Role of Dok-1 and Dok-2 in myeloid homeostasis and suppression of leukemia

Epigenetics: prenatal exposure to genistein leaves a permanent signature on the hematopoietic lineage

Recent studies demonstrate that maternal diet during pregnancy results in long-lasting effects on the progeny. Supplementation of maternal diet with genistein, a phytoestrogen ubiquitous in the daily diet, altered coat color of agouti mice due to epigenetic changes. We studied hematopoiesis of mice prenatally exposed to genistein (270 mg/kg feed) compared with that of mice prenatally exposed to phytoestrogen-poor feed and observed a significant increase in granulopoiesis, erythropoiesis, and mild macrocytosis at the adult age of 12 wk. Genistein exposure was associated with hypermethylation of certain repetitive elements, which coincided with a significant down-regulation of estrogen-responsive genes and genes involved in hematopoiesis in bone marrow cells of genistein-exposed mice, as assessed by microarray technology. Although genistein exposure did not affect global methylation in fetal liver of fetuses at embryonic day 14.5, it accelerated the switch from primitive to definitive erythroid lineage. Taken together, our data demonstrate that prenatal exposure to genistein affects fetal erythropoiesis and exerts lifelong alterations in gene expression and DNA methylation of hematopoietic cells.

Endogenous oncogenic Nras mutation promotes aberrant GM-CSF signaling in granulocytic/monocytic precursors in a murine model of chronic myelomonocytic leukemia

Oncogenic NRAS mutations are frequently identified in myeloid diseases involving monocyte lineage. However, its role in the genesis of these diseases remains elusive. We report a mouse bone marrow transplantation model harboring an oncogenic G12D mutation in the Nras locus. Approximately 95% of recipient mice develop a myeloproliferative disease resembling the myeloproliferative variant of chronic myelomonocytic leukemia (CMML), with a prolonged latency and acquisition of multiple genetic alterations, including uniparental disomy of oncogenic Nras allele. Based on single-cell profiling of phospho-proteins, a novel population of CMML cells is identified to display aberrant granulocyte-macrophage colony stimulating factor (GM-CSF) signaling in both the extracellular signal-regulated kinase (ERK) 1/2 and signal transducer and activator of transcription 5 (Stat5) pathways. This abnormal signaling is acquired during CMML development. Further study suggests that aberrant Ras/ERK signaling leads to expansion of granulocytic/monocytic precursors, which are highly responsive to GM-CSF. Hyperactivation of Stat5 in CMML cells is mainly through expansion of these precursors rather than up-regulation of surface expression of GM-CSF receptors. Our results provide insights into the aberrant cytokine signaling in oncogenic NRAS-associated myeloid diseases.

Mice lacking Dok-1, Dok-2, and Dok-3 succumb to aggressive histiocytic sarcoma

Histiocytic sarcoma (HS), a rare hematological malignancy, is an aggressive neoplasm that responds poorly to therapy. The molecular etiology and pathology of this disease remain unclear, hampering the development of an effective therapy, and there remains a need for more, and more realistic, animal models. HS cells typically show a histiocytic (ie, tissue macrophage-like) morphology and express histiocyte/macrophage markers in the absence of lymphocyte markers. In this study, we report that Dok-1(-/-)Dok-2(-/-)Dok-3(-/-) mice develop HS, but do not exhibit elevated incidence of other types of tumors. These mutant mice showed earlier mortality than wild-type (WT) or the other mutant mice, and this mortality was associated with HS. In total, 17 of 21 tumor-bearing Dok-1(-/-)Dok-2(-/-)Dok-3(-/-) mice necropsied at 25-66 weeks of age showed multiple organ spread, with osteolytic lesions and orthotopic invasion from the bone marrow to skeletal muscle. Tumors from the mice were transplantable. In addition, all Dok-1(-/-)Dok-2(-/-)Dok-3(-/-) mice, but only a small proportion of Dok-3(-/-) mice and no Dok-1(-/-)Dok-2(-/-) mice, exhibited abnormal accumulation of macrophages in the lung on necropsy at 8-12 weeks of age. Macrophages derived from Dok-1(-/-)Dok-2(-/-)Dok-3(-/-) mice displayed an exaggerated proliferative response to macrophage colony-stimulating factor (M-CSF) or granulocyte- macrophage colony-stimulating factor (GM-CSF) compared with WT and mutant controls. Together, these findings indicate that Dok-1, Dok-2, and Dok-3 cooperatively suppress aggressive HS, and commend Dok-1(-/-)Dok-2(-/-)Dok-3(-/-) mice as a useful model for the study of this neoplasia.

The proximal signaling network of the BCR-ABL1 oncogene shows a modular organization

BCR-ABL1 is a fusion tyrosine kinase, which causes multiple types of leukemia. We used an integrated proteomic approach that includes label-free quantitative protein complex and phosphorylation profiling by mass spectrometry to systematically characterize the proximal signaling network of this oncogenic kinase. The proximal BCR-ABL1 signaling network shows a modular and layered organization with an inner core of three leukemia transformation-relevant adaptor protein complexes (Grb2/Gab2/Shc1 complex, CrkI complex and Dok1/Dok2 complex). We introduced an 'interaction directionality' analysis, which annotates static protein networks with information on the directionality of phosphorylation-dependent interactions. In this analysis, the observed network structure was consistent with a step-wise phosphorylation-dependent assembly of the Grb2/Gab2/Shc1 and the Dok1/Dok2 complexes on the BCR-ABL1 core. The CrkI complex demonstrated a different directionality, which supports a candidate assembly on the Nedd9 (Hef1, CasL) scaffold. As adaptor protein family members can compensate for each other in leukemic transformation, we compared members of the Dok and Crk protein families and found both overlapping and differential binding patterns. We identified an additional level of regulation for the CrkII protein via binding to 14-3-3 proteins, which was independent from its inhibitory phosphorylation. We also identified novel components of the inner core complexes, including the kinases Pragmin (Sgk223) and Lrrk1 (Lrrk2 paralog). Pragmin was found as a component of the CrkI complex and is a potential link between BCR-ABL1/CrkI and RhoA signaling. Lrrk1 is an unusual kinase with a GTPase domain. We detected Lrrk1 as a component of the Grb2/Gab2/Shc1 complex and found that it functionally interacts with the regulator of small GTPases Arap1 (Centd2) and possibly participates in the mitogen-activated protein kinase response to cellular stresses. This modular and phosphorylation-driven interaction network provides a framework for the integration of pleiotropic signaling effects of BCR-ABL1 toward leukemic transformation.

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.

Downstream of tyrosine kinase/docking protein 6, as a novel substrate of tropomyosin-related kinase C receptor, is involved in neurotrophin 3-mediated neurite outgrowth in mouse cortex neurons

BACKGROUND

The downstream of tyrosine kinase/docking protein (Dok) adaptor protein family has seven members, Dok1 to Dok7, that act as substrates of multiple receptor tyrosine kinase and non-receptor tyrosine kinase. The tropomyosin-related kinase (Trk) receptor family, which has three members (TrkA, TrkB and TrkC), are receptor tyrosine kinases that play pivotal roles in many stages of nervous system development, such as differentiation, migration, axon and dendrite projection and neuron patterning. Upon related neurotrophin growth factor stimulation, dimerisation and autophosphorylation of Trk receptors can occur, recruiting adaptor proteins to mediate signal transduction.

RESULTS

In this report, by using yeast two-hybrid assays, glutathione S-transferase (GST) precipitation assays and coimmunoprecipitation (Co-IP) experiments, we demonstrate that Dok6 selectively binds to the NPQY motif of TrkC through its phosphotyrosine-binding (PTB) domain in a kinase activity-dependent manner. We further confirmed their interaction by coimmunoprecipitation and colocalisation in E18.5 mouse cortex neurons, which provided more in vivo evidence. Next, we demonstrated that Dok6 is involved in neurite outgrowth in mouse cortex neurons via the RNAi method. Knockdown of Dok6 decreased neurite outgrowth in cortical neurons upon neurotrophin 3 (NT-3) stimulation.

CONCLUSIONS

We conclude that Dok6 interacts with the NPQY motif of the TrkC receptor through its PTB domain in a kinase activity-dependent manner, and works as a novel substrate of the TrkC receptor involved in NT-3-mediated neurite outgrowth in mouse cortex neurons.

Identification of DOK genes as lung tumor suppressors

Genome-wide analyses of human lung adenocarcinoma have identified regions of consistent copy-number gain or loss, but in many cases the oncogenes and tumor suppressors presumed to reside in these loci remain to be determined. Here we identify the downstream of tyrosine kinase (Dok) family members Dok1, Dok2 and Dok3 as lung tumor suppressors. Single, double or triple compound loss of these genes in mice results in lung cancer, with penetrance and latency dependent on the number of lost Dok alleles. Cancer development is preceded by an aberrant expansion and signaling profile of alveolar type II cells and bronchioalveolar stem cells. In human lung adenocarcinoma, we identify DOK2 as a target of copy-number loss and mRNA downregulation and find that DOK2 suppresses lung cancer cell proliferation in vitro and in vivo. Given the genomic localization of DOK2, we propose it as an 8p21.3 haploinsufficient human lung tumor suppressor.

Tailoring T-cell receptor signals by proximal negative feedback mechanisms

The T-cell receptor (TCR) signalling machinery is central in determining the response of a T cell (establishing immunity or tolerance) following exposure to antigen. This process is made difficult by the narrow margin of self and non-self discrimination, and by the complexity of the genetic programmes that are induced for each outcome. Recent studies have identified novel negative feedback mechanisms that are rapidly induced by TCR engagement and that have key roles in the regulation of signal triggering and propagation. In vitro and in vivo data suggest that they are important in determining ligand discrimination by the TCR and in regulating signal output in response to antigen.

Dok-7/MuSK signaling and a congenital myasthenic syndrome

Skeletal muscle contraction is controlled by motor neurons, which contact the muscle at the neuromuscular junction (NMJ). The formation and maintenance of the NMJ, which includes the aggregation of densely packed clusters of acetylcholine receptor (AChR) opposite the motor nerve terminal, is orchestrated by muscle-specific receptor tyrosine kinase, MuSK. Recently, a MuSK-interacting cytoplasmic adaptor-like protein Dok-7 was identified and its localization at the postsynaptic region of the NMJ was revealed. Mice lacking Dok-7 have a phenotype indistinguishable from MuSK-deficient mice, and fail to form both AChR clusters and NMJs. In cultured myotubes, Dok-7 is required for MuSK activation and AChR clustering. Thus, Dok-7 is essential for neuromuscular synaptogenesis and it appears that the regulatory interaction of Dok-7 with MuSK is integrally involved in this process. In humans there are both autoimmune and genetic causes of defective neuromuscular transmission that gives rise to the fatigable muscle weakness known as myasthenia. DOK7 has been found to be a major locus for mutations that underlie a genetic form of myasthenia with a characteristic 'limb girdle' pattern of muscle weakness (DOK7 CMS). Patients with DOK7 CMS have small, simplified NMJs but normal AChR function. The most common mutation causes a COOH-terminal truncation, which greatly impairs Dok-7's ability to activate MuSK. Recently, a series of differing DOK7 mutations have been identified, which affect not only the COOH-terminal region but also the NH2-terminal moiety. The study of these mutations may help understand the underlying pathogenic mechanism of DOK7 CMS.

RasGAPs: a crucial regulator of extracellular stimuli for homeostasis of cellular functions

Ras and its GTPase activating proteins (GAPs) are among the crucial regulators of extracelluar ligands. Information about these regulators has been elucidated during the course of studies in signal transduction over the last two decades. RasGAPs such as p120GAP and neurofibromin have been studied extensively for their roles as either "negative" regulators or effectors of Ras. Accumulating evidence suggests that these molecules are crucial regulators of extracellular stimuli that serve to maintain the homeostasis of cellular functions. This compendium highlights cellular functions of RasGAPs and their signaling characteristics from the viewpoint of homeostasis, including our recent finding of the phenotype of R-RasGAP mutant mice whose GAP activity is down-regulated.

An integrin phosphorylation switch: the effect of beta3 integrin tail phosphorylation on Dok1 and talin binding

Integrins play a fundamental role in cell migration and adhesion; knowledge of how they are regulated and controlled is vital for understanding these processes. Recent work showed that Dok1 negatively regulates integrin activation, presumably by competition with talin. To understand how this occurs, we used NMR spectroscopy and x-ray crystallography to investigate the molecular details of interactions with integrins. The binding affinities of beta3 integrin tails for the Dok1 and talin phosphotyrosine binding domains were quantified using 15N-1H hetero-nuclear single quantum correlation titrations, revealing that the unphosphorylated integrin tail binds more strongly to talin than Dok1. Chemical shift mapping showed that unlike talin, Dok1 exclusively interacts with the canonical NPXY motif of the beta3 integrin tail. Upon phosphorylation of Tyr 747 in the beta3 integrin tail, however, Dok1 then binds much more strongly than talin. Thus, we show that phosphorylation of Tyr 747 provides a switch for integrin ligand binding. This switch may represent an in vivo mechanism for control of integrin receptor activation. These results have implications for the control of integrin signaling by proteins containing phosphotyrosine binding domains.

Ligation of CD200R by CD200 is not required for normal murine myelopoiesis

CD200R is an inhibitory receptor involved in the regulation of myeloid cells. It recruits Dok-1 and Dok-2, which are potent inhibitors of the Ras signalling pathway used by colony-stimulating factor (CSF) receptors. Dok-1/Dok-2 double knockout (DKO) mice develop leukaemia at 10-12 months of age. We investigated whether disturbed CD200R signalling could be responsible for this phenotype. Therefore, we studied whether CD200(-/-) mice have altered myelopoiesis and develop leukaemia. We report that CD200R is expressed on haematopoietic progenitor cells. However, CD200(-/-) mice have normal numbers of myeloid progenitors in the bone marrow and these cells have normal proliferative capacity. These results indicate that the development of leukaemia in Dok-1/Dok-2 DKO mice is not solely due to an absence of CD200R signalling. In addition, we show that the previously reported enhanced numbers of myeloid cells do not occur in all CD200(-/-) mice. We determined whether variations in the numbers of peripheral myeloid cells were due to an enhanced response to granulocyte-CSF (G-CSF) or an inflammatory stimulus. Mobilisation of immature neutrophils via G-CSF and infiltration of mature neutrophils and macrophages upon thioglycolate injection were not altered in CD200(-/-) mice. We conclude that CD200(-/-) mice exhibit normal myelopoiesis and that development of leukaemia in Dok-1/Dok-2 DKO mice is not caused by a lack of CD200-mediated CD200R signalling.

Dok-3 sequesters Grb2 and inhibits the Ras-Erk pathway downstream of protein-tyrosine kinases

Adaptor proteins are essential in coordinating recruitment and, in a few cases, restraint of various effectors during cellular signaling. Dok-1, Dok-2 and Dok-3 comprise a closely related family of adaptor, which negatively regulates mitogen-activated protein kinase Erk downstream of protein-tyrosine kinases (PTKs). Recruitment of p120 rasGAP, a potent inhibitor of Ras, by Dok-1 and Dok-2 appears critical in the negative regulation of the Ras-Erk pathway. However, as Dok-3 does not bind rasGAP, it has been unclear how Dok-3 inhibits Erk downstream of PTKs. Here, we identified Grb2 as a Dok-3-binding protein upon its tyrosine phosphorylation. This interaction required the intact binding motifs of the Grb2 SH2 domain, and a mutant (Dok-3-FF) having a Tyr/Phe substitution at these motifs failed to inhibit Ras and Erk activation downstream of a cytoplasmic PTK Src. Because Grb2 forms a stable complex with Sos, a crucial activator of Ras, these data suggest that Dok-3 restrains Grb2 and inhibits the ability of the Grb2-Sos complex to activate Ras. Indeed, forced expression of Dok-3, but not Dok-3-FF, inhibited the recruitment of the Grb2-Sos complex to Shc downstream of Src, which is an essential event for activation of the Ras-Erk pathway. These findings indicate that Dok-3 sequesters Grb2 from Shc and inhibits the Ras-Erk pathway downstream of PTKs.

Stem cell c-KIT and HOXB4 genes: critical roles and mechanisms in self-renewal, proliferation, and differentiation

Hematopoietic stem cells (HSCs) possess a distinct ability to perpetuate through self-renewal and to generate progeny that differentiate into mature cells of myeloid and lymphoid lineages. A better understanding of the molecular mechanisms by which HSCs replicate and differentiate from the perspective of developing new approaches for HSC transplantation is necessary for further advances. The interaction of the receptor tyrosine kinase--c-KIT--with its ligand stem cell factor plays a key role in HSC survival, mitogenesis, proliferation, differentiation, adhesion, homing, migration, and functional activation. Evidence that activating site-directed point mutations in the c-KIT gene contributes to its ligand-independent constitutive activation, which induces enhanced proliferation of HSCs, is accumulating. Similarly, and equally important, self-renewal is a process by which HSCs generate daughter cells via division. Self-renewal is necessary for retaining the HSC pool. Therefore, elucidating the molecular machinery that governs self-renewal is of key importance. The transcription factor, HOXB4 is a key molecule that has been reported to induce the in vitro expansion of HSCs via self-renewal. However, critical downstream effector molecules of HOXB4 remain to be determined. This concisely reviewed information on c-KIT and HOXB4 helps us to update our understanding of their function and mechanism of action in self-renewal, proliferation, and differentiation of HSCs, particularly modulation by c-KIT mutant interactions, and HOXB4 overexpression showing certain therapeutic implications.

Dok-3 plays a nonredundant role in negative regulation of B-cell activation

p62(dok) and Dok-3 are members of the Dok family of adaptors found in B cells, with the former cloned as a substrate of the p210(bcr/abl) oncoprotein in Ph + chronic myelogenous leukemia. A role for p62(dok) in FcgammaRIIB-mediated negative regulation of B-cell proliferation had been established previously. Here, we generated Dok-3(-/-) mice to assess the function of Dok-3 in B cells. Mice lacking Dok-3 have normal B-cell development but possess higher level of IgM antibodies in their sera. In comparison to wild-type mice, Dok-3(-/-) mice mounted significantly enhanced humoral immune responses to T cell-independent type I and II antigens. Dok-3-deficient B cells hyperproliferated, exhibited elevated level of calcium signaling as well as enhanced activation of NF-kappaB, JNK, and p38MAPK in response to B-cell receptor (BCR) engagement. In the absence of Dok-3, the localization of the inhibitory phosphatase SHIP-1 to the plasma membrane is intact while its phosphorylation is compromised, suggesting that Dok-3 could function to facilitate or sustain the activation of SHIP-1. The phenotype and responses of Dok-3(-/-) mice and B cells could be differentiated from those of the Dok-1(-/-) counterparts. Hence, we propose that Dok-3 plays a distinct and nonredundant role in the negative regulation of BCR 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.

T cell receptor for antigen induces linker for activation of T cell-dependent activation of a negative signaling complex involving Dok-2, SHIP-1, and Grb-2

Adaptor proteins positively or negatively regulate the T cell receptor for antigen (TCR) signaling cascade. We report that after TCR stimulation, the inhibitory adaptor downstream of kinase (Dok)-2 and its homologue Dok-1 are involved in a multimolecular complex including the lipid phosphatase Src homology 2 domain–containing inositol polyphosphate 5′-phosphatase (SHIP)-1 and Grb-2 which interacts with the membrane signaling scaffold linker for activation of T cells (LAT). Knockdown of LAT and SHIP-1 expression indicated that SHIP-1 favored recruitment of Dok-2 to LAT. Knockdown of Dok-2 and Dok-1 revealed their negative control on Akt and, unexpectedly, on Zap-70 activation. Our findings support the view that Dok-1 and -2 are critical elements of a LAT-dependent negative feedback loop that attenuates early TCR signal. Dok-1 and -2 may therefore exert a critical role in shaping the immune response and as gatekeepers for T cell tolerance.

All-trans retinoic acid induces p62DOK1 and p56DOK2 expression which enhances induced differentiation and G0 arrest of HL-60 leukemia cells

p62(DOK1) (DOK1) and p56(DOK2) (DOK2) are sequence homologs that act as docking proteins downstream of receptor or nonreceptor tyrosine kinases. Originally identified in chronic myelogenous leukemia cells as a highly phosphorylated substrate for the chimeric p210(bcr-abl) protein, DOK1 was suspected to play a role in leukemogenesis. However, p62(DOK1-/-) fibroblast knockout cells were found to have enhanced MAPK signaling and proliferation due to growth factors, suggesting negative regulatory capabilities for DOK1. The role of DOK1 and DOK2 in leukemogeneis thus is enigmatic. The data in this report show that both the DOK1 and the DOK2 adaptor proteins are constitutively expressed in the myelomonoblastic leukemia cell line, HL-60, and that expression of both proteins is induced by the chemotherapeutic differentiation causing agents, all-trans retinoic acid (atRA) and 1,25-dihydroxyvitamin D3 (VD3). Ectopic expression of either protein enhances atRA- or VD3-induced growth arrest, differentiation, and G(0)/G(1) cell cycle arrest and results in increased ERK1/2 phosphorylation. DOK1 and DOK2 are similarly effective in these capabilities. The data provide evidence that DOK1 and DOK2 proteins have a similar role in regulating cell proliferation and differentiation and are positive regulators of the MAPK signaling pathway in this context.

A nuclear export signal and phosphorylation regulate Dok1 subcellular localization and functions

Dok1 is believed to be a mainly cytoplasmic adaptor protein which down-regulates mitogen-activated protein kinase activation, inhibits cell proliferation and transformation, and promotes cell spreading and cell migration. Here we show that Dok1 shuttles between the nucleus and cytoplasm. Treatment of cells with leptomycin B (LMB), a specific inhibitor of the nuclear export signal (NES)-dependent receptor CRM1, causes nuclear accumulation of Dok1. We have identified a functional NES (348LLKAKLTDPKED359) that plays a major role in the cytoplasmic localization of Dok1. Src-induced tyrosine phosphorylation prevented the LMB-mediated nuclear accumulation of Dok1. Dok1 cytoplasmic localization is also dependent on IKKbeta. Serum starvation or maintaining cells in suspension favor Dok1 nuclear localization, while serum stimulation, exposure to growth factor, or cell adhesion to a substrate induce cytoplasmic localization. Functionally, nuclear NES-mutant Dok1 had impaired ability to inhibit cell proliferation and to promote cell spreading and cell motility. Taken together, our results provide the first evidence that Dok1 transits through the nucleus and is actively exported into the cytoplasm by the CRM1 nuclear export system. Nuclear export modulated by external stimuli and phosphorylation may be a mechanism by which Dok1 is maintained in the cytoplasm and membrane, thus regulating its signaling functions.

The duplicitous nature of the Lyn tyrosine kinase in growth factor signaling

The Lyn tyrosine kinase is a unique member of the Src family of non-receptor protein tyrosine kinases whose principal role is to regulate signals through inhibitory receptors thereby promoting signal attenuation. Lyn is renowned for its role in B cell antigen receptor and FcepsilonRI signaling; however, it is becoming increasingly apparent that Lyn also functions in signal transduction from growth factor receptors including the receptors for GM-CSF, IL-3, IL-5, SCF, erythropoietin, CSF-1, G-CSF, thrombopoietin and Flt3 ligand. Numerous studies have implicated Lyn in growth factor receptor signal amplification, while a number also suggest that Lyn participates in negative regulation of growth factor signaling. Indeed Lyn-deficient mice are hyper-responsive to myeloid growth factors and develop a myeloproliferative disorder that predisposes the mice to macrophage tumours, with loss of negative regulation through SHP-1 and SHIP-1 thought to be the major contributing factor to this phenotype. Developing a clear understanding of Lyn's role in establishing signaling thresholds in growth factor receptor signal amplification and signal inhibition may have important implications in the management of leukemias that may depend on Lyn activity.

Dok5 is substrate of TrkB and TrkC receptors and involved in neurotrophin induced MAPK activation

Tropomyosin-related kinase (Trk) family receptors are a group of high affinity receptors for neurotrophin growth factors, which have pivotal functions in many physiological processes of nervous system. Trk receptors can dimerize and autophosphorylate upon neurotrophin stimulation, then recruit multiple adaptor proteins to transduct signal. In this report, we identified Dok5, a member of Dok family, as a new substrate of TrkB/C receptors. In yeast two-hybrid assay, Dok5 can interact with intracellular domain of TrkB and TrkC receptor through its PTB domain, but not with that of TrkA receptor. The interaction was then confirmed by GST pull-down assay and Co-IP experiment. Dok5 co-localized with TrkB and TrkC in differentiated PC12 cells, providing another evidence for their interaction. By using mutational analysis, we characterized that Dok5 PTB domain bound to Trk receptor NPQY motif in a kinase-activity-dependent manner. Furthermore, competition experiment indicated that Dok5 competed with N-shc for binding to the receptors at the same site. Finally, we showed that Dok5 was involved in the activation of MAPK pathway induced by neurotrophin stimulation. Taken together, these results suggest that Dok5 acts as substrate of TrkB/C receptors and is involved in neurotrophin induced MAPK signal pathway activation.

Dok-1 independently attenuates Ras/mitogen-activated protein kinase and Src/c-myc pathways to inhibit platelet-derived growth factor-induced mitogenesis

The Dok adaptor proteins play key regulatory roles in receptor and non-receptor kinase-initiated signaling pathways. Dok-1, the prototype member of this family, negatively regulates cell proliferation elicited by numerous growth factors, including platelet-derived growth factor (PDGF). However, how Dok-1 exerts its negative effect on mitogenesis has remained elusive. Using Dok-1 knockout cells and Dok-1 mutants deficient in binding to specific Dok-1-interacting proteins, we show that Dok-1 interferes with PDGF-stimulated c-myc induction and Ras/mitogen-activated protein kinase (MAPK) activation by tethering different signaling components to the cell membrane. Specifically, Dok-1 attenuates PDGF-elicited c-myc induction by recruiting Csk to active Src kinases, whereupon their activities and consequent c-myc induction are diminished. On the other hand, Dok-1 negatively regulates PDGF-induced MAPK activation by acting on Ras-GAP and at least one other Dok-1-interacting protein. Importantly, we demonstrate that Dok-1's actions on both of these signaling pathways contribute to its inhibitory effect on mitogenesis. Our data suggest a mechanistic basis for the inhibitory effect of Dok-1 on growth factor-induced mitogenesis and its role as a tumor suppressor.

Subversion of protein kinase C alpha signaling in hematopoietic progenitor cells results in the generation of a B-cell chronic lymphocytic leukemia-like population in vivo

B-cell chronic lymphocytic leukemia (B-CLL) is characterized by the accumulation of long-lived mature B cells with the distinctive phenotype CD19(hi) CD5+ CD23+ IgM(lo), which are refractory to apoptosis. An increased level of apoptosis has been observed on treatment of human B-CLL cells with protein kinase C (PKC) inhibitors, suggesting that this family of protein kinases mediate survival signals within B-CLL cells. Therefore, to investigate the ability of individual PKC isoforms to transform developing B cells, we stably expressed plasmids encoding PKC mutants in fetal liver-derived hematopoietic progenitor cells (HPC) from wild-type mice and then cultured them in B-cell generation systems in vitro and in vivo. Surprisingly, we noted that expression of a plasmid-encoding dominant-negative PKC alpha (PKC alpha-KR) in HPCs and subsequent culture both in vitro and in vivo resulted in the generation of a population of cells that displayed an enhanced proliferative capacity over untransfected cells and phenotypically resemble human B-CLL cells. In the absence of growth factors and stroma, these B-CLL-like cells undergo cell cycle arrest and, consistent with their ability to escape growth factor withdrawal-induced apoptosis, exhibited elevated levels of Bcl-2 expression. These studies therefore identify a unique oncogenic trigger for the development of a B-CLL-like disease resulting from the subversion of PKC alpha signaling. Our findings uncover novel avenues not only for the study of the induction of leukemic B cells but also for the development of therapeutic drugs to combat PKC alpha-regulated transformation events.

Drosophila Dok is required for embryonic dorsal closure

Embryonic dorsal closure (DC) in Drosophila is a series of morphogenetic movements involving the bilateral dorsal movement of the epidermis (cell stretching) and dorsal suturing of the leading edge (LE) cells to enclose the viscera. The Syk family tyrosine kinase Shark plays a crucial role in this Jun amino-terminal kinase (JNK)-dependent process, where it acts upstream of JNK in LE cells. Using a yeast two-hybrid screen, the unique Drosophila homolog of the downstream of kinase (Dok) family, Ddok,was identified by its ability to bind Shark SH2 domains in a tyrosine phosphorylation-dependent fashion. In cultured S2 embryonic cells, Ddok tyrosine phosphorylation is Src dependent; Shark associates with Ddok and Ddok localizes at the cell cortex, together with a portion of the Shark protein. The embryonic expression pattern of Ddok resembles the expression pattern of Shark. Ddok loss-of-function mutant (DdokPG155)germ-line clones possess DC defects, including the loss of JNK-dependent expression of dpp mRNA in LE cells, and decreased epidermal F-actin staining and LE actin cable formation. Epistatic analysis indicates that Ddok functions upstream of shark to activate JNK signaling during DC. Consistent with these observations, Ddok mutant embryos exhibit decreased levels of tyrosine phosphorylated Shark at the cell periphery of LE and epidermal cells. As there are six mammalian Dok family members that exhibit some functional redundancy, analysis of the regulation of DC by Ddok is expected to provide novel insights into the function of the Dok adapter proteins.

Dok1 expression and mutation in Burkitt's lymphoma cell lines

Dok1 is an adaptor tyrosine kinase substrate with tumor-suppressive activity. The gene encoding Dok1 maps to human chromosome 2p13, which is frequently rearranged in human tumors. We have previously reported a frameshift mutation of this gene and the down-regulation of its expression in chronic lymphocytic leukemia. In this study, we have determined the expression levels of Dok1 in Burkitt's lymphoma (BL) cell lines, lymphoblastoid cell lines from patients with X-linked lymphoproliferative (XLP-LCL), or from control healthy donors. We have also screened for Dok1 gene mutations by heteroduplex analysis and direct sequencing. Dok1 expression was down-regulated in all BL and XLP-LCL cell lines in comparison to the control cells. No Dok1 mutation or polymorphism was found in the coding region of Dok1 in the three types of cells. However, DNA sequence analysis revealed the presence of four nucleotide changes in Dok1 gene, T(90172)C (intron 1), C(89487)T and (89433)InsCTCT (intron 2), and A(87714)G (3' UTR). T(90172)C and (89433)InsCTCT that were detected in about 7% of BL, 9% of XLP-LCL and 4% of normal samples may represent a common polymorphism. C(89487)T and A(87714)G changes were detected in 9 and 6% of analyzed BL lines, respectively, but never in the control and XLP-LCL cells, indicating that these nucleotide substitution occurred during tumor development. Interestingly, the C(89487)T variant is associated with a significantly lower level of Dok1 expression compared to the control samples. A positive association was also found between the presence of EBV in BL and the Dok1 genetic variation. Our data show that Dok1 expression and structure are affected in a subset of Burkitt's lymphoma samples, suggesting its possible role in this type of cancer.

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.

Dok-1 and Dok-2 are negative regulators of lipopolysaccharide-induced signaling

Endotoxin, a bacterial lipopolysaccharide (LPS), causes fatal septic shock via Toll-like receptor (TLR)4 on effector cells of innate immunity like macrophages, where it activates nuclear factor κB (NF-κB) and mitogen-activated protein (MAP) kinases to induce proinflammatory cytokines such as tumor necrosis factor (TNF)-α. Dok-1 and Dok-2 are adaptor proteins that negatively regulate Ras–Erk signaling downstream of protein tyrosine kinases (PTKs). Here, we demonstrate that LPS rapidly induced the tyrosine phosphorylation and adaptor function of these proteins. The stimulation with LPS of macrophages from mice lacking Dok-1 or Dok-2 induced elevated Erk activation, but not the other MAP kinases or NF-κB, resulting in hyperproduction of TNF-α and nitric oxide. Furthermore, the mutant mice showed hyperproduction of TNF-α and hypersensitivity to LPS. However, macrophages from these mutant mice reacted normally to other pathogenic molecules, CpG oligodeoxynucleotides, poly(I:C) ribonucleotides, or Pam₃CSK₄ lipopeptide, which activated cognate TLRs but induced no tyrosine phosphorylation of Dok-1 or Dok-2. Forced expression of either adaptor, but not a mutant having a Tyr/Phe substitution, in macrophages inhibited LPS-induced Erk activation and TNF-α production. Thus, Dok-1 and Dok-2 are essential negative regulators downstream of TLR4, implying a novel PTK-dependent pathway in innate immunity.

Chronic myelomonocytic leukemia: lost in classification?

Chronic myelomonocytic leukemia (CMML) comprises a spectrum of disease variably considered as a myelodysplastic (MDS) and/or myeloproliferative (MPD) disorder. Now classified by the WHO within a separate nosological group from MDS or MPD, the reality is that there is a dynamic of evolution through increasing monocyte counts in one-third of patients. The principal clinical difference between CMML and other MPD is the presence of ineffective hematopoiesis, manifesting as more frequent anemia and thrombocytopenia in CMML. A fundamental biological characteristic shared with MPD is progenitor hypersensitivity to growth factors, but the pathways mediating this likely differ, as does the lineage specificity. Activation of the STAT pathway in MPD contrasts with frequent RAS pathway activation in CMML. Therapy of CMML is unsatisfactory, with the median age dictating that supportive care and control of myeloproliferation remains the mainstay for the majority. Intensive chemotherapy alone is of little benefit, and stem cell transplantation is the only curative modality in the small number of eligible patients, although outcome remains suboptimal. A deeper understanding of the biological basis of CMML may lead to targeted therapy analogous to the evolving management of MPD best exemplified for chronic myeloid leukemia.