Evidence that Llck-mediated phosphorylation of p56dok and p62dok may play a role in CD2 signaling

A chiral fluorescent Ir(iii) complex that targets the GPX4 and ErbB pathways to induce cellular ferroptosis

Ferroptosis has recently emerged as a non-apoptotic form of programmed cell death and promising target for anticancer treatment. However, it is challenging to discover ferroptosis inducers with both highly selective tumour targeting and low cytotoxicity to normal cells. Here, we report an Ir(iii) complex, Ir1, that contains a novel chiral pyridine RAS-selective lethal ligand (Py-RSL). This complex effectively inhibits glutathione peroxidase 4 (GPX4) and ferroptosis suppressor protein 1 (FSP1) to induce ferroptosis in human fibrosarcoma (HT-1080) cells. Notably, metal coordination not only endows Ir1 with fluorescent properties for convenient cellular real-time tracking but also efficiently reduces the off-target toxicity of the Py-RSL ligand. Furthermore, label-free quantitative proteomic profiling revealed that Ir1 simultaneously inhibits the ErbB signalling pathway to enhance tumour suppression. Our work is the first to report a ferroptosis-inducing iridium complex with dual mechanisms of inhibition and provides a highly selective and efficient route to develop new ferroptosis-inducing metallodrugs.

Introduction to DOK2 and its potential role in cancer

Cancer is a complex, multifactorial disease that modern medicine ultimately aims to overcome. Downstream of tyrosine kinase 2 (DOK2) is a well-known tumor suppressor gene, and a member of the downstream protein DOK family of tyrosine kinases. Through a search of original literature indexed in PubMed and other databases, the present review aims to extricate the mechanisms by which DOK2 acts on cancer, thereby identifying more reliable and effective therapeutic targets to promote enhanced methods of cancer prevention and treatment. The review focuses on the role of DOK2 in multiple tumor types in the lungs, intestines, liver, and breast. Additionally, we discuss the potential mechanisms of action of DOK2 and the downstream consequences via the Ras/MPAK/ERK or PI3K/AKT/mTOR signaling pathways.

Herpes simplex virus 1 infection of T cells causes VP11/12-dependent phosphorylation and degradation of the cellular protein Dok-2

Previous studies have shown that HSV-1 infection of lymphocytes induces the tyrosine phosphorylation of several proteins that might correspond to viral or host proteins. VP11/12, a viral tegument protein, is the major HSV-induced tyrosine phosphorylated protein identified thus far. In this report, we demonstrated that the cellular adaptor proteins Dok-2 and Dok-1 are tyrosine phosphorylated upon HSV-1 infection. In addition, HSV-1 induced the selective degradation of Dok-2. Finally, we provide evidence that Dok-2 interacts with VP11/12, and that HSV-induced tyrosine phosphorylation and degradation of Dok-2 require VP11/12. Inactivation of either the Src Family Kinases binding motifs or the SHC binding motif of VP11/12 eliminated the interaction of Dok-2 with VP11/12. Elimination of the binding of Dok-2 to VP11/12 prevented Dok-2 phosphorylation and degradation. We propose that HSV-induced Dok phosphorylation and Dok-2 degradation is an immune evasion mechanism to inactivate T cells that might play an important role in HSV pathogenesis.

Dok proteins are recruited to the phagosome and degraded in a GP63-dependent manner during Leishmania major infection

Three adaptor molecules of the Dok family, Dok-1, Dok-2 and Dok-3 are expressed in macrophages and are involved in the negative regulation of signaling in response to lipopolysaccharide and various cytokines and growth factors. We investigated the role and the fate of these proteins following infection with Leishmania major promastigotes in macrophages. The protozoan parasite L. major causes cutaneous leishmaniasis and is known for its capacity to alter host-cell signaling and function. Dok-1/Dok-2(-/-) bone marrow-derived macrophages displayed normal uptake of L. major promastigotes. Following Leishmania infection, Dok-1 was barely detectable by confocal microscopy. By contrast, phagocytosis of latex beads or zymosan led to the recruitment of Dok-1 to phagosomes. In the absence of the Leishmania pathogenesis-associated metalloprotease GP63, Dok-1 was also, partially, recruited to phagosomes containing L. major promastigotes. Further biochemical analyses revealed that similar to Dok-1, Dok-2 and Dok-3 were targets of GP63. Moreover, we showed that upon infection with wild-type or Δgp63 L. major promastigotes, production of nitric oxide and tumor necrosis factor by interferon-γ-primed Dok-1/Dok-2(-/-) macrophages was reduced compared to WT macrophages. These results suggest that Dok proteins may be important regulators of macrophage responses to Leishmania infection.

BRK targets Dok1 for ubiquitin-mediated proteasomal degradation to promote cell proliferation and migration

Breast tumor kinase (BRK), also known as protein tyrosine kinase 6 (PTK6), is a non-receptor tyrosine kinase overexpressed in more that 60% of human breast carcinomas. The overexpression of BRK has been shown to sensitize mammary epithelial cells to mitogenic signaling and to promote cell proliferation and tumor formation. The molecular mechanisms of BRK have been unveiled by the identification and characterization of BRK target proteins. Downstream of tyrosine kinases 1 or Dok1 is a scaffolding protein and a substrate of several tyrosine kinases. Herein we show that BRK interacts with and phosphorylates Dok1 specifically on Y362. We demonstrate that this phosphorylation by BRK significantly downregulates Dok1 in a ubiquitin-proteasome-mediated mechanism. Together, these results suggest a novel mechanism of action of BRK in the promotion of tumor formation, which involves the targeting of tumor suppressor Dok1 for degradation through the ubiquitin proteasomal pathway.

Dok-1 overexpression promotes development of γδ natural killer T cells

In T cells, two members of the Dok family, Dok-1 and Dok-2, are predominantly expressed. Recent evidence suggests that they play a negative role in T-cell signaling. In order to define whether Dok proteins regulate T-cell development, we have generated transgenic mice overexpressing Dok-1 in thymocytes and peripheral T cells. We show that overexpression of Dok-1 retards the transition from the CD4(-) CD8(-) to CD4(+) CD8(+) stage. Moreover, there is a specific expansion of PLZF-expressing Vγ1.1(+) Vδ6.3(+) T cells. This subset of γδ T cells acquires innate characteristics including rapid IL-4 production following stimulation and requiring SLAM-associated adaptor protein (SAP) for their development. Moreover, Dok-1 overexpression promotes the generation of an innate-like CD8(+) T-cell population that expresses Eomesodermin. Altogether, these findings identify a novel role for Dok-1 in the regulation of thymic differentiation and in particular, in the development of PLZF(+) γδ T cells.

The inositol 5-phosphatase SHIP-1 and adaptors Dok-1 and 2 play central roles in CD4-mediated inhibitory signaling

CD4 functions to enhance the sensitivity of T cells to antigenic peptide/MHC class II. However, if aggregated in isolation, e.g. in the absence of T cell receptor (TCR), CD4 can transduce yet undefined signals that lead to T cell unresponsiveness to antigen and apoptosis. In Human Immunodeficiency Virus-1 (HIV-1) disease, CD4(+) T cell loss can result from gp120-induced CD4 signaling in uninfected cells. We show here that CD4 aggregation leads to Lck-dependent phosphorylation of the RasGAP adaptors Downstream of kinase-1/2 (Dok-1/2) and the inositol 5-phosphatase-1 (SHIP-1) and association of the two molecules. Studies using SHIP-1 shRNA, knockout mice and decoy inhibitors further indicate that CD4-mediated inhibition of TCR-mediated T cell activation is SHIP-1 and Dok-1/2 dependent, and involves SHIP-1 hydrolysis of Phosphatidylinositol 3,4,5-trisphosophate (PI(3,4,5)P3) needed for TCR signaling. Our studies provide evidence for a novel mechanism by which ill-timed CD4-mediated signals activated by ligands such as HIV-1 gp120 lead to disarmament of the immune system.

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.

Molecular signature of cell cycle exit induced in human T lymphoblasts by IL-2 withdrawal

Background

The molecular mechanisms of cell cycle exit are poorly understood. Studies on lymphocytes at cell cycle exit after growth factor deprivation have predominantly focused on the initiation of apoptosis. We aimed to study gene expression profile of primary and immortalised IL-2-dependent human T cells forced to exit the cell cycle by growth factor withdrawal, before apoptosis could be evidenced.

Results

By the Affymetrix microarrays HG-U133 2.0 Plus, 53 genes were distinguished as differentially expressed before and soon after IL-2 deprivation. Among those, PIM1, BCL2, IL-8, HBEGF, DUSP6, OSM, CISH, SOCS2, SOCS3, LIF and IL13 were down-regulated and RPS24, SQSTM1, TMEM1, LRRC8D, ECOP, YY1AP1, C1orf63, ASAH1, SLC25A46 and MIA3 were up-regulated. Genes linked to transcription, cell cycle, cell growth, proliferation and differentiation, cell adhesion, and immune functions were found to be overrepresented within the set of the differentially expressed genes.

Conclusion

Cell cycle exit of the growth factor-deprived T lymphocytes is characterised by a signature of differentially expressed genes. A coordinate repression of a set of genes known to be induced during T cell activation is observed. However, growth arrest following exit from the cell cycle is actively controlled by several up-regulated genes that enforce the non-dividing state. The identification of genes involved in cell cycle exit and quiescence provides new hints for further studies on the molecular mechanisms regulating the non-dividing state of a cell, the mechanisms closely related to cancer development and to many biological processes.

CD45 recruits adapter protein DOK-1 and negatively regulates JAK-STAT signaling in hematopoietic cells

It has been extensively documented that CD45 positively regulates T cell receptor-mediated signaling through the activation of Src-family kinases. The mechanism whereby CD45 negatively regulates the JAK/STAT pathway, however, has not been fully elucidated. Here we describe the mechanism by which CD45 negatively regulates the JAK/STAT pathway through the recruitment of the inhibitory molecule Downstream of Kinase 1 (DOK-1) in hematopoietic cells. We present evidences that CD45 recruits DOK-1 to associate with tyrosine-phosphorylated DOK-1, and that the DOK-1-Y296F mutant completely abrogates its interaction with CD45. Moreover, CD45 expression is required for DOK-1 targeting to the plasma membrane in response to anti-CD3 stimulation. Functional studies further showed that stable expression of DOK-1 in K562 cells markedly decreased both JAK-2 and STAT-3/5 phosphorylation following IL-3 and IFN-alpha stimulation. Likewise, stable expression of DOK-1 in Jurkat cells significantly decreased JAK-2 phosphorylation. Similarly, both IL-3 and IFN-alpha-induced JAK-2 phosphorylations were significantly increased in CD45 deficient Jurkat cells. Consistently, silencing of the DOK-1 gene resulted in rescue of MAP kinases and JAKs activities in CD45 positive Jurkat cells. Accordingly, CD45 recruits adaptor DOK-1 to the proximal plasma membrane to serve as a downstream effector, resulting in negative regulation of the JAK/STAT signaling pathway.

Dual acylation and lipid raft association of Src-family protein tyrosine kinases are required for SDF-1/CXCL12-mediated chemotaxis in the Jurkat human T cell lymphoma cell line

Chemokines play pivotal roles in regulating a wide variety of biological processes by modulating cell migration and recruitment. Deregulation of chemokine signaling can alter cell recruitment, contributing to the pathogenic states associated with autoimmune disease, inflammatory disorders, and sepsis. During chemotaxis, lipid rafts and their resident signaling molecules have been demonstrated to partition to different parts of the cell. Herein, we investigated the role of lipid raft resident Src-family kinases (SFK) in stromal cell-derived factor 1/CXCL12-mediated chemotaxis. We have shown that Lck-deficient J.CaM 1.6 cells are defective in CXCL12-mediated chemotaxis in contrast to their parental counterpart, Jurkat cells. Ectopic expression of the SFK hematopoietic cell kinase (Hck) in J.CaM 1.6 cells reconstituted CXCL12 responsiveness. The requirement of lipid raft association of SFK was assessed using both isoforms of Hck: the dually acylated p59(Hck) isoform that is targeted to lipid rafts and the monoacylated p61(Hck) isoform that is nonraft-associated. We have shown using several gain and loss of acylation alleles that dual acylation of Hck was required for CXCL12-mediated chemotaxis in J.CaM 1.6 cells. These results highlight the importance of the unique microenvironment provided by lipid rafts and their specific contribution in providing specificity to CXCL12 signaling.

Differential regulation of adapter proteins Dok2 and Dok1 in platelets, leading to an association of Dok2 with integrin alphaIIbbeta3

BACKGROUND

We previously demonstrated that Dok2 is rapidly phosphorylated on tyrosine residues in platelets in response to thrombin, the immunoreceptor tyrosine-based activation motif-coupled collagen receptor glycoprotein (GP) VI, and by integrin alphaIIbbeta3.

OBJECTIVES AND METHODS

In this study we further delineate the regulation of phosphorylation of Dok2 and compare this to the related adapter Dok1.

RESULTS

We demonstrate expression of Dok1 in platelets and the unexpected observation that the adapter protein undergoes tyrosine phosphorylation in response to thrombin but not to GPVI or integrin alphaIIbbeta3. Furthermore, Dok1 phosphorylation is transient, peaking at 30 s and returning to basal by 5 min, whereas Dok2 phosphorylation is delayed but sustained. Dok2 phosphorylation, but not that of Dok1, is inhibited by Src kinase inhibitors and by chelation of intracellular calcium. Further, phosphorylation of Dok2 by thrombin and integrin alphaIIbbeta3 in mouse platelets is independent of Syk and phospholipase Cgamma2. Additionally, Dok2 coimmunoprecipitates with integrin alphaIIbbeta3 downstream of Src kinases.

CONCLUSIONS

These results demonstrate differential modes of regulation of Dok1 and Dok2 in platelets. Further, they raise the interesting possibility that Dok2 plays an important role in integrin outside-in signaling through a physical and functional interaction with integrin alphaIIbbeta3.

Downstream of Tyrosine Kinases-1 and Src Homology 2-Containing Inositol 5′-Phosphatase Are Required for Regulation of CD4+CD25+ T Cell Development

The adaptor protein, downstream of tyrosine kinases-1 (Dok-1), and the phosphatase SHIP are both tyrosine phosphorylated in response to T cell stimulation. However, a function for these molecules in T cell development has not been defined. To clarify the role of Dok-1 and SHIP in T cell development in vivo, we compared the T cell phenotype of wild-type, Dok-1 knockout (KO), SHIP KO, and Dok-1/SHIP double-knockout (DKO) mice. Dok-1/SHIP DKO mice were runted and had a shorter life span compared with either Dok-1 KO or SHIP KO mice. Thymocyte numbers from Dok-1/SHIP DKO mice were reduced by 90%. Surface expression of both CD25 and CD69 was elevated on freshly isolated splenic CD4(+) T cells from SHIP KO and Dok-1/SHIP DKO, suggesting these cells were constitutively activated. However, these T cells did not proliferate or produce IL-2 after stimulation. Interestingly, the CD4(+) T cells from SHIP KO and Dok-1/SHIP DKO mice produced higher levels of TGF-beta, expressed Foxp3, and inhibited IL-2 production by CD3-stimulated CD4(+)CD25(-) T cells in vitro. These findings suggest Dok-1 and SHIP function in pathways that influence regulatory T cell development.

Phosphotyrosine Binding-Mediated Oligomerization of Downstream of Tyrosine Kinase (Dok)-1 and Dok-2 Is Involved in CD2-Induced Dok Phosphorylation

To date, five members of the downstream of tyrosine kinase (Dok) family have been characterized. In T cells, two members, Dok-1 and Dok-2, are expressed. CD2 or CD28 stimulation, but not CD3/TCR stimulation, induces Dok phosphorylation. Recent evidence suggests that they act as negative regulators of the CD2 and CD28 signaling pathways. To identify the molecular mechanisms involved in Dok-mediated inhibition, we have identified proteins that bind to the phosphotyrosine-binding (PTB) domain of Dok-1 and Dok-2. We showed that the Dok PTB domain mediates phosphotyrosine-dependent homotypic and heterotypic interactions of Dok-1 and Dok-2. Moreover, in CD2-stimulated Jurkat cells, Dok-1 coimmunoprecipitates with tyrosine-phosphorylated Dok-2. To study the involvement of PTB-mediated oligomerization in Dok function, we have generated Jurkat clones overexpressing Dok-1 or Dok-2 with a mutation that prevents oligomerization (in either the PTB domain or Tyr146 of Dok-1 and Tyr139 of Dok-2). These mutations abrogate CD2-induced phosphorylation and the ability of Dok-1 or Dok-2 to inhibit CD2-induced ERK1/2 and NFAT activation. Moreover, overexpression of Dok-1Y146F or Dok-2Y139F interferes with CD2-induced phosphorylation of endogenous Dok, whereas overexpression of PTB mutant or wild-type Dok does not. Taken together, these data indicate that PTB-mediated oligomerization of Dok-1 and Dok-2 represents an essential step for Dok phosphorylation and function.

Human T lymphotropic virus type-1 p30II alters cellular gene expression to selectively enhance signaling pathways that activate T lymphocytes

Background

Human T-lymphotropic virus type-1 (HTLV-1) is a deltaretrovirus that causes adult T-cell leukemia/lymphoma and is implicated in a variety of lymphocyte-mediated disorders. HTLV-1 contains both regulatory and accessory genes in four pX open reading frames. pX ORF-II encodes two proteins, p13^II and p30^II, which are incompletely defined in the virus life cycle or HTLV-1 pathogenesis. Proviral clones of the virus with pX ORF-II mutations diminish the ability of the virus to maintain viral loads in vivo. Exogenous expression of p30^II differentially modulates CREB and Tax-responsive element-mediated transcription through its interaction with CREB-binding protein/p300 and represses tax/rex RNA nuclear export.

Results

Herein, we further characterized the role of p30^II in regulation of cellular gene expression, using stable p30^II expression system employing lentiviral vectors to test cellular gene expression with Affymetrix U133A arrays, representing ~33,000 human genes. Reporter assays in Jurkat T cells and RT-PCR in Jurkat and primary CD4+ T-lymphocytes were used to confirm selected gene expression patterns. Our data reveals alterations of interrelated pathways of cell proliferation, T-cell signaling, apoptosis and cell cycle in p30^II expressing Jurkat T cells. In all categories, p30^II appeared to be an overall repressor of cellular gene expression, while selectively increasing the expression of certain key regulatory genes.

Conclusions

We are the first to demonstrate that p30^II, while repressing the expression of many genes, selectively activates key gene pathways involved in T-cell signaling/activation. Collectively, our data suggests that this complex retrovirus, associated with lymphoproliferative diseases, relies upon accessory gene products to modify cellular environment to promote clonal expansion of the virus genome and thus maintain proviral loads in vivo.

Stimulation of peripheral blood and intestinal mucosa cells by synthetic CpG oligodeoxynucleotides

The breakdown of tolerance to autologous bacterial flora has been implicated as a major factor contributing to the initiation and perpetuation of chronic inflammation in inflammatory bowel diseases (IBD). To test whether bacterial DNA is at the origin of inflammation in IBD, we have examined the response of lamina propria (LPMC) or peripheral mononuclear cells (PBMC) and purified T cells from IBD patients and control patients to stimulations with a set of oligodeoxynucleotides (ODNs) characterized by the presence or absence of cytosine-guanosine dinucleotides (CpG) and/or 3' poly-guanosine (poly-G) extension. Furthermore we have evaluated the costimulatory activities of these ODNs on T cells activated via CD2 or CD3 pathway. We demonstrated that CpG ODNs induce higher proliferation of LPMC from inflammatory intestinal mucosa compared to healthy mucosa. We confirmed that CpG ODNs do not directly costimulate peripheral blood T cells activated by CD3 pathway. Finally, we revealed that CpG or non-CpG ODNs with 3' poly-G extension inhibit completely CD2 activation of purified PB or LP T-cells whereas only CpG ODNs without poly-G extension enhance proliferation and IFN-gamma production of PB T cells stimulated by CD2 pathway only in presence of NK and NK T cells. Our data suggest that NK T cells may be the primary target of ODNs and play a crucial role in indirect T-cell activation by ODN.

Stromal cell-derived factor-1alpha/CXCL12-induced chemotaxis of T cells involves activation of the RasGAP-associated docking protein p62Dok-1

Events mediating stromal cell-derived factor-1 (SDF-1alpha/CXCL12) chemotaxis of lymphocytes are not completely known. We evaluated intracellular signaling through RasGAP-associated protein p62Dok-1 (downstream of tyrosine kinase [Dok-1]) and associated proteins. SDF-1alpha/CXCL12 stimulated Dok-1 tyrosine phosphorylation and association with RasGAP, adaptor protein p46Nck, and Crk-L in Jurkat T cells. The phosphorylation of Dok-1 was blocked by pretreatment of cells with the src kinase inhibitor PP2. Src kinase family member Lck was implicated. SDF-1alpha/CXCL12 did not phosphorylate Dok-1 in J.CaM1.6 cells, a Jurkat derivative not expressing Lck, but did phosphorylate Dok-1 in J.CaM1.6 cells expressing Lck. SDF-1alpha/CXCL12 induced the tyrosine phosphorylation of Pyk2 and the association of Pyk2 with zeta chain-associated protein-70 kilodaltons (Zap-70) and Vav. SDF-1alpha/CXCL12 enhanced the association of RasGAP with Pyk2. CXCR4-expressing NIH3T3 and Baf3 cells transfected with full-length Dok-1 cDNA were suppressed in their responses to SDF-1alpha/CXCL12-induced chemotaxis; mitogen-activated protein (MAP) kinase activity was also decreased. Chemotaxis to SDF-1/CXCL12 was significantly enhanced in Dok-1(-/-) CD4+ and CD8+ splenic T cells. These results implicate Dok-1, Nck, Crk-L, and Src kinases-especially Lck, Pyk2, Zap-70, Vav, and Ras-GAP-in intracellular signaling by SDF-1alpha/CXCL12, and they suggest that Dok-1 plays an important role in SDF-1alpha/CXCL12-induced chemotaxis in T cells.

Frameshift mutation in the Dok1 gene in chronic lymphocytic leukemia

B-cell chronic lymphocytic leukemia (B-CLL) is a malignant disease characterized by an accumulation of monoclonal CD5+ mature B cells, with a high percentage of cells arrested in the G0/G1 phase of the cell cycle, and a particular resistance toward apoptosis-inducing agents. Dok1 (downstream of tyrosine kinases) is an abundant Ras-GTPase-activating protein (Ras-GAP)-associated tyrosine kinase substrate, which negatively regulates cell proliferation, downregulates MAP kinase activation and promotes cell migration. The gene encoding Dok1 maps to human chromosome 2p13, a region previously found to be rearranged in B-CLL. We have screened the Dok1 gene for mutations from 46 individuals with B-CLL using heteroduplex analysis. A four-nucleotide GGCC deletion in the coding region was found in the leukemia cells from one patient. This mutation causes a frameshift leading to protein truncation at the carboxyl-terminus, with the acquisition of a novel amino-acid sequence. In contrast to the wild-type Dok1 protein, which has cytoplasmic/membrane localization, the mutant Dok1 is a nuclear protein containing a functional bipartite nuclear localization signal. Whereas overexpression of wild-type Dok1 inhibited PDGF-induced MAP kinase activation, this inhibition was not observed with the mutant Dok1. Furthermore the mutant Dok1 forms heterodimers with Dok1 wild type and the association can be enhanced by Lck-mediated tyrosine-phosphorylation. This is the first example of a Dok1 mutation in B-CLL and the data suggest that Dok1 might play a role in leukemogenesis.

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.

Interaction of SAP-1, a transmembrane-type protein-tyrosine phosphatase, with the tyrosine kinase Lck. Roles in regulation of T cell function

SAP-1 is a transmembrane-type protein-tyrosine phosphatase that is expressed in most tissues but whose physiological functions remain unknown. The cytoplasmic region of SAP-1 has now been shown to bind directly the tyrosine kinase Lck. Overexpression of wild-type SAP-1, but not that of a catalytically inactive mutant of SAP-1, inhibited both the basal and the T cell antigen receptor (TCR)-stimulated activity of Lck in human Jurkat T cell lines. Lck served as a direct substrate for dephosphorylation by SAP-1 in vitro. Overexpression of wild-type SAP-1 in Jurkat cells also: (i) inhibited both the activation of mitogen-activated protein kinase and the increase in cell surface expression of CD69 induced by TCR stimulation; (ii) reduced the extent of the TCR-induced increase in the tyrosine phosphorylation of ZAP-70 or that of LAT; (iii) reduced both the basal level of tyrosine phosphorylation of p62dok, as well as the increase in the phosphorylation of this protein induced by CD2 stimulation; and (iv) inhibited cell migration. These results thus suggest that the direct interaction of SAP-1 with Lck results in inhibition of the kinase activity of the latter and a consequent negative regulation of T cell function.

Gr-1+ myeloid cells lacking T cell protein tyrosine phosphatase inhibit lymphocyte proliferation by an IFN-gamma- and nitric oxide-dependent mechanism

The T cell protein tyrosine phosphatase is involved in the immune system regulation, as evidenced by defective function and development of several hemopoietic cell populations in T cell protein tyrosine phosphatase (TC-PTP)-deficient mice. In particular, B and T cell proliferation is greatly inhibited when total splenocytes are stimulated by LPS or anti-CD3 mAb. To define the functional defect of TC-PTP(-/-) lymphocytes, we isolated T and B cells from the spleen of TC-PTP(-/-) mice. We show that the proliferative response of lymphocytes was greatly increased when cultured as a purified population, indicating that an inhibitory population is present in TC-PTP(-/-) spleen. However, TC-PTP(-/-) lymphocytes have a 2- to 3-fold lower proliferation rate compared with TC-PTP(+/+) lymphocytes, suggesting that, as shown previously in embryonic fibroblasts, TC-PTP is involved in the control of cell cycle in lymphocytes. We have characterized phenotypically and functionally the inhibitory population present in the spleen of TC-PTP(-/-) mice. We show that a Gr-1(+)-enriched cell population isolated from TC-PTP(-/-) mice suppresses the CD3-induced proliferation of T cells in coculture in vitro. The specific inhibition of NO synthesis with N(G)-monomethyl-L-arginine.monoacetate restored splenocyte responses, and there is a strict correlation between NO levels and the degree of suppression. Neutralization of IFN-gamma with specific mAb almost completely abolished the inhibitory activity of Gr-1(+) cells and concomitantly high levels of NO secretion. Moreover, inhibition of lymphocyte proliferative responses required cell-cell contact to achieve sufficient levels of NO. These findings demonstrate an important function of TC-PTP in the induction of the NO pathway that mediates inhibition of T cell proliferation.

DOK4 and DOK5: new Dok-related genes expressed in human T cells

Dok proteins are adapter proteins involved in signal transduction. Several intracellular proteins expressed in lymphocytes meet the criteria of membrane-associated adapter proteins such as members of the Dok family. To understand the role and the formation of multiprotein networks involving Dok proteins in T lymphocytes, we search for potential additional members of this family. Here, we describe the two new human dok-related genes DOK4 and DOK5 and present data showing the expression of DOK4 and DOK5 genes in T cells. These genes are the orthologues of mouse Dok4 and Dok5 genes. Based on analysis of phylogenetic trees and exon/intron structure of Dok family members, DOK4 and DOK5 define a subfamily within dok genes distinct from DOK1, DOK2 and DOK3. So, Dok-4 and Dok-5 molecules constitute a new group of adapter proteins in T cells, requiring further functional analysis.

The distinct capacity of Fyn and Lck to phosphorylate Sam68 in T cells is essentially governed by SH3/SH2-catalytic domain linker interactions

Sam68 phosphorylation correlates with Fyn but not Lck expression in T cells. This substrate has been used here to explore the possible basis of the specificity of Fyn versus Lck. We show that this specificity is not based on a spatial segregation of the two kinases, since a chimeric Lck molecule containing the membrane anchoring domain of Fyn does not phosphorylate Sam68. Moreover, a Sam68 molecule targeted to the plasma membrane by the farnesylation signal of c-Ha-Ras remains poorly phosphorylated by Lck. In T cells, Fyn appears to be the active Src kinase in rafts, but Sam68 is not expressed in rafts, and its distinct phosphorylation by Fyn and Lck is not affected by raft dispersion. The Fyn/Lck specificity does not reflect a higher kinase activity of Fyn in general, as both Fyn and Lck are similarly recognized by an anti-active Src antibody. Both also strongly phosphorylate another Src substrate in vivo. Mainly, Lck phosphorylates Sam68 when the interaction between the SH3 domain and the SH2-catalytic domain linker is altered in heterologous Src molecules or after mutating key residues in the linker that increase the accessibility of the SH3 domain. Thus, the distinct potential of Fyn and Lck to phosphorylate Sam68 is likely controlled by the interaction of the kinase SH3 domain with the linker and Sam68, possibly on a competitive binding basis.

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.

Dok-related protein negatively regulates T cell development via its RasGTPase-activating protein and Nck docking sites

Downstream of kinase (Dok)-related protein (DokR, also known as p56(dok)/FRIP/Dok-R) is implicated in cytokine and immunoreceptor signaling in myeloid and T cells. Tyrosine phosphorylation induces DokR to bind the signal relay molecules, RasGTPase-activating protein (RasGAP) and Nck. Here, we have examined the function of DokR during hematopoietic development and the requirement for RasGAP and Nck binding sites in its biological function. Retroviral-mediated expression of DokR in bone marrow cells dramatically inhibited their capacity to form colonies in vitro in response to the cytokines macrophage colony-stimulating factor and stem cell factor, whereas responses to interleukin-3 and granulocyte macrophage colony-stimulating factor were only weakly affected. When introduced into lethally irradiated mice, hematopoietic cells expressing DokR showed a drastically reduced capacity to repopulate lymphoid tissues. Most notably, DokR dramatically reduced repopulation of the thymus, in part by reducing the number of T cell precursors seeding in the thymus, but equally, through inhibiting the transition of CD4(-)CD8(-) to CD4(+)CD8(+) T cells. Consequently, the number of mature peripheral T cells was markedly reduced. In contrast, a minimal effect on B cell and myeloid lineage development was observed. Importantly, functional RasGAP and Nck binding sites were found to be essential for the biological effects of DokR in vitro and in vivo.

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.

Phosphotyrosine-binding domains in signal transduction

Protein phosphorylation provides molecular control of complex physiological events within cells. In many cases, phosphorylation on specific amino acids directly controls the assembly of multi-protein complexes by recruiting phospho-specific binding modules. Here, the function, structure, and cell biology of phosphotyrosine-binding domains is discussed.

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.

Phosphoinositide 3-kinase-dependent membrane recruitment of p62(dok) is essential for its negative effect on mitogen-activated protein (MAP) kinase activation

A major pathway by which growth factors, such as platelet-derived growth factor (PDGF), regulate cell proliferation is via the receptor tyrosine kinase/Ras/mitogen-activated protein kinase (MAPK) signaling cascade. The output of this pathway is subjected to tight regulation of both positive and negative regulators. One such regulator is p62(dok), the prototype of a newly identified family of adaptor proteins. We recently provided evidence, through the use of p62(dok)-deficient cells, that p62(dok) acts as a negative regulator of growth factor-induced cell proliferation and the Ras/MAPK pathway. We show here that reintroduction of p62(dok) into p62(dok)-(/)- cells can suppress the increased cell proliferation and prolonged MAPK activity seen in these cells, and that plasma membrane recruitment of p62(dok) is essential for its function. We also show that the PDGF-triggered plasma membrane translocation of p62(dok) requires activation of phosphoinositide 3-kinase (PI3-kinase) and binding of its pleckstrin homology (PH) domain to 3'-phosphorylated phosphoinositides. Furthermore, we demonstrate that p62(dok) can exert its negative effect on the PDGFR/MAPK pathway independently of its ability to associate with RasGAP and Nck. We conclude that p62(dok) functions as a negative regulator of the PDGFR/Ras/MAPK signaling pathway through a mechanism involving PI3-kinase-dependent recruitment of p62(dok) to the plasma membrane.

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.