Activation of the Syk tyrosine kinase is insufficient for downstream signal transduction in B lymphocytes

Unperturbed Immune Function despite Mutation of C-Terminal Tyrosines in Syk Previously Implicated in Signaling and Activity Regulation

The nonreceptor tyrosine kinase Syk, a central regulator of immune cell differentiation and activation, is a promising drug target for treatment of leukemia and allergic and inflammatory diseases. The clinical failure of Syk inhibitors underscores the importance of understanding the regulation of Syk function and activity. A series of previous studies emphasized the importance of three C-terminal tyrosines in Syk for kinase activity regulation, as docking sites for downstream effector molecules, and for Ca²⁺ mobilization. Here, we investigated the roles of these C-terminal tyrosines in the mouse. Surprisingly, expression of a triple tyrosine-to-phenylalanine human Syk mutant, SYK(Y3F), was not associated with discernible signaling defects either in reconstituted DT40 cells or in B or mast cells from mice expressing SYK(Y3F) instead of wild-type Syk. Remarkably, lymphocyte differentiation, calcium mobilization, and 2,4,6-trinitrophenyl (TNP)-specific immune responses were unperturbed in SYK(Y3F) mice. These results emphasize the capacity of immune cells to compensate for specific molecular defects, likely using redundant intermolecular interactions, and highlight the importance of in vivo analyses for understanding cellular signaling mechanisms.

Follicular lymphoma and the immune system: from pathogenesis to antibody therapy

Follicular lymphoma (FL) is a B-cell tumor arising in germinal centers and retaining features of its normal B-cell counterpart. Lymphomagenesis appears stepwise from the t(14;18) translocation, through FL-like cells, to FL in situ, then to overt FL. Surface Ig is mandatory and carries a striking V-region modification because of introduction of glycan addition sites during somatic mutation. These are positively selected and acquire unusual high mannoses, which interact with lectins. The Ig-associated mannoses appear essential for FL, providing a disease- specific target for antibody attack. Antibody therapy is currently focused on anti-CD20 (rituximab), which appears to rely predominantly on the Fcγ module recruiting suitably activated macrophages. Immunogloblulin and, to some extent, CD20, can each escape antibody attack in vitro by modulation, but this is difficult to demonstrate clinically. Instead, studies of anti-CD20 therapy of FL suggest that effector modulation, similar to that seen in the suppression of autoimmune inflammation by infusions of normal human IgG, may be important. Both antigenic and effector modulations might be minimized by repeated small doses of more potent antibodies. Clearly, mechanisms of attack vary with the malignancy, the target molecule, and the antibody design, offering opportunities for optimizing this promising strategy.

Autoinhibition and adapter function of Syk

Development, survival, and activation of B lymphocytes are controlled by signals emanating from the B-cell antigen receptor (BCR). The BCR has an autonomous signaling function also known as tonic signaling that allows for long-term survival of B cells in the immune system. Upon binding of antigen to the BCR, the tonic signal is amplified and diversified, leading to alteration in gene expression and B-cell activation. The spleen tyrosine kinase (Syk) intimately cooperates with the signaling subunits of the BCR and plays a central role in the amplification and diversification of BCR signals. In this review, we discuss the molecular mechanisms by which Syk activity is inhibited and activated at the BCR. Importantly, Syk acts not only as a kinase that phosphorylates downstream substrates but also as an adapter that can bind to a diverse set of signaling proteins. Depending on its interactions and localization, Syk can signal opposing cell fate decisions such as proliferation or differentiation of B cells.

Tyrosines in the carboxyl terminus regulate Syk kinase activity and function

The Syk tyrosine kinase family plays an essential role in immunoreceptor tyrosine-based activation motif (ITAM) signaling. The binding of Syk to tyrosine-phosphorylated ITAM subunits of immunoreceptors, such as FcepsilonRI on mast cells, results in a conformational change, with an increase of enzymatic activity of Syk. This conformational change exposes the COOH-terminal tail of Syk, which has three conserved Tyr residues (Tyr-623, Tyr-624, and Tyr-625 of rat Syk). To understand the role of these residues in signaling, wild-type and mutant Syk with these three Tyr mutated to Phe was expressed in Syk-deficient mast cells. There was decreased FcepsilonRI-induced degranulation, nuclear factor for T cell activation and NFkappaB activation with the mutated Syk together with reduced phosphorylation of MAP kinases p38 and p42/44 ERK. In non-stimulated cells, the mutated Syk was more tyrosine phosphorylated predominantly as a result of autophosphorylation. In vitro, there was reduced binding of mutated Syk to phosphorylated ITAM due to this increased phosphorylation. This mutated Syk from non-stimulated cells had significantly reduced kinase activity toward an exogenous substrate, whereas its autophosphorylation capacity was not affected. However, the kinase activity and the autophosphorylation capacity of this mutated Syk were dramatically decreased when the protein was dephosphorylated before the in vitro kinase reaction. Furthermore, mutation of these tyrosines in the COOH-terminal region of Syk transforms it to an enzyme, similar to its homolog ZAP-70, which depends on other tyrosine kinases for optimal activation. In testing Syk mutated singly at each one of the tyrosines, Tyr-624 but especially Tyr-625 had the major role in these reactions. Therefore, these results indicate that these tyrosines in the tail region play a critical role in regulating the kinase activity and function of Syk.

Swiprosin-1/EFhd2 controls B cell receptor signaling through the assembly of the B cell receptor, Syk, and phospholipase C gamma2 in membrane rafts

Compartmentalization of the BCR in membrane rafts is important for its signaling capacity. Swiprosin-1/EFhd2 (Swip-1) is an EF-hand and coiled-coil-containing adaptor protein with predicted Src homology 3 (SH3) binding sites that we identified in membrane rafts. We showed previously that Swip-1 amplifies BCR-induced apoptosis; however, the mechanism of this amplification was unknown. To address this question, we overexpressed Swip-1 and found that Swip-1 amplified the BCR-induced calcium flux in WEHI231, B62.1, and Bal17 cells. Conversely, the BCR-elicited calcium flux was strongly attenuated in Swip-1-silenced WEHI231 cells, and this was due to a decreased calcium mobilization from intracellular stores. Complementation of Swip-1 expression in Swip-1-silenced WEHI231 cells restored the BCR-induced calcium flux and enhanced spleen tyrosine kinase (Syk) tyrosine phosphorylation and activity as well as SLP65/BLNK/BASH and phospholipase C gamma2 (PLCgamma2) tyrosine phosphorylation. Furthermore, Swip-1 induced the constitutive association of the BCR itself, Syk, and PLCgamma2 with membrane rafts. Concomitantly, Swip-1 stabilized the association of BCR with tyrosine-phosphorylated proteins, specifically Syk and PLCgamma2, and enhanced the constitutive interaction of Syk and PLCgamma2 with Lyn. Interestingly, Swip-1 bound to the rSH3 domains of the Src kinases Lyn and Fgr, as well as to that of PLCgamma. Deletion of the predicted SH3-binding region in Swip-1 diminished its association and that of Syk and PLCgamma2 with membrane rafts, reduced its interaction with the SH3 domain of PLCgamma, and diminished the BCR-induced calcium flux. Hence, Swip-1 provides a membrane scaffold that is required for the Syk-, SLP-65-, and PLCgamma2-dependent BCR-induced calcium flux.

Tumor cell dormancy: implications for the biology and treatment of breast cancer

Despite progress made in the therapy of solid tumors such as breast cancer, the prognosis of patients even with small primary tumors is still limited by metastatic relapse often long after removal of the primary tumor. Therefore, it has been hypothesized that primary tumors shed tumor cells already at an early stage into the blood circulation. A subset of these disseminated tumor cells may persist in a state of so-called "dormancy". Based on cell culture and animal models, dormancy can occur at two different stages. Single dormant cells are defined as cells with a lack of proliferation and apoptosis with the cells undergoing cell cycle arrest. The micrometastasis model defines tumor cell dormancy as a state of balanced apoptosis and proliferation of micrometastasis resulting in no net increase of tumor mass. Mechanisms leading to a growth activation of dormant tumor cells and the outgrowth of manifest metastases are not completely understood. Genetic predisposition of the dormant cells as well as immunological and angiogenetic influences of the surrounding environment may contribute to this phenomenon. In this review, we summarize findings on different factors for tumor cell dormancy and potential therapeutic implications that should help to reduce metastatic relapse in cancer patients.

Mouse models of non-Hodgkin lymphoma reveal Syk as an important therapeutic target

We have generated mouse models of non-Hodgkin lymphoma (NHL) that rely on the cooperation between MYC overexpression and B-cell antigen receptor (BCR) signaling for the initiation and maintenance of B-cell lymphomas. Using these mouse models of NHL, we have focused on the identification of BCR-derived signal effectors that are important for the maintenance of NHL tumors. In the present study, we concentrate on Spleen tyrosine kinase (Syk), a nonreceptor tyrosine kinase required to transduce BCR-dependent signals. Using a genetic approach, we showed that Syk expression is required for the survival of murine NHL-like tumors in vitro and that tumor cells deficient in Syk fail to expand in vivo. In addition, a pharmacologic inhibitor of Syk was able to induce apoptosis of transformed B cells in vitro and led to tumor regression in vivo. Finally, we show that genetic or pharmacologic inhibition of Syk activity in human NHL cell lines are generally consistent with results found in the mouse models, suggesting that targeting Syk may be a viable therapeutic strategy.

The kinase Syk as an adaptor controlling sustained calcium signalling and B-cell development

Upon B-cell antigen receptor (BCR) activation, the protein tyrosine kinase Syk phosphorylates the adaptor protein SH2 domain-containing leukocyte protein of 65 kDa (SLP-65), thus coupling the BCR to diverse signalling pathways. Here, we report that SLP-65 is not only a downstream target and substrate of Syk but also a direct binding-partner and activator of this kinase. This positive feedback is mediated by the binding of the SH2 domain of SLP-65 to an autophosphorylated tyrosine of Syk. The mutant B cells that cannot form the Syk/SLP-65 complex are defective in BCR-induced extracellular signal-regulated kinase, nuclear factor kappa B and nuclear factor of activated T cells, but not Akt activation, and are blocked in B-cell development. Furthermore, we show that formation of the Syk/SLP-65 complex is required for sustained Ca(2+) responses in activated B cells. We suggest that after activation and internalization of the BCR, Syk remains active as part of a membrane-bound Syk/SLP-65 complex controlling sustained signalling and calcium influx.

Cancer vaccines and tumor dormancy: a long-term struggle between host antitumor immunity and persistent cancer cells?

Tumor dormancy is a phenomenon characterized by the persistence of residual cancer cells for long periods in the host. Evidence has emerged that a balance exists between the immune response and dormant tumor cells. This review presents our current understanding of the immune relationship between host and dormant tumor cells and the mechanism developed by these cells to escape host antitumor immunity. Implications of this immune escape for cancer vaccine strategy are considered.