Outside-In Signaling of Soluble and Solid-Phase Fibrinogen Through Integrin ΙΙbβ3 Is Different and Cooperative With Each Other in a Megakaryoblastic Leukemia Cell Line, CMK

Syk facilitates phagosome-lysosome fusion by regulating actin-remodeling in complement-mediated phagocytosis

Effective phagocytosis is crucial for host defense against pathogens. Macrophages entrap pathogens into a phagosome and subsequently acidic lysosomes fuse to the phagosome. Previous studies showed the pivotal role of actin-remodeling mediated by phosphoinositide-related signaling in phagosome formation, but the mechanisms of phagosome-lysosome fusion remain unexplored. Here we show that in complement-mediated phagocytosis, phagosome-lysosome fusion requires the disappearance of F-actin structure surrounding the phagosome and a tyrosine kinase Syk plays a key role in this process. Using macrophage-like differentiated HL60 and Syk-knockout (Syk-KO) HL60 cells, we found that Syk-KO cells showed insufficient phagosome acidification caused by impaired fusion with lysosomes and permitted the survival of Candida albicans in complement-mediated phagocytosis. Phagosome tracking analysis showed that during phagosome internalization process, F-actin surrounding phagosomes disappeared in both parental and Syk-KO cells but this structure was reconstructed immediately only in Syk-KO cells. In addition, F-actin-stabilizing agent induced a similar impairment of phagosome-lysosome fusion. Collectively, Syk-derived signaling facilitates phagosome-lysosome fusion by regulating actin-remodeling.

The Possible Importance of β3 Integrins for Leukemogenesis and Chemoresistance in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is an aggressive bone marrow malignancy where the immature leukemia cells communicate with neighboring cells through constitutive cytokine release and through their cell surface adhesion molecules. The primary AML cells express various integrins. These heterodimeric molecules containing an α and a β chain are cell surface molecules that bind extracellular matrix molecules, cell surface molecules and soluble mediators. The β3 integrin (ITGB3) chain can form heterodimers only with the two α chains αIIb and αV. These integrins are among the most promiscuous and bind to a large number of ligands, including extracellular matrix molecules, cell surface molecules and soluble mediators. Recent studies suggest that the two β3 integrins are important for leukemogenesis and chemosensitivity in human AML. Firstly, αIIb and β3 are both important for adhesion of AML cells to vitronectin and fibronectin. Secondly, β3 is important for the development of murine AML and also for the homing and maintenance of the proliferation for xenografted primary human AML cells, and for maintaining a stem cell transcriptional program. These last effects seem to be mediated through Syk kinase. The β3 expression seems to be regulated by HomeboxA9 (HoxA9) and HoxA10, and the increased β3 expression then activates spleen tyrosine kinase (Syk) and thereby contributes to cytokine hypersensitivity and activation of β2 integrins. Finally, high integrin αV/β3 expression is associated with an adverse prognosis in AML and decreased sensitivity to the kinase inhibitor sorafenib; this integrin can also be essential for osteopontin-induced sorafenib resistance in AML. In the present article, we review the experimental and clinical evidence for a role of β3 integrins for leukemogenesis and chemosensitivity in AML.

Integrin αIIbβ3 outside-in signaling

Integrin α_IIbβ₃ is a highly abundant heterodimeric platelet receptor that can transmit information bidirectionally across the plasma membrane, and plays a critical role in hemostasis and thrombosis. Upon platelet activation, inside-out signaling pathways increase the affinity of α_IIbβ₃ for fibrinogen and other ligands. Ligand binding and integrin clustering subsequently stimulate outside-in signaling, which initiates and amplifies a range of cellular events driving essential platelet processes such as spreading, thrombus consolidation, and clot retraction. Integrin α_IIbβ₃ has served as an excellent model for the study of integrin biology, and it has become clear that integrin outside-in signaling is highly complex and involves a vast array of enzymes, signaling adaptors, and cytoskeletal components. In this review, we provide a concise but comprehensive overview of α_IIbβ₃ outside-in signaling, focusing on the key players involved, and how they cooperate to orchestrate this critical aspect of platelet biology. We also discuss gaps in the current understanding of α_IIbβ₃ outside-in signaling and highlight avenues for future investigation.

Integrin αvβ3 enhances the suppressive effect of interferon-γ on hematopoietic stem cells

Hematopoietic homeostasis depends on the maintenance of hematopoietic stem cells (HSCs), which are regulated within a specialized bone marrow (BM) niche. When HSC sense external stimuli, their adhesion status may be critical for determining HSC cell fate. The cell surface molecule, integrin αvβ3, is activated through HSC adhesion to extracellular matrix and niche cells. Integrin β3 signaling maintains HSCs within the niche. Here, we showed the synergistic negative regulation of the pro-inflammatory cytokine interferon-γ (IFNγ) and β3 integrin signaling in murine HSC function by a novel definitive phenotyping of HSCs. Integrin αvβ3 suppressed HSC function in the presence of IFNγ and impaired integrin β3 signaling mitigated IFNγ-dependent negative action on HSCs. During IFNγ stimulation, integrin β3 signaling enhanced STAT1-mediated gene expression via serine phosphorylation. These findings show that integrin β3 signaling intensifies the suppressive effect of IFNγ on HSCs, which indicates that cell adhesion via integrin αvβ3 within the BM niche acts as a context-dependent signal modulator to regulate the HSC function under both steady-state and inflammatory conditions.

Central role of PI3K-SYK interaction in fibrinogen-induced lamellipodia and filopodia formation in platelets

The WAVE complex‐1, a complex of WAVE, Abi1, NAP1, PIR121, HSPC300, RacGTP and Arp2/3 proteins, and WASP complex‐1, a complex of WASP, Cdc42, PIP2, and Arp2/3 proteins, are involved in lamellipodia and filopodia formation, respectively. It is known that the two complexes have opposite dynamics. Furthermore, Rac has two guanine nucleotide exchange factors, Vav and Sos, whose role in activating Rac is not well understood. In this work, by the construction of signaling network, analysis, and mathematical modeling, I show that Sos generates a pulse of WAVE complex‐1, decreasing the response time of WAVE complex‐1 formation upon the stimulation of platelets by fibrinogen. Furthermore, I also show that the dynamics of WAVE and WASP complexes depends on PI3K–SYK interaction. In the absence of this interaction, the WAVE complex‐1 does not form and the WASP complex‐1 remains at the initial, sustained level. Thus, I show the significance of the two protein/protein complexes: Sos and PI3K–SYK interaction, in fibrinogen‐induced lamellipodia and filopodia formation in platelets.

Separation of in-vitro-derived megakaryocytes and platelets using spinning-membrane filtration

In-vitro-derived platelets (PLTs) could potentially overcome problems associated with donated PLTs, including contamination and alloimmunization. Although several groups have produced functional PLTs from stem cells in vitro, the challenge of developing this technology to yield transfusable PLT units has yet to be addressed. The asynchronous nature of in vitro PLT generation makes a single harvest point infeasible for collecting PLTs as soon as they are formed. The current standard of performing manual centrifugations to separate PLTs from nucleated cells at multiple points during culture is labor-intensive, imprecise, and difficult to standardize in accordance with current Good Manufacturing Practices (cGMP). In an effort to develop a more effective method, we adapted a commercially-available, spinning-membrane filtration device to separate in-vitro-derived PLTs from nucleated cells and recover immature megakaryocytes (MKs), the precursor cells to PLTs, for continued culture. Processing a mixture of in-vitro-derived MKs and PLTs on the adapted device yielded a pure PLT population and did not induce PLT pre-activation. MKs recovered from the separation process were unaffected with respect to viability and ploidy, and were able to generate PLTs after reseeding in culture. Being able to efficiently harvest in-vitro-derived PLTs brings this technology one step closer to clinical relevance.

Dynamic combinatorial interactions of RUNX1 and cooperating partners regulates megakaryocytic differentiation in cell line models

Specific interactions of transcription factors (TFs) with their targets are crucial for specifying gene expression programs during cell differentiation. How specificity is maintained despite limited selectivity of individual TF-DNA interactions is not fully understood. RUNX1 TF is among the most frequently mutated genes in human leukemia and an important regulator of megakaryopoiesis. We used megakaryocytic cell lines to characterize the network of RUNX1 targets and cooperating TFs in differentiating megakaryocytes and demonstrated how dynamic partnerships between RUNX1 and cooperating TFs facilitated regulatory plasticity and specificity during this process. After differentiation onset, RUNX1 directly activated a large number of genes through interaction with preexisting and de novo binding sites. Recruitment of RUNX1 to de novo occupied sites occurred at H3K4me1-marked preprogrammed enhancers. A significant number of these de novo bound sites lacked RUNX motif but were occupied by AP-1 TFs. Reciprocally, AP-1 TFs were up-regulated by RUNX1 after 12-O-tetradecanoylphorbol-13-acetate induction and recruited to RUNX1-occupied sites lacking AP-1 motifs. At other differentiation stages, additional combinatorial interactions occurred between RUNX1 and its coregulators, GATA1 and ETS. The findings suggest that in differentiating megakaryocytic cell lines, RUNX1 cooperates with GATA1, AP-1, and ETS to orchestrate cell-specific transcription programs through dynamic TF partnerships.

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.

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.

Targeting Syk as a treatment for allergic and autoimmune disorders

Recent advances in our understanding of allergic and autoimmune disorders have begun to translate into novel, effective and safe medicines for these common maladies. Examples include an anti-IgE monoclonal antibody recently approved for severe asthmatics and the TNF-alpha antagonists that have demonstrated their ability to suppress rheumatoid arthritis, Crohn's disease and other chronic inflammatory processes. However, protein therapies are difficult and expensive to develop, manufacture and administer. Clearly, there is also a need for small-molecule inhibitors of novel targets that have safe and effective characteristics. Syk is an intracellular protein tyrosine kinase that was discovered 15 years ago as a key mediator of immunoreceptor signalling in a host of inflammatory cells including B cells, mast cells, macrophages and neutrophils. These immunoreceptors, including Fc receptors and the B-cell receptor, are important for both allergic diseases and antibody-mediated autoimmune diseases and thus pharmacologically interfering with Syk could conceivably treat these disorders. In addition, as Syk is positioned upstream in the cell signalling pathway, therapies targeting Syk may be more advantageous relative to drugs that inhibit a single downstream event. Syk inhibition during an allergic or asthmatic response will block three mast cell functions: the release of preformed mediators such as histamine, the production of lipid mediators such as leukotrienes and prostaglandins and the secretion of cytokines. In contrast, commonly used antihistamines or leukotriene receptor antagonists target only a single mediator of this complex cascade. Despite its expression in platelets and other non-haematopoietic cells, the role of Syk in regulating vascular homeostasis and other housekeeping functions is minimal or masked by redundant Syk-independent pathways. This suggests that targeting Syk would be an optimal approach to effectively treat a multitude of chronic inflammatory diseases without undue toxicity.

Activation of integrin alpha IIb beta 3 in the glycoprotein Ib-high population of a megakaryocytic cell line, CMK, by inside-out signaling

Affinity/avidity state of integrin alpha IIb beta 3 is regulated by intracellular inside-out signaling. Although several megakaryocytic cell lines have been established, soluble ligand binding to alpha IIb beta 3 expressed in these cells by cellular agonists has not been demonstrated. We have re-examined agonist-induced alpha IIb beta 3 activation on megakaryocytic cell lines with a marker of the late stage of megakaryocytic differentiation, glycoprotein Ib (GPIb). Activation of alpha IIb beta 3 was assessed by PAC1 and soluble fibrinogen binding to the cells. We found that alpha IIb beta 3 expressed in CMK cells with high GPIb expression was activated by a phorbor ester, phorbol myristate acetate (PMA). Although the population of the GPIbhigh cells was <0.5% of the total cells, incubation with a nucleoside analog, ribavirin, efficiently increased the PMA-reactive GPIbhigh cells. Not only PMA but also a calcium ionophore, A23187, induced alpha IIb beta 3 activation, and PMA and A23187 had an additive effect on alpha IIb beta 3 activation. Ligand binding to the activated alpha IIb beta 3 in the GPIbhigh CMK cells is totally abolished by an alpha IIb beta 3-specific antagonist, and inhibited by wortmannin, cytochalasin-D and prostaglandin E1, and the effects of these inhibitors on alpha IIb beta 3 activation in the GPIbhigh CMK cells were compatible with those in platelets. We have also demonstrated that the ribavirin-treated CMK cells express PKC-alpha, -beta, -delta and -theta, and suggested that PKC-alpha and/or -beta appear to be responsible for PMA-induced activation of alpha IIb beta 3 in CMK cells.

Sustained activation of MEK1-ERK1/2 pathway in membrane skeleton occurs dependently on cell adhesion in megakaryocytic differentiation

A human megakaryoblastic cell line, CMK, was treated with 12-o-tetradecanoylphorbol-13-acetate (TPA) for differentiation-induction. We examined TPA-induced activation of the MEK1-ERK1/2 pathway in the 100,000g Triton X-insoluble fraction of CMK cells as the membrane skeleton and researched the relation of the MEK1-ERK1/2 activation with integrin expression. We found that this activation was divided into two phases: the first activation occurred transiently in the membrane skeleton fraction of the suspended cell status and diminished after 1h; and the second sustained activation was maintained by cell adhesion. TPA-treated CMK cells revealed increased expression of integrins alphaIIb and beta3 only when the cell adhesion persisted, regardless of the difference of culture substratum. Sustained activation of the MEK1-ERK1/2 pathway is generated in the membrane skeleton by continuous cell adhesion and seems to be essential to TPA-induced megakaryocytic differentiation of CMK cells.

Agonist-induced aggregation of Chinese hamster ovary cells coexpressing the human receptors for fibrinogen (integrin alphaIIbbeta3) and the platelet-activating factor: dissociation between adhesion and aggregation

This work reports the establishment of a Chinese hamster ovary (CHO) cell line stably coexpressing the human alphaIIbbeta3 integrin and the platelet-activating factor receptor (PAFR). These cells aggregate in response to PAF in a Ca(++), alphaIIbbeta3, and soluble fibrinogen (Fg)-dependent manner that is prevented by PAF antagonists or alphaIIbbeta3 blockade. The aggregating response is accompanied by enhanced binding of fibrinogen and the activation-dependent IgM PAC1. This model has permitted us to identify, for the first time, intracellular signals distinctly associated with either alphaIIbbeta3-mediated adhesion or aggregation. Nonreceptor activation of protein kinase C (PKC) by phorbol ester produced cellular adhesion and spreading onto immobilized Fg, but it was not a sufficient signal to provoke cellular aggregation. Moreover, inhibition of PKC impeded the PAF stimulation of cellular adhesion, whereas the aggregation was not prevented. The PAF-induced cellular aggregation was distinctly associated with signaling events arising from the liganded Fg receptor and the agonist-induced stimulation of a calcium/calmodulin-dependent signaling pathway. Sustained tyrosine phosphorylation of both mitogen-activated protein kinase (MAPK) and an approximately 100-kd protein was associated with the PAF-induced aggregation, whereas phosphorylation of focal adhesion kinase (FAK) was preferably associated with cellular adherence and spreading onto immobilized Fg.

Syk expression and novel function in a wide variety of tissues

Syk protein-tyrosine kinase has been implicated in a variety of hematopoietic cell responses, in particular immunoreceptor signaling events that mediate diverse cellular responses including proliferation, differentiation, and phagocytosis. On the other hand, Syk exhibits a more widespread expression pattern in nonhematopoietic cells like fibroblasts, epithelial cells, breast tissue, hepatocytes, neuronal cells, and vascular endothelial cells and has been shown to be functionally important on these cell types. Thus, Syk appears to play a general physiological function in a wide variety of cells. In this article, we briefly review the current literature regarding the expression and novel function of Syk in various cells and tissues.

Adhesion via CD43 induces Syk activation and cell proliferation in TF-1 cells

The effect of adhesion via CD43 (leukosialin, sialophorin) on cell proliferation and phosphorylation signaling were examined in a growth factor-dependent hematopoietic progenitor cell line, TF-1. TF-1 cells promptly resulted in death after withdrawal of growth factors. However, the viable cell number increased when TF-1 cells were cultured on anti-CD43 monoclonal antibody-coated plates. In this case, sustained activation of protein tyrosine kinase Syk and extracellular signal-regulated kinase (Erk) 1/2 were detected. Overexpression of exogenous Syk on TF-1 cells by the adenovirus vector system induced enhancement of the cell proliferation accompanied with enhancement of the Erk activation by a dominant-positive effect. The signal transducer and activator of transcription (STAT) 5 seemed not to be associated with the CD43-mediated cell proliferation. These results indicated that adhesion via CD43 induces the proliferation of TF-1 cells in the absence of growth factors in part by Syk-dependent Erk 1/2 signaling.

Syk protein-tyrosine kinase is involved in neuron-like differentiation of embryonal carcinoma P19 cells

Syk has been implicated in activated immunoreceptors to downstream signaling events in hematopoietic cells. Here we report that Syk is expressed in neuron-like cells and involved in neuron-like differentiation of embryonal carcinoma P19 cells. Immunoblot, RT-PCR, and Northern analysis indicated that Syk is expressed in mouse brain, PC12 and P19 cells. In addition, Syk was found to be tyrosine phosphorylated during neuron-like differentiation of P19 cells. Furthermore, adenovirus-mediated overexpression of Syk induced supernumerary neurite formation and extracellular signal-regulated kinase (ERK) activation in P19 cells. These results suggest that Syk plays an important role in signaling steps leading to ERK activation in P19 cells.

Pyk2 and Syk participate in functional activation of granulocytic HL-60 cells in a different manner

The roles of the protein tyrosine kinases Pyk2 (also called RAFTK or CAK β) and Syk in the process of functional activation of human myeloid cells were examined. During granulocytic differentiation of HL-60 cells with dimethyl sulfoxide (DMSO), the amounts of Pyk2 and β2 integrin increased, whereas the amount of Syk was abundant before differentiation and did not change during differentiation. When the granulocytic cells were stimulated withN-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP), tyrosine phosphorylation of Pyk2 occurred promptly and subsequent association of Pyk2 with β2 integrin was detected. In contrast, Syk was not tyrosine phosphorylated by fMLP stimulation but constitutively associated with β2 integrin. Stimulation with fMLP also caused the alteration of β2 integrin to an activated form, a finding that was confirmed by the observation of fMLP-induced cell attachment on fibrinogen-coated dishes and inhibition of this attachment by pretreatment with anti-β2 integrin antibody. Cell attachment to fibrinogen caused the enhanced tyrosine phosphorylation of Pyk2 and the initial tyrosine phosphorylation of Syk, which was also inhibited by pretreatment with anti-β2 integrin antibody. In vitro kinase assays revealed that Pyk2 and Syk represented kinase activities to induce tyrosine phosphorylation of several molecules in the anti-β2 integrin immunoprecipitates of the attached cells. These results showed that Pyk2 is involved in the functional activation of granulocytic cells in 2 signaling pathways: an fMLP receptor–mediated “inside-out” signaling pathway that might cause β2 integrin activation and a subsequent β2 integrin–mediated “outside-in” signaling pathway. Syk was activated in relation to cell attachment to fibrinogen as a result of “outside-in” signaling, although it was already associated with β2 integrin before fMLP stimulation.

PAR1 activation initiates integrin engagement and outside-in signalling in megakaryoblastic CHRF-288 cells

To better understand the means by which cells such as human platelets regulate the binding of the integrin alphaIIbbeta3 to fibrinogen, we have examined agonist-initiated inside-out and outside-in signalling in CHRF-288 cells, a megakaryoblastic cell line that expresses alphaIIbbeta3 and the human thrombin receptor, PAR1. The results show several notable similarities and differences. (1) Activation of PAR1 caused CHRF-288 cells to adhere and spread on immobilized fibrinogen in an alphaIIbbeta3-dependent manner, but did not support the binding of soluble fibrinogen or PAC-1, an antibody specific for activated alphaIIbbeta3. (2) Direct activation of protein kinase C with PMA or disruption of the actin cytoskeleton with low concentrations of cytochalasin D also caused CHRF-288 cells to adhere to fibrinogen. (3) Despite the failure to bind soluble fibrinogen, activation of PAR1 in CHRF-288 cells caused phosphoinositide hydrolysis, arachidonate mobilization and the phosphorylation of p42MAPK, phospholipase A2 and the Rac exchange protein, Vav, all of which occur in platelets. PAR1 activation also caused an increase in cytosolic Ca2+, which, when prevented, blocked adhesion to fibrinogen. (4) Finally, as in platelets, adhesion of CHRF-288 cells to fibrinogen was followed by a burst of integrin-dependent ('outside-in') signalling, marked by FAK phosphorylation and a more prolonged phosphorylation of p42MAPK. However, in contrast to platelets, adhesion to fibrinogen had no effect on Vav phosphorylation. Collectively, these observations show that signalling initiated through PAR1 in CHRF-288 cells can support alphaIIbbeta3 binding to immobilized ligand, but not the full integrin activation needed to bind soluble ligand. This would suggest that there has been an increase in integrin avidity without an accompanying increase in affinity. Such increases in avidity are thought to be due to integrin clustering, which would also explain the results obtained with cytochalasin D. The failure of alphaIIbbeta3 to achieve the high affinity state in CHRF-288 cells was not due to the failure of PAR1 activation to initiate a number of signalling events that normally accompany platelet activation nor did it prevent at least some forms of outside-in signalling. However, at least one marker of outside-in signalling, the augmentation of Vav phosphorylation seen during platelet aggregation, did not occur in CHRF-288 cells.