Protein-tyrosine kinase, Syk, is required for CXCL12-induced polarization of B cells

B cell receptor signaling and associated pathways in the pathogenesis of chronic lymphocytic leukemia

B cell antigen receptor (BCR) signaling is a key driver of growth and survival in both normal and malignant B cells. Several lines of evidence support an important pathogenic role of the BCR in chronic lymphocytic leukemia (CLL). The significant improvement of CLL patients' survival with the use of various BCR pathway targeting inhibitors, supports a crucial involvement of BCR signaling in the pathogenesis of CLL. Although the treatment landscape of CLL has significantly evolved in recent years, no agent has clearly demonstrated efficacy in patients with treatment-refractory CLL in the long run. To identify new drug targets and mechanisms of drug action in neoplastic B cells, a detailed understanding of the molecular mechanisms of leukemic transformation as well as CLL cell survival is required. In the last decades, studies of genetically modified CLL mouse models in line with CLL patient studies provided a variety of exciting data about BCR and BCR-associated kinases in their role in CLL pathogenesis as well as disease progression. BCR surface expression was identified as a particularly important factor regulating CLL cell survival. Also, BCR-associated kinases were shown to provide a crosstalk of the CLL cells with their tumor microenvironment, which highlights the significance of the cells' milieu in the assessment of disease progression and treatment. In this review, we summarize the major findings of recent CLL mouse as well as patient studies in regard to the BCR signalosome and discuss its relevance in the clinics.

RhoA as a Key Regulator of Innate and Adaptive Immunity

RhoA is a ubiquitously expressed cytoplasmic protein that belongs to the family of small GTPases. RhoA acts as a molecular switch that is activated in response to binding of chemokines, cytokines, and growth factors, and via mDia and the ROCK signaling cascade regulates the activation of cytoskeletal proteins, and other factors. This review aims to summarize our current knowledge on the role of RhoA as a general key regulator of immune cell differentiation and function. The contribution of RhoA for the primary functions of innate immune cell types, namely neutrophils, macrophages, and conventional dendritic cells (DC) to (i) get activated by pathogen-derived and endogenous danger signals, (ii) migrate to sites of infection and inflammation, and (iii) internalize pathogens has been fairly established. In activated DC, which constitute the most potent antigen-presenting cells of the immune system, RhoA is also important for the presentation of pathogen-derived antigen and the formation of an immunological synapse between DC and antigen-specific T cells as a prerequisite to induce adaptive T cell responses. In T cells and B cells as the effector cells of the adaptive immune system Rho signaling is pivotal for activation and migration. More recently, mutations of Rho and Rho-modulating factors have been identified to predispose for autoimmune diseases and as causative for hematopoietic malignancies.

Spleen Tyrosine Kinase Modulates Fibrous Airway Obliteration and Associated Lymphoid Neogenesis After Transplantation

Chronic lung allograft dysfunction, the major cause of death following lung transplantation, usually manifests as irreversible airflow obstruction associated with obliterative bronchiolitis (OB), a lesion characterized by chronic inflammation, lymphoid neogenesis, fibroproliferation and small airway obliteration. Spleen tyrosine kinase (Syk), a tyrosine kinase that regulates B cell function and innate immunity, has been implicated in the pathogenesis of chronic inflammation and tissue repair. This study evaluated the role of Syk in development of OB, using an intrapulmonary tracheal transplant model of OB with the conditional Syk-knockout Syk(flox/flox) //rosa26-CreER(T2) mice and a Syk-selective inhibitor, GSK2230413. BALB/c trachea allografts were transplanted into Syk-knockout (Syk(del/del) ) mice or wild-type C57BL/6 recipients treated with GSK2230413. At day 28, histological analysis revealed that in the Syk(del/del) and GSK2230413-treated C57BL/6 recipients, the graft lumen remained open compared with allografts transplanted into Syk-expressing (Syk(flox/flox) ) and placebo control-treated C57BL/6 recipients. Immunofluorescence showed lymphoid neogenesis with distinct B and T cell zones in control mice. In contrast, lymphoid neogenesis was absent and few B or T cells were found in Syk(del/del) and GSK2230413-treated mice. These observations suggest that inhibition of Syk may be a potential therapeutic strategy for the management of OB following lung transplantation.

Phosphorylation of spleen tyrosine kinase at tyrosine 348 (pSyk³⁴⁸) may be a marker of advanced phase of Chronic Myeloid Leukemia (CML)

We investigated Syk as a potential marker of CML progression. We observed a significant over-expression of Syk mRNA and constitutive phosphorylation of Syk Y348 in blast cells from six AP or BP-CML, but not in 15 CML in chronic phase. We could follow in vivo the recurrence of pSyk(348) throughout blast cell escape, despite observing storage of dasatinib in blast cells. A combination of dasatinib and R406 did not improve therapeutic efficacy in vitro. Our results strongly suggest that Syk activation could be a relevant biomarker of disease progression and dasatinib resistance but is probably not a molecular target.

Pim kinases in hematological malignancies: where are we now and where are we going?

The proviral insertion in murine (PIM) lymphoma proteins are a serine/threonine kinase family composed of three isoformes: Pim-1, Pim-2 and Pim-3. They play a critical role in the control of cell proliferation, survival, homing and migration. Recently, overexpression of Pim kinases has been reported in human tumors, mainly in hematologic malignancies. In vitro and in vivo studies have confirmed their oncogenic potential. Indeed, PIM kinases have shown to be involved in tumorgenesis, to enhance tumor growth and to induce chemo-resistance, which is why they have become an attractive therapeutic target for cancer therapy. Novel molecules inhibiting Pim kinases have been evaluated in preclinical studies, demonstrating to be effective and with a favorable toxicity profile. Given the promising results, some of these compounds are currently under investigation in clinical trials. Herein, we provide an overview of the biological activity of PIM-kinases, their role in hematologic malignancies and future therapeutic opportunities.

Blockade of Syk ameliorates the development of murine sclerodermatous chronic graft-versus-host disease

BACKGROUND

Murine sclerodermatous chronic graft-versus-host disease (Scl-cGVHD) is a model for human Scl-cGVHD and systemic sclerosis (SSc). Syk is expressed in most of hematopoietic cells, fibroblasts, and endothelial cells. Syk is a protein tyrosine kinase that has an important role in transmitting signals from a variety of cell surface receptors.

OBJECTIVE

This study aims to investigate the effect of R788 (fostamatinib sodium), an oral prodrug that is rapidly converted to a potent inhibitor of Syk, R406, on Scl-cGVHD.

METHODS

R788 was orally administered twice a day to allogeneic recipients from day 14 to day 42 after bone marrow transplantation (BMT). In vitro, proliferation of GVHD-derived CD4(+) T cells and CD11b(+) cells was analyzed by R406.

RESULTS

Allogeneic BMT increased Syk phosphorylation in T, B, and CD11b(+) cells. The administration of R788 attenuated severity and fibrosis of Scl-cGVHD. The elevated expressions of CXCR4 on T cells, B cells, and CD11b(+) cells were significantly down-regulated by R788 treatment. R788 reduced memory CD4(+) T cells (CD44(hi)CD62L(-)CD4(+)). R406 inhibited proliferation of GVHD CD4(+) T cells and CD11b(+) cells in vitro. In addition, R788 treatment, inhibited proliferation of CD11b(+) cells in Scl-cGVHD mice. R788 treatment also reduced skin mRNA expressions of MCP-1, MIP-1α, IFN-γ, IL-13, IL-17A, and TGF-β1, but not influenced RANTES, CXCL12, and TFN-α.

CONCLUSION

Blockade of Syk suppressed migration factor of immune cells and antigen-specific memory CD4(+) T cells and proliferation and activation of GVHD CD4(+) T cells and CD11b(+) cells. The current studies suggested that Syk inhibitor is a potential candidate for use in treating patients with Scl-cGVHD and SSc.

Development of novel CXCR4-based therapeutics

INTRODUCTION

During embryogenesis, CXCR4, a chemokine receptor, and its ligand, stromal cell-derived factor-1 (SDF-1/CXCL12), are critically involved in the development of the hematopoietic, nerve and endothelial tissues by regulating tissue progenitor cell migration, homing and survival. In adult life, the CXCR4 axis serves as the key factor for stem and immune cell trafficking. More importantly, CXCR4-CXCL12 axis plays a critical role in HIV, stem cell mobilization, autoimmune diseases, cancer and tissue regeneration. Targeting the CXCR4-CXCL12 axis, therefore, is an attractive therapeutic approach in various diseases.

AREAS COVERED

In this review, we update current knowledge about CXCR4-CXCL12 biology, therapeutic approaches and therapeutic agents. The data presented was collected from http://www.ncbi.nlm.nih.gov/pubmed , http://clinicaltrials.gov/ , http://bloodjournal.hematologylibrary.org/ .

EXPERT OPINION

Development of CXCR4 antagonists with increased affinity, extended pharmacokinetics and/or pharmacodynamics and with the capacity to differentially target CXCR4 may lead to a development of novel therapeutics for HIV, cancer, tissue regeneration and stem cell collection.

SYK regulates B-cell migration by phosphorylation of the F-actin interacting protein SWAP-70

B-cell migration into and within lymphoid tissues is not only central to the humoral immune response but also for the development of malignancies and autoimmunity. We previously demonstrated that SWAP-70, an F-actin-binding, Rho GTPase-interacting protein strongly expressed in activated B cells, is necessary for normal B-cell migration in vivo. SWAP-70 regulates integrin-mediated adhesion and cell attachment. Here we show that upon B-cell activation, SWAP-70 is extensively posttranslationally modified and becomes tyrosine phosphorylated by SYK at position 517. This phosphorylation inhibits binding of SWAP-70 to F-actin. Phospho-site mutants of SWAP-70 disrupt B-cell polarization in a dominant-negative fashion in vitro and impair migration in vivo. After CXCL12 stimulation of B cells SYK becomes activated and SWAP-70 is phosphorylated in a SYK-dependent manner. Use of the highly specific SYK inhibitor BAY61-3606 showed SYK activity is necessary for normal chemotaxis and B-cell polarization in vitro and for entry of B cells into lymph nodes in vivo. These findings demonstrate a novel requirement for SYK in migration and polarization of naive recirculating B cells and show that SWAP-70 is an important target of SYK in this pathway.

Spleen tyrosine kinase inhibition prevents chemokine- and integrin-mediated stromal protective effects in chronic lymphocytic leukemia

The microenvironment provides essential growth and survival signals to chronic lymphocytic leukemia (CLL) cells and contributes to their resistance to cytotoxic agents. Pharmacologic inhibition of spleen tyrosine kinase (SYK), a key mediator of B-cell receptor (BCR) signaling, induces apoptosis in primary CLL cells and prevents stroma contact-mediated cell survival. This report demonstrates a role of SYK in molecularly defined pathways that mediate the CLL-microenvironmental crosstalk independent from the BCR. Chemokine and integrin stimulation induced SYK phosphorylation, SYK-dependent Akt phosphorylation, and F-actin formation in primary CLL cells. Inhibition of SYK by 2 pharmacologic inhibitors and siRNA-knockdown abrogated downstream SYK signaling and morphologic changes induced by these stimuli. CLL cell migration toward CXCL12, the major homing attractor, and CLL cell adhesion to VCAM-1, a major integrin ligand expressed on stromal cells, were markedly reduced by SYK inhibition. In combination with fludarabine, the SYK inhibitor R406 abrogated stroma-mediated drug resistance by preventing up-regulation of the antiapoptotic factor Mcl-1 in CLL cells. SYK blockade in CLL is a promising therapeutic principle not only for its inhibition of the BCR signaling pathway, but also by inhibiting protective stroma signals in a manner entirely independent of BCR signaling.

PRL-2 increases Epo and IL-3 responses in hematopoietic cells

Dual specificity protein tyrosine phosphatase PRL-2 is overexpressed in pediatric acute myeloid leukemia (AML) and is located at human chromosome 1p35, a region often rearranged or amplified in malignant lymphoma and B-cell chronic lymphocytic leukemia (B-CLL). Little is known of the significance of PRL-2 expression in hematopoietic malignancies. Herein we demonstrated that ectopic expression of PRL-2 in murine pre-B-cell line Baf3ER and mouse bone marrow cells induced key features associated with malignant progression and metastasis. PRL-2-transfected Baf3ER cells had augmented growth responses to hematopoietic growth factors Epo or IL-3 with shortened cell cycle, reduced requirement (5x) for Epo in cell survival, increased cell migration (3x), reduced cell adhesion (5x), and conversion to an immature cell morphology in association with increased expression (3x) of stem cell marker Bmi-1. When transduced into mouse bone marrow cells, PRL-2 increased Epo-induced colony formation (4x) and gave rise to larger colonies. These observations provide evidences implicating PRL-2 as a pathogenic molecule in hematopoietic malignancies and suggest its potential as a novel therapeutic target.

Protein tyrosine kinase, syk: a key player in phagocytic cells

Spleen tyrosine kinase (Syk) is a non-receptor protein tyrosine kinase expressed in a wide range of haematopoietic cells. At the initial stage of investigation, main exploring was toward its functions in platelets and in classical immunoreceptor signalling. However, Syk has now been recognized as a key player in both innate and adaptive immunity. Especially, in phagocytosis, Syk plays essential roles in signalling evoked by various types of receptors such as FcgammaR, CR3, Dectin-1 and apoptotic cell-recognizing receptor. A variety of upstream immunoreceptor tyrosine-based activation motif-like molecules have been found and are still in the course of new studies. On the contrary, downstream effectors to explain diverse function of Syk are still under exploration. As its novel function, we propose the role of Syk in the regulation of alpha-tubulin acetylation. Further investigation on the effectors of Syk would give us more information in relation to therapeutic molecular targets.

Convergence of immunoreceptor and integrin signaling

A common signaling pathway is known to operate downstream of immunoreceptors, such as the T-cell, B-cell, or Fc receptors, following engagement by their respective ligands. This pathway involves Src family kinase-mediated tyrosine phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) that recruit and activate spleen tyrosine kinase (Syk) or Zap70 (zeta-associated protein of 70 kDa) kinases, which in turn activate a variety of downstream signals. Evidence has been building from a variety of sources, particularly mouse models, that molecules involved in the immunoreceptor signaling pathway are also required for signals initiated by integrins. Integrins are the major cell surface receptors that mediate adhesion of leukocytes to a variety of extracellular matrix proteins and counter-receptors expressed on endothelial cells. Integrin ligation is a critical step in the activation of leukocyte effector functions (such as neutrophil degranulation or lymphocyte proliferation). Integrin signaling through pathways common to those utilized by immunoreceptors provides a mechanism by which leukocyte adhesion can regulate activation of cellular responses. In animal models, integrin-mediated signal transduction plays a critical role in inflammatory disease. In this review, we discuss the convergence of immunoreceptor and integrin signaling, focusing on how these pathways modulate leukocyte activation.

Syk-dependent actin dynamics regulate endocytic trafficking and processing of antigens internalized through the B-cell receptor

Antigen binding to the B-cell receptor (BCR) induces multiple signaling cascades that ultimately lead to B lymphocyte activation. In addition, the BCR regulates the key trafficking events that allow the antigen to reach endocytic compartments devoted to antigen processing, i.e., that are enriched for major histocompatibility factor class II (MHC II) and accessory molecules such as H2-DM. Here, we analyze the role in antigen processing and presentation of the tyrosine kinase Syk, which is activated upon BCR engagement. We show that convergence of MHC II- and H2-DM-containing compartments with the vesicles that transport BCR-uptaken antigens is impaired in cells lacking Syk activity. This defect in endocytic trafficking compromises the ability of Syk-deficient cells to form MHC II-peptide complexes from BCR-internalized antigens. Altered endocytic trafficking is associated to a failure of Syk-deficient cells to properly reorganize their actin cytoskeleton in response to BCR engagement. We propose that, by modulating the actin dynamics induced upon BCR stimulation, Syk regulates the positioning and transport of the vesicles that carry the molecules required for antigen processing and presentation.

The expanding role for ITAM-based signaling pathways in immune cells

The immunoreceptor tyrosine-based activation motif (ITAM) is the primary signaling domain used by classical immunoreceptors, such as the antigen receptors on B and T lymphocytes and the Fc receptors (FcRs) on myeloid cells. The ITAM is contained in the intracellular region of subunits associated with these receptors, often in pairs, or is part of the cytoplasmic domain of the receptors themselves. Data from many investigators have demonstrated that ITAMs are both necessary and sufficient for initiation of signaling downstream of all immunoreceptors. More recent reports indicate that ITAM signaling is used by additional receptors beyond the classical immunoreceptors: Cell adhesion molecules (integrins and PSGL-1), chemokine receptors (CXCR4), plexins, and lectin receptors all mediate immune cell function through ITAM-like signaling pathways. This convergence of intracellular signaling pathways in leukocytes illuminates the importance of tyrosine-based activation motifs in the immune system and suggests that inhibitors of ITAM signaling may have broader effects than originally envisioned.

Effects of Peripheral Cannabinoid Receptor Ligands on Motility and Polarization in Neutrophil-like HL60 Cells and Human Neutrophils

The possible role of the peripheral cannabinoid receptor (CB2) in neutrophil migration was investigated by using human promyelocytic HL60 cells differentiated into neutrophil-like cells and human neutrophils isolated from whole blood. Cell surface expression of CB2 on HL60 cells, on neutrophil-like HL60 cells, and on human neutrophils was confirmed by flow cytometry. Upon stimulation with either of the CB2 ligands JWH015 and 2-arachidonoylglycerol (2-AG), neutrophil-like HL60 cells rapidly extended and retracted one or more pseudopods containing F-actin in different directions instead of developing front/rear polarity typically exhibited by migrating leukocytes. Activity of the Rho-GTPase RhoA decreased in response to CB2 stimulation, whereas Rac1, Rac2, and Cdc42 activity increased. Moreover, treatment of cells with RhoA-dependent protein kinase (p160-ROCK) inhibitor Y27632 yielded cytoskeletal organization similar to that of CB2-stimulated cells. In human neutrophils, neither JWH015 nor 2-AG induced motility or morphologic alterations. However, pretreatment of neutrophils with these ligands disrupted N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP)-induced front/rear polarization and migration and also substantially suppressed fMLP-induced RhoA activity. These results suggest that CB2 might play a role in regulating excessive inflammatory response by controlling RhoA activation, thereby suppressing neutrophil migration.

Protein-tyrosine kinase Syk is required for pathogen engulfment in complement-mediated phagocytosis

The protein tyrosine kinase Syk plays a central role in Fcgamma receptor-mediated phagocytosis in the adaptive immune system. We show here that Syk also plays an essential role in complement-mediated phagocytosis in innate immunity. Macrophage-like differentiated HL60 cells and C3bi-opsonized zymosan comprised the pathogen-phagocyte system. C3bi-opsonized zymosan particles promptly attached to the cells and were subsequently engulfed via complement receptor 3. During this process, Syk became tyrosine phosphorylated and accumulated around the nascent phagosomes. The transfer of Syk-siRNA or dominant-negative Syk (DN-Syk) into HL60 cells resulted in impaired phagocytosis. Quenching assays using fluorescent zymosan revealed that most of the attached zymosan particles were located inside parental HL60 cells, whereas few were ingested by the mutant cells. These data indicated that Syk is required for the engulfment of C3bi-opsonized zymosan. During C3bi-zymosan-induced phagocytosis, actin accumulation occurred around phagosomes and was followed by depolymerization, and further RhoA was activated together with tyrosine phosphorylation of Vav. These responses including the actin remodeling were suppressed in Syk-siRNA- or DN-Syk-expressing cells. Our results demonstrated that Syk plays an indispensable role in complement-mediated phagocytosis by regulating both actin dynamics and the RhoA activation pathway and that these functions of Syk lead to phagosome formation and pathogen engulfment.

The Syk tyrosine kinase: a new negative regulator in tumor growth and progression

The spleen tyrosine kinase Syk was long thought to be a hematopoietic cell-specific signaling molecule. Recent evidence demonstrated that it is also expressed by many non-hematopoietic cell types and that it plays a negative role in cancer. A significant drop in its expression was first observed during breast cancer progression, but an anomalous Syk expression has now also been evidenced in many other tumor types. Mechanistic studies using Syk re-expression demonstrated its suppressive function in tumorigenesis and metastasis formation, which is surprising for a tyrosine kinase. Loss of Syk expression is regulated, albeit not exclusively, by its promoter hypermethylation. The molecular mechanism of its tumor-suppressive function remains largely unknown; the identification of its activators and effectors in non-hematopoietic cells will be a challenge for the years to come. An increasing number of clinical studies reveal a correlation between reduced Syk expression and an increased risk for metastasis formation, and assign Syk as a potential new prognostic marker in different tumor types.

5-Hydroxytryptamine as a potent migration enhancer of human aortic endothelial cells

The purpose of the present study was to investigate whether 5-hydroxytryptamine (5-HT, serotonin) affects migration of vascular endothelial cells. 5-HT significantly enhanced migration of human aortic endothelial cells (HAECs), and this enhancement was completely inhibited by GR 55562, a 5-HT1 receptor antagonist, and fluoxetine, a 5-HT transporter inhibitor, but was not affected by ketanserin, a 5-HT2 receptor antagonist. 5-HT stimulation increased RhoA and ERK activity of HAECs, and inhibitors of RhoA (Y-27632 and H-1152) and inhibitors of MEK (U0126 and PD98059) abolished the 5-HT-induced increase in migration velocity. Inhibition of Rho kinase by Y-27632 blocked stress fiber formation and rear release of HAECs. Thus, 5-HT has a potent enhancing action on migration of HAECs through activating the RhoA and ERK pathways following 5-HT1 receptor stimulation.

5-Hydroxytryptamine augments migration of human aortic smooth muscle cells through activation of RhoA and ERK

The purpose of this study was to elucidate the mechanism of 5-hydroxytryptamine (5-HT, serotonin) action on migration of vascular smooth muscle cells. Migration of cultured human aortic smooth muscle cells (HASMCs), evaluated using time-lapse microscopy, was significantly enhanced by 5-HT at concentrations of 1-100 nM. The enhancing effect of 5-HT on cell migration was markedly inhibited in the presence of ketanserin, a 5-HT2 receptor antagonist, but not by GR 55562, a 5-HT1 receptor antagonist. Activities of RhoA and ERK were increased by 5-HT, and the increase in cell migration by 5-HT was abolished in the presence of U0126, a MEK1/2 inhibitor, or Y-27632, a Rho-kinase inhibitor. Activation of ERK was strongly inhibited by Y-27632. 5-HT-induced formation of stress fiber and detachment of uropod (trailing edge) were abolished by Y-27632. Thus, 5-HT has a potent enhancing action on migration of HASMCs due to an increase in stress fiber formation by 5-HT2 receptor stimulation followed by activation of the Rho-kinase and ERK pathways.