Hirohei Yamamura

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.

B cell responses to oxidative stress

B-lymphocytes are exposed to a reduction/oxidation environment during activation or inflammatory process, and the antioxidant systems are functional to protect themselves against harmful reactive oxygen species (ROS). The crucial roles of thioredoxin-2 (Trx-2) and a DNA repair enzyme APE/Ref-1 in mitochondria are reported in B-lymphocytes. Furthermore, ROS stimulate different signaling pathways in many cellular responses. Their effects often cause some diseases or are utilized for the treatment of other diseases. For example, the cells derived from Fanconi anemia (FA) patients are intolerant of oxidative stress and the therapeutic effect of anti-CD20 monoclonal antibody rituximab on B cell lymphoproliferative disorders is due to the generation of ROS. To clarify the oxidative stress-induced signaling pathways, we stimulated a B cell line with various concentrations of H(2)O(2). As a result, a protein tyrosine kinase, Syk was involved in the induction of G2/M arrest and protection of cells from apoptosis. Syk might inhibit the activation of caspase-9 through Akt thereby protecting cells from oxidative stress-induced apoptosis. On the other hand, Syk-dependent PLC-gamma2 activation was required for acceleration towards apoptosis following oxidative stress. These findings suggest that oxidative stress-induced Syk activation triggers the activation of different pathways, such as pro-apoptotic or survival pathways, and that the balance of these pathways is a key factor in determining the fate of the cells exposed to oxidative stress. In contrast, the stimulation with the millimolar concentrations of H(2)O(2) rapidly led to necrosis in which tyrosine phosphorylation of FAK was involved at the downstream of Lyn and Syk.

Negative regulation of Lyn protein-tyrosine kinase by c-Cbl ubiquitin-protein ligase in FcεRI-mediated mast cell activation

Background: Recent studies have demonstrated that c‐Cbl functions as a ubiquitin‐protein ligase toward immune receptors and non‐receptor protein‐tyrosine kinase Syk by facilitating their ubiquitination and subsequent targeting to proteasomes. However, it was not clear whether Src family kinase Lyn is regulated by the Cbl family of ubiquitin‐protein ligases.

Lysosome is a primary organelle in B cell receptor-mediated apoptosis: An indispensable role of Syk in lysosomal function

To investigate the mechanism of B cell receptor (BCR)‐mediated apoptosis, we utilized immature B cell lines, DT40 and WEHI‐231. In both cell lines, BCR‐crosslinking caused the increase in lysosomal pH with early apoptotic changes characterized by chromatin condensation and phosphatidylserine exposure. This increase was detected in c‐Abl‐deficient DT40 cells but not in Syk‐deficient cells, which corresponded to the fact that the former cells but not the latter revealed BCR‐induced apoptosis. In contrast, BCR‐crosslinking caused no apparent change in mitochondrial transmembrane potential. Therefore, the lysosomal change might be a primary event in BCR‐induced apoptosis in DT40 cells. The increased activity of cathepsin B and apoptosis‐preventing effect of a cathepsin inhibitor suggested a significant role of lysosomal enzymes in this apoptosis. By microscopic studies, lysosomes of wild‐type DT40 cells fused to BCR‐carrying endosomes became enlarged and accumulated one another. In contrast, these changes of lysosomal dynamics did not occur in Syk‐deficient cells but transfer of wild‐type Syk restored the lysosomal changes and apoptosis. These results demonstrated that the lysosomal change accompanied with the activation of lysosomal enzymes is a primary step in BCR‐crosslinking‐mediated apoptosis and Syk is responsible for this step through the fusion of BCR‐carrying endosomes to lysosomes.

Activation of Syk Protein Tyrosine Kinase in Response to Osmotic Stress Requires Interaction with p21-Activated Protein Kinase Pak2/γ-PAK

ABSTRACT The p21-activated serine/threonine protein kinase Pak2/γ-PAK and the nonreceptor type of protein tyrosine kinase Syk are known to be activated when the cells are exposed to osmotic stress. The purpose of the present study was to examine whether Pak2 and Syk functionally cooperate in cellular signaling. Cotransfection studies revealed that Pak2 associates with Syk in COS cells. The constitutively active form of Cdc42 increases the association of Pak2 with Syk. Pak2 coexpressed with an inactive form of Cdc42 or kinase-inactive Pak2 interacts to a lesser extent with Syk, suggesting that Pak2-Syk association is enhanced by Pak2 activation. Interaction with Pak2 enhances the intrinsic kinase activity of Syk. This is supported by in vitro studies showing that Pak2 phosphorylates and activates Syk. Treatment of cells with sorbitol to induce hyperosmolarity results in the translocation of Pak2 and Syk to the region surrounding the nucleus and in dramatic enhancement of their association. Furthermore, cotransfection of Pak2 and Syk leads to the activation of c-Jun N-terminal kinase (JNK) under hyperosmotic conditions. Pak2 short interfering RNA suppresses sorbitol-mediated activation of endogenous Syk and JNK, thus identifying a novel pathway for JNK activation by Cdc42. These results demonstrate that Pak2 and Syk positively cooperate to regulate cellular responses to stress.

A novel phenoxazine derivative suppresses surface IgM expression in DT40 B cell line

2‐amino‐4, 4α‐dihydro‐4α, 7‐dimethyl‐3H‐phenoxazine‐3‐one (Phx) has been demonstrated to be an actinomycin D‐like phenoxazine, and to display anti‐tumour activity. In this study, we report on the effect of Phx on B cell antigen receptor (BCR) and receptor‐mediated signalling in DT40 B cells. Treatment of B cells with Phx for 12 h inhibited BCR‐stimulated tyrosine phosphorylation of cellular proteins. B cells exposed to Phx exhibited down‐regulation of surface IgM which is part of BCR. In contracts with actinomycin D, Phx rapidly reduced the expression of IgM without decreasing the expression of other signalling molecules. Analysis with confocal microscopy demonstrated that Phx treatment reduced IgM expression both at the cell surface and inside the cell. Treatment of B cells with Phx resulted in the reduction of IgM secretion. Since MG‐132, a proteasomal inhibitor, restored IgM contents to the control levels, Phx has the specific effect of accelerating IgM degradation. These results suggest that Phx down‐regulates the expression of IgM and inhibits BCR‐mediated signalling and IgM secretion. Phx may be useful as an immunosuppressive agent for therapeutic purposes.

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.