Requirement of phospholipase C-gamma 2 activation in surface immunoglobulin M-induced B cell apoptosis

Shc contains two Grb2 binding sites needed for efficient formation of complexes with SOS in B lymphocytes

Cross-linking of the B-cell antigen receptor (BCR) induces tyrosine phosphorylation of Shc, which is believed to lead to the activation of Ras. Previous work has shown that tyrosine-phosphorylated Shc forms complexes with another adapter protein, Grb2, and the Ras guanine nucleotide exchange factor SOS. Here, we demonstrate that phosphorylation of Shc by the hematopoietic cell-specific tyrosine kinase Syk induces binding of Grb2 to Shc, suggesting that Syk phosphorylates Shc in stimulated B cells. Surprisingly, Syk-phosphorylated Shc possesses two Grb2 binding sites rather than the one site that has been previously reported. Both of these sites are required for efficient formation of Shc-Grb2-SOS complexes in vitro and in vivo. We suggest that two Grb2 proteins anchored by a single Shc protein bind simultaneously to one SOS molecule, resulting in a complex that is more stable than a complex containing only a single Grb2 protein bound to one SOS molecule. This model is consistent with our observation that BCR stimulation greatly increases the amount of SOS associated with Grb2.

Tumor Dormancy and Cell Signaling. V. Regrowth of the BCL1 Tumor After Dormancy Is Established

The majority of BALB/c mice immunized with the BCL1 lymphoma-derived idiotype (Id+) IgM and subsequently challenged with BCL1 tumor cells develop a state of tumor dormancy. The vast majority of dormant lymphoma cells are in cell cycle arrest, but there are also residual replicating cells. In the present studies, we attempted to define features of both the dormant lymphoma cells and the host that lead to escape from dormancy. Escape from dormancy occurs at a steady rate over a 2-year period, suggesting that it is a stochastic process. We found that, in the majority of mice, escape was due to the emergence of genetic variants that were no longer susceptible to the anti-Id–mediated induction of dormancy. Ten percent of these variants were Id−; the remainder were Id+ but could grow in the presence of anti-Id antibodies, suggesting that there were mutations in molecules involved in one or more mIg-mediated negative-signaling pathways. In two of five such escapees, alterations in either Syk, HS1, and/or Lyn were observed. In a small percentage of mice, a low titer of circulating anti-Id antibody before tumor challenge correlated with a subsequent, more rapid loss of dormancy.

Genetic evidence for involvement of type 1, type 2 and type 3 inositol 1,4,5-trisphosphate receptors in signal transduction through the B-cell antigen receptor

Stimulation of B-cell antigen receptor (BCR) induces a rapid increase in cytoplasmic free calcium due to its release from intracellular stores and influx from the extracellular environment. Inositol 1,4,5-trisphosphate receptors (IP3Rs) are ligand-gated channels that release intracellular calcium stores in response to the second messenger, inositol 1,4,5-trisphosphate. Most hematopoietic cells, including B cells, express at least two of the three different types of IP3R. We demonstrate here that B cells in which a single type of IP3R has been deleted still mobilize calcium in response to BCR stimulation, whereas this calcium mobilization is abrogated in B cells lacking all three types of IP3R. Calcium mobilization by a transfected G protein-coupled receptor (muscarinic M1 receptor) was also abolished in only triple-deficient cells. Capacitative Ca2+ entry, stimulated by thapsigargin, remains unaffected by loss of all three types of IP3R. These data establish that IP3Rs are essential and functionally redundant mediators for both BCR- and muscarinic receptor-induced calcium mobilization, but not for thapsigargin-induced Ca2+ influx. We further show that the BCR-induced apoptosis is significantly inhibited by loss of all three types of IP3R, suggesting an important role for Ca2+ in the process of apoptosis.

Phospholipase C-gamma1: regulation of enzyme function and role in growth factor-dependent signal transduction

Phospholipase C(gamma)1 (PLC-gamma1), a tyrosine kinase substrate, is a multi-domain molecule that modulates the intracellular levels of the second messenger molecules: Ca2+ and diacylglycerol. Although a wide variety of growth factor receptor tyrosine kinases phosphorylate and activate PLC-gamma1, the biological role and necessity of this signal transduction element in mitogenesis has remained unclear. Recent results, however, point to a more essential role than was suggested by initial studies. Also, biochemical studies have indicated a putative means for the intramolecular repression of PLC-gamma1 activity and provide a means for interpreting activation signals through a derepression mechanism.

Molecular mechanisms in B cell antigen receptor signaling

Recent gene-targeting experiments have highlighted the importance of the intracellular protein tyrosine kinases Lyn, Syk, and Btk in BCR signal transduction and B cell development. In addition, the interactions of these kinases and their regulatory mechanisms have been reported. Activation loop phosphorylation of these kinases is critical for their participation in signal propagation. Several substrates have been identified for these kinases and this has led to elucidation of the mechanisms by which these kinases mediate the downstream signaling events that lead to cellular responses of B cells.

B-Cell Antigen Receptor–Induced Apoptosis Requires Both Igα and Igβ

The response of a B cell to antigen is dependent on the surface expression of a clonotypic B-cell receptor complex (BCR) consisting of membrane-bound Ig and disulfide-linked heterodimers of Igα/β. Studies of Igα or Igβ have shown that the immunoreceptor tyrosine-based activation motif (ITAM) found in each cytoplasmic tail is capable of inducing most receptor signaling events. However, Igα, Igβ, and most of the other receptor chains that contain ITAMs, including CD3ε, CD3γ, TCRζ, and FcεRIγ, are found as components of multimeric and heterogenous complexes. In such a complex it is possible that cooperativity between individual chains imparts functional capacities to the intact receptor that are not predicted from the properties of its constituents. Therefore, we developed a novel system in which we could form and then aggregate dimers, representative of partial receptor complexes, which contained either Igα alone, Igβ alone, or the two chains together and then examine their ability to induce apoptosis in the immature B-cell line, WEHI-231. Here we present evidence that heterodimers of Igα and Igβ efficiently induced apoptosis while homodimers of either chain did not. Apoptosis was associated with the inductive tyrosine phosphorylation of a very restricted set of proteins including the tyrosine kinase Syk. These findings may provide insight into the mechanisms by which the BCR, and other such multimeric receptor complexes, initiate both apoptotic and proliferative responses to antigen.

B-Cell Antigen Receptor–Induced Apoptosis Requires Both Igα and Igβ

The response of a B cell to antigen is dependent on the surface expression of a clonotypic B-cell receptor complex (BCR) consisting of membrane-bound Ig and disulfide-linked heterodimers of Igα/β. Studies of Igα or Igβ have shown that the immunoreceptor tyrosine-based activation motif (ITAM) found in each cytoplasmic tail is capable of inducing most receptor signaling events. However, Igα, Igβ, and most of the other receptor chains that contain ITAMs, including CD3ε, CD3γ, TCRζ, and FcεRIγ, are found as components of multimeric and heterogenous complexes. In such a complex it is possible that cooperativity between individual chains imparts functional capacities to the intact receptor that are not predicted from the properties of its constituents. Therefore, we developed a novel system in which we could form and then aggregate dimers, representative of partial receptor complexes, which contained either Igα alone, Igβ alone, or the two chains together and then examine their ability to induce apoptosis in the immature B-cell line, WEHI-231. Here we present evidence that heterodimers of Igα and Igβ efficiently induced apoptosis while homodimers of either chain did not. Apoptosis was associated with the inductive tyrosine phosphorylation of a very restricted set of proteins including the tyrosine kinase Syk. These findings may provide insight into the mechanisms by which the BCR, and other such multimeric receptor complexes, initiate both apoptotic and proliferative responses to antigen.

Role of protein kinase activity in apoptosis

The transmission of signals from the plasma membrane to the nucleus involves a number of different pathways all of which have in common protein modification. The modification is primarily in the form of phosphorylation which leads to the activation of a series of protein kinases. It is now evident that these pathways are common to stimuli that lead to mitogenic and apoptotic responses. Even the same stimuli under different physiological conditions can cause either cell proliferation or apoptosis. Activation of specific protein kinases can in some circumstances protect against cell death, while in others it protects the cell against apoptosis. Some of the pathways involved lead to activation of transcription factors and the subsequent induction of genes involved in the process of cell death or proliferation. In other cases, such as for the tumour suppressor gene product p53, activation may be initiated both at the level of gene expression or through pre-existing proteins. Yet in others, while the initial steps in the pathway are ill-defined, it is clear that downstream activation of a series of cystein proteases is instrumental in pushing the cell towards apoptosis. In this report we review the involvement of protein kinases at several different levels in the control of cell behaviour.

Protein kinase C mu (PKC mu) associates with the B cell antigen receptor complex and regulates lymphocyte signaling

We have identified a Ser/Thr kinase associated with the B cell receptor (BCR) complex as protein kinase C mu (PKC mu). PKC mu activity is up-regulated after cross-linking the BCR and CD19 on B cells, and PKC mu co-precipitates with Syk and phospholipase C-gamma 1/2 (PLC gamma 1/2). In vitro phosphorylation of fusion proteins showed that both Syk and PLC gamma 1 are potential substrates of PKC mu in vivo. Analysis of mutants of the chicken B cell line DT40 deficient in either Syk, Lyn, Btk, or PLC gamma 2 revealed that BCR-induced activation of PKC mu, like activation of PLC gamma 2, requires Syk and is partially regulated by Btk, but is Lyn independent. PKC mu can down-regulate the ability of Syk to phosphorylate PLC gamma 1 in vitro. Thus, PKC mu may function in a negative feedback loop regulating BCR-initiated signaling cascades.

BTK as a mediator of radiation-induced apoptosis in DT-40 lymphoma B cells

Bruton's tyrosine kinase (BTK) is a member of the SRC-related TEC family of protein tyrosine kinases (PTKs). DT-40 lymphoma B cells, rendered BTK-deficient through targeted disruption of the btk gene by homologous recombination knockout, did not undergo radiation-induced apoptosis, but cells with disrupted lyn or syk genes did. Introduction of the wild-type, or a SRC homology 2 domain or a plecstrin homology domain mutant (but not a kinase domain mutant), human btk gene into BTK-deficient cells restored the apoptotic response to radiation. Thus, BTK is the PTK responsible for triggering radiation-induced apoptosis of lymphoma B cells, and its kinase domain is indispensable for the apoptotic response.

Activation mediated by RP105 but not CD40 makes normal B cells susceptible to anti-IgM-induced apoptosis: a role for Fc receptor coligation

Signals through the B cell antigen receptor lead to a variety of cellular events such as activation, anergy, and apoptosis. B cells select these outcomes to establish and maintain self-tolerance, and to mount adequate antibody responses. However, it is not fully understood how one and the same signal causes such different consequences. In the present study, we have studied the effect of activation signals on the outcome of responses to antigen receptor ligation. Two distinct growth-promoting signals were used to activate B cells. Ligation of either RP105, a newly discovered B cell surface molecule, or the CD40 molecule, drove B cells to proliferate. Resultant blastic cells were then exposed to anti-immunoglobulin M (IgM). Blast cells that had been stimulated with anti-RP105 ceased growing and underwent apoptosis after cross-linking of surface IgM. Coligation of the Fc gamma receptor IIB with surface IgM augmented, rather than aborted, this response. In contrast to RP105-activated B cells, blast cells that had been activated by CD40 ligation were unaltered by anti-IgM. On the other hand, CD40-activated B cells became extremely susceptible to Fas-mediated apoptosis, whereas RP105-activated B cells were much less sensitive. Anti-IgM-induced apoptosis in RP105 blasts was independent of Fas, because it was demonstrable with Fas-deficient MRL-lpr/lpr mice. These results demonstrate that the nature of an initial activation signal has a great influence on the fate of activated B cells after (re)engagement of the antigen receptor. RP105, as well as CD40, may be important in this life/death decision.

A role for Bruton's tyrosine kinase in B cell antigen receptor-mediated activation of phospholipase C-gamma 2

Defects in the gene encoding Bruton's tyrosine kinase (Btk) result in a disease called X-linked agammaglobulinemia, in which there is a profound decrease of mature B cells due to a block in B cell development. Recent studies have shown that Btk is tyrosine phosphorylated and activated upon B cell antigen receptor (BCR) stimulation. To elucidate the functions of this kinase, we examined BCR signaling of DT40 B cells deficient in Btk. Tyrosine phosphorylation of phospholipase C (PLC)-gamma 2 upon receptor stimulation was significantly reduced in the mutant cells, leading to the loss of both BCR-coupled phosphatidylinositol hydrolysis and calcium mobilization. Pleckstrin homology and Src-homology 2 domains of Btk were required for PLC-gamma 2 activation. Since Syk is also required for the BCR-induced PLC-gamma 2 activation, our findings indicate that PLC-gamma 2 activation is regulated by Btk and Syk through their concerted actions.

Negative selection of human germinal center B cells by prolonged BCR cross-linking

The antigen receptors on T and B lymphocytes can transduce both agonist and antagonist signals leading either to activation/survival or anergy/death. The outcome of B lymphocyte antigen receptor (BCR) triggering depends upon multiple parameters which include (a) antigen concentration and valency, (b) duration of BCR occupancy, (c) receptor affinity, and (d) B cell differentiation stages. Herein, using anti-immunoglobulin kappa and lambda light chain antibodies, we analyzed the response of human naive, germinal center (GC) or memory B cells to BCR cross-linking regardless of heavy chain Ig isotype or intrinsic BCR specificity. We show that after CD40-activation, anti-BCR (kappa + gamma) can elicit an intracellular calcium flux on both GC and non-GC cells. However, prolonged BCR cross-linking induces death of CD40-activated GC B cells but enhances proliferation of naive or memory cells. Anti-kappa antibody only kills kappa + GC B cells without affecting surrounding gamma + GC B cells, thus demonstrating that BCR-mediated killing of GC B lymphocytes is a direct effect that does not involve a paracrine mechanism. BCR-mediated killing of CD40-activated GC B cells could be partially antagonized by the addition of IL-4. Moreover, in the presence of IL-4, prestimulation through CD40 could prevent subsequent anti-Ig-mediated cell death, suggesting a specific role of this combination in selection of GC B cells. This report provides evidence that in human, susceptibility to BCR killing is regulated along peripheral B cell differentiation pathway.