target for other cellular tyrosine kinase(s) in addition to the IR. Replacing Tyr(362) with phenylalanine abolished the interaction between p62(dok) and Nck. Mutations at Tyr(362/398) of p62(dok) disrupted the interaction between p62(dok) and GAP and decreased the inhibitory effect of p62(dok) on the insulin-stimulated activation of Ras and Akt, but not mitogen-activated protein kinase. Furthermore, the inhibitory effect of p62(dok) on Akt phosphorylation could be blocked by coexpression of a constitutively active Ras. Taken together, our findings indicate that p62(dok) is a direct substrate for the IR tyrosine kinase and that phosphorylation at Tyr(362) and Tyr(398) plays an essential role for p62(dok) to interact with its effectors and negatively regulate the insulin signaling pathway.

Inhibition of the motility and growth of B16F10 mouse melanoma cells by dominant negative mutants of Dok-1

Dok-1 (p62(Dok)) is a multiple-site docking protein that acts downstream of receptor and nonreceptor tyrosine kinases. Although it has been proposed to contribute to the control of cell growth and migration through association with the Ras GTPase-activating protein and the adapter protein Nck, the role of Dok-1 remains largely unknown. The functions of Dok-1 have now been investigated by the generation of two different COOH-terminal truncation mutants of this protein: one (DokPH+PTB) containing the pleckstrin homology and phosphotyrosine-binding domains, and the other (DokPH) composed only of the pleckstrin homology domain. Both of these mutant proteins were shown to act in a dominant negative manner. Overexpression of each of the mutants in highly metastatic B16F10 mouse melanoma cells thus both inhibited the tyrosine phosphorylation of endogenous Dok-1 induced by cell adhesion as well as reduced the association of the endogenous protein with cellular membranes and the cytoskeleton. Overexpression of DokPH+PTB in these cells also markedly reduced both the rates of cell spreading, migration, and growth as well as the extent of Ras activation. The effects of DokPH on these processes were less pronounced than were those of DokPH+PTB, indicating the importance of the phosphotyrosine-binding domain. These results suggest that at least in B16F10 cells, Dok-1 positively regulates not only cell spreading and migration but also cell growth and Ras activity.

p62dok negatively regulates CD2 signaling in Jurkat cells

p62(dok) belongs to a newly identified family of adaptor proteins. In T cells, the two members that are predominantly expressed, p56(dok) and p62(dok), are tyrosine phosphorylated upon CD2 or CD28 stimulation, but not upon CD3 ligation. Little is known about the biological role of Dok proteins in T cells. In this study, to evaluate the importance of p62(dok) in T cell function, we generated Jurkat clones overexpressing p62(dok). Our results demonstrate that overexpression of p62(dok) in Jurkat cells has a dramatic negative effect on CD2-mediated signaling. The p62(dok)-mediated inhibition affects several biochemical events initiated by CD2 ligation, such as the increase of intracellular Ca(2+), phospholipase C gamma 1 activation, and extracellular signal-regulated kinase 1/2 activation. Importantly, these cellular events are not affected in the signaling cascade induced by engagement of the CD3/TCR complex. However, both CD3- and CD2-induced NF-AT activation and IL-2 secretion are impaired in p62(dok)-overexpressing cells. In addition, we show that CD2 but not CD3 stimulation induces p62(dok) and Ras GTPase-activating protein recruitment to the plasma membrane. These results suggest that p62(dok) plays a negative role at multiple steps in the CD2 signaling pathway. We propose that p62(dok) may represent an important negative regulator in the modulation of the response mediated by the TCR.

Domain-dependent function of the rasGAP-binding protein p62Dok in cell signaling

p62Dok, the rasGAP-binding protein, is a common target of protein-tyrosine kinases. It is one of the major tyrosine-phosphorylated molecules in v-Src-transformed cells. Dok consists of an amino-terminal Pleckstrin homology domain, a putative phosphotyrosine binding domain, and a carboxyl-terminal tail containing multiple tyrosine phosphorylation sites. The importance and function of these sequences in Dok signaling remain largely unknown. We have demonstrated here that the expression of Dok can inhibit cellular transformation by the Src tyrosine kinase. Both the phosphotyrosine binding domain and the carboxyl-terminal tail of Dok (in particular residues 336-363) are necessary for such activity. Using a combinatorial peptide library approach, we have shown that the Dok phosphotyrosine binding domain binds phosphopeptides with the consensus motif of Y/MXXNXL-phosphotyrosine. Furthermore, Dok can homodimerize through its phosphotyrosine binding domain and Tyr(146) at the amino-terminal region. Mutations of this domain or Tyr(146) that block homodimerization significantly reduce the ability of Dok to inhibit Src transformation. Our results suggest that Dok oligomerization through its multiple domains plays a critical role in Dok signaling in response to tyrosine kinase activation.

Signaling network of the Btk family kinases

The Btk family kinases represent new members of non-receptor tyrosine kinases, which include Btk/Atk, Itk/Emt/Tsk, Bmx/Etk, and Tec. They are characterized by having four structural modules: PH (pleckstrin homology) domain, SH3 (Src homology 3) domain, SH2 (Src homology 2) domain and kinase (Src homology 1) domain. Increasing evidence suggests that, like Src-family kinases, Btk family kinases play central but diverse modulatory roles in various cellular processes. They participate in signal transduction in response to virtually all types of extracellular stimuli which are transmitted by growth factor receptors, cytokine receptors, G-protein coupled receptors, antigen-receptors and integrins. They are regulated by many non-receptor tyrosine kinases such as Src, Jak, Syk and FAK family kinases. In turn, they regulate many of major signaling pathways including those of PI3K, PLCgamma and PKC. Both genetic and biochemical approaches have been used to dissect the signaling pathways and elucidate their roles in growth, differentiation and apoptosis. An emerging new role of this family of kinases is cytoskeletal reorganization and cell motility. The physiological importance of these kinases was amply demonstrated by their link to the development of immunodeficiency diseases, due to germ-line mutations. The present article attempts to review the structure and functions of Btk family kinases by summarizing our current knowledge on the interacting partners associated with the different modules of the kinases and the diverse signaling pathways in which they are involved.