Requirement for JNK-dependent upregulation of BimL in anti-IgM-induced apoptosis in murine B lymphoma cell lines WEHI-231 and CH31

Induction of Apoptosis by the Nonstructural Protein 4 and 10 of Porcine Reproductive and Respiratory Syndrome Virus

Infection by most viruses triggers apoptosis in host cells, and viruses manipulate this cell response to promote viral replication, virus spread, and cell killing. Porcine reproductive and respiratory syndrome virus (PRRSV) has been shown to induce apoptosis both in vitro and in vivo, while the regulatory roles of PRRSV-encoded products in apoptosis are not fully understood. In the present study, we first showed a biphasic apoptosis regulation by a highly pathogenic PRRSV strain JXwn06. It was indicated that PRRSV infection delays apoptosis at early infection but activates apoptosis at late infection in MARC-145 cells. In PRRSV-infected MARC-145 cells, procaspase-8, -9 and -12 were activated at late infection, demonstrating the involvements of death receptor pathway, mitochondrial pathway and endoplasmic reticulum (ER) stress pathway in inducing apoptosis. PRRSV was also shown to induce a similar apoptosis process in pulmonary alveolar macrophages (PAMs) with an early initiation. Next, the PRRSV-encoded apoptosis inducers were screened, indicating that the nonstructural protein (Nsp) 4 and Nsp10 of PRRSV are pro-apoptotic. In the presence of Nsp4, it was confirmed that procaspase-8, -9 and -12 were cleaved, and Nsp4 facilitates the cleavage of procaspase-9 by activating B-cell lymphoma 2 interacting mediator of cell death (Bim), a pro-apoptotic protein. In addition, Nsp4 was shown to induce the degradation of an anti-apoptotic protein, B-cell lymphoma-extra large (Bcl-xL). Nsp10 was shown to activate procaspase-8 and -9 but procaspase-12 and to upregulate the expression of BH3-only pro-apoptotic protein BH3 interacting-domain death agonist (Bid) and its active form, truncated Bid (tBid). Clearly, the participation of both activated caspase-8 and Bid is required for Nsp10-induced apoptosis, indicating a crosstalk between extrinsic- and mitochondria-dependent pathways. Together, our findings suggest that PRRSV infection regulates apoptosis in a two-phase manner and activates all three apoptotic pathways; the Nsp4 and Nsp10 of PRRSV function as apoptosis inducers with different molecular basis.

BCR-signaling-induced cell death demonstrates dependency on multiple BH3-only proteins in a murine model of B-cell lymphoma

Genetic recombination during B-cell development regularly results in the generation of autoreactive, potentially pathogenic B-cell receptors (BCRs). Consequently, multiple mechanisms link inappropriate BCR specificity to clonal deletion. Similar pathways remain in malignant B cells, offering the potential for targeting BCR signaling. Recently, small molecule inhibitors have realized this potential and, therefore, a deeper understanding of BCR-induced signaling networks in malignant cells is vital. The BH3-only protein Bim has a key role in BCR-induced apoptosis, but it has long been proposed that additional BH3-only proteins also contribute, although conclusive proof has been lacking. Here, we comprehensively characterized the mechanism of BCR-induced apoptosis in Eμ-Myc murine lymphoma cells. We demonstrate the upregulation of Bim, Bik, and Noxa during BCR signaling in vitro and that intrinsic apoptosis has a prominent role in anti-BCR antibody therapy in vivo. Furthermore, lymphomas deficient in these individual BH3-only proteins display significant protection from BCR-induced cell death, whereas combined loss of Noxa and Bim offers enhanced protection in comparison with loss of Bim alone. Some but not all of these effects were reversed upon inhibition of Syk or MEK. These observations indicate that BCR signaling elicits maximal cell death through upregulation of multiple BH3-only proteins; namely Bim, Bik, and Noxa.

In-Depth Analyses of B Cell Signaling Through Tandem Mass Spectrometry of Phosphopeptides Enriched by PolyMAC

Tandem mass spectrometry (MS/MS) has enabled researchers to analyze complex biological samples since the original concept inception. It facilitates the identification and quantification of modifications within tens of thousands of proteins in a single large-scale proteomic experiment. Phosphorylation analysis, as one of the most common and important post-translational modifications, has particularly benefited from such progress in the field. Here we showcase the technique through in-depth analyses of B cell signaling based on quantitative phosphoproteomics. As a complement to the previously described PolyMAC-Ti (polymer-based metal ion affinity capture using titanium) reagent, we introduce here PolyMAC-Fe, which utilizes a different metal ion, Fe(III). An extensive comparison using the different available MS/MS fragmentations techniques was made between PolyMAC-Fe, PolyMAC-Ti and IMAC (immobilized metal ion affinity chromatography) reagents in terms of specificity, reproducibility and type of phosphopeptides being enriched. PolyMAC-Fe based chelation demonstrated good selectivity and unique specificity toward phosphopeptides, making it useful in specialized applications. We have combined PolyMAC-Ti and PolyMAC-Fe, along with SILAC-based quantitation and large-scale fractionation, for quantitative B cell phosphoproteomic analyses. The complementary approach allowed us to identify a larger percentage of multiply phosphorylated peptides than with PolyMAC-Ti alone. Overall, out of 13,794 unique phosphorylation sites identified, close to 20% were dependent on BCR signaling. These sites were further mapped to a variety of major signaling networks, offering more detailed information about the biochemistry of B cell receptor engagement.

Role of c-Jun N-terminal kinase activation in apoptosis induced by removal of the growth factors

Apoptosis plays a crucial role for generation of lymphocyte repertoire, clonal contraction, and elimination of virus-infected cells. Since IL-3-dependent pro-B cell line Baf-3 resulted in rapid induction of apoptotic cell death upon IL-3 withdrawal, it would by very valuable for analysis of apoptosis induction by growth factor deprivation. First, we confirmed that Baf-3 cells underwent loss of mitochondrial membrane potential (ΔΨm) and apoptosis in a time-dependent manner when they were cultured in the RPMI-1640 medium without IL-3. Induction of apoptosis and loss of ΔΨm was determined by DiOC6 and annexin V staining method using flow cytometer, respectively. Deprivation of IL-3 induced upregulation of proapoptotic molecule Bax, in conjunction with slight down-regulation of anti-apoptotic molecule Bcl-xL, which was assessed by Western blotting. Since Bcl-xL-overexpressing Baf-3 cells showed some resistance to IL-3-deprivation, Bcl-xL prevents apoptosis induced by IL-3 withdrawal. Finally, a sustained JNK1 activation was observed prior to induction of apoptosis upon IL-3 deprivation. Dominat-negative form of JNK1 and JNK inhibitor sp600125 partially inhibit the apoptosis upon IL-3 deprivation, suggesting that a sustained JNK1 activation was involved in the induction of apoptosis. Together, IL-3 deprivation of IL-3-dependent cell line Baf-3 induces a sustained JNK1 activation, followed by a decline of the ratio of Bcl-xL to Bax, leading to loss of DCm, and finally apoptosis.

Thy28 protects against anti-CD3-mediated thymic cell death in vivo

Apoptotic cell death plays a pivotal role in the development and/or maintenance of several tissues including thymus. Deregulated thymic cell death is associated with autoimmune diseases including experimental autoimmune encephalomyelitis (EAE), a prototype murine model for analysis of human multiple sclerosis. Because Thy28 expression is modulated during thymocyte development, we tested whether Thy28 affects induction of EAE as effectively as antigen-induced thymocyte deletion using Thy28 transgenic (TG) mice. Thy28 TG mice showed partial resistance to anti-CD3 monoclonal antibody (mAb)-induced thymic cell death in vivo, as assessed by annexin V-expression and loss of mitochondrial membrane potential. The resistance to anti-CD3 mAb-induced cell death in Thy28 TG mice appeared to correlate with a decreased c-Jun N-terminal kinase phosphorylation and reduced down-regulation of Bcl-xL. Moreover, thymic hyperplasia was detected in Thy28 TG mice, although thymocyte development was unaltered. Development of peripheral lymphoid tissues including spleen and lymph nodes was also unaltered. Thy28 TG spleen T cells showed an increased production of IFN-γ, but not IL-17, in response to both anti-CD3 and anti-CD28 mAbs. Finally, Thy28 TG mice displayed accelerated induction of EAE as assessed by disease incidence, clinical score, and pathology following immunization with myelin oligodendrocyte glycoprotein compared with control WT mice. These findings suggest that modulation of Thy28 expression plays a crucial role in the determination of thymic cell fate, which may contribute to the development of EAE through proinflammatory cytokine production.

Kinase-independent feedback of the TAK1/TAB1 complex on BCL10 turnover and NF-κB activation

Antigen receptors activate pathways that control cell survival, proliferation, and differentiation. Two important targets of antigen receptors, NF-κB and Jun N-terminal kinase (JNK), are activated downstream of CARMA1, a scaffolding protein that nucleates a complex including BCL10, MALT1, and other IκB kinase (IKK)-signalosome components. Somatic mutations that constitutively activate CARMA1 occur frequently in diffuse large B cell lymphoma (DLBCL) and mediate essential survival signals. Mechanisms that downregulate this pathway might thus yield important therapeutic targets. Stimulation of antigen receptors induces not only BCL10 activation but also its degradation downstream of CARMA1, thereby ultimately limiting signals to its downstream targets. Here, using lymphocyte cell models, we identify a kinase-independent requirement for TAK1 and its adaptor, TAB1, in antigen receptor-induced BCL10 degradation. We show that TAK1 acts as an adaptor for E3 ubiquitin ligases that target BCL10 for degradation. Functionally, TAK1 overexpression restrains CARMA1-dependent activation of NF-κB by reducing BCL10 levels. TAK1 also promotes counterselection of NF-κB-addicted DLBCL lines by a dual mechanism involving kinase-independent degradation of BCL10 and kinase-dependent activation of JNK. Thus, by directly promoting BCL10 degradation, TAK1 counterbalances NF-κB and JNK signals essential for the activation and survival of lymphocytes and CARMA1-addicted lymphoma types.

Transgenic overexpression of G5PR that is normally augmented in centrocytes impairs the enrichment of high-affinity antigen-specific B cells, increases peritoneal B-1a cells, and induces autoimmunity in aged female mice

To investigate signals that control B cell selection, we examined expression of G5PR, a regulatory subunit of the serine/threonine protein phosphatase 2A, which suppresses JNK phosphorylation. G5PR is upregulated in activated B cells, in Ki67-negative centrocytes at germinal centers (GCs), and in purified B220(+)Fas(+)GL7(+) mature GC B cells following Ag immunization. G5PR rescues transformed B cells from BCR-mediated activation-induced cell death by suppression of late-phase JNK activation. In G5PR-transgenic (G5PR(Tg)) mice, G5PR overexpression leads to an augmented generation of GC B cells via an increase in non-Ag-specific B cells and a consequent reduction in the proportion of Ag-specific B cells and high-affinity Ab production after immunization with nitrophenyl-conjugated chicken γ-globulin. G5PR overexpression impaired the affinity-maturation of Ag-specific B cells, presumably by diluting the numbers of high-affinity B cells. However, aged nonimmunized female G5PR(Tg) mice showed an increase in the numbers of peritoneal B-1a cells and the generation of autoantibodies. G5PR overexpression did not affect the proliferation of B-1a and B-2 cells but rescued B-1a cells from activation-induced cell death in vitro. G5PR might play a pivotal role in B cell selection not only for B-2 cells but also for B-1 cells in peripheral lymphoid organs.

Thy28 partially prevents apoptosis induction following engagement of membrane immunoglobulin in WEHI-231 B lymphoma cells

Thy28 protein is conserved among plants, bacteria, and mammalian cells. Nuclear Thy28 protein is substantially expressed in testis, liver, and immune cells such as lymphocytes. Lymphocyte apoptosis plays a crucial role in homeostasis and formation of a diverse lymphocyte repertoire. In this study, we examined whether Thy28 affects induction of apoptosis in WEHI-231 B lymphoma cells following engagement of membrane immunoglobulin (mIg). Once they were established, the Thy28-overexpressing WEHI-231 cells showed similar expression levels of IgM and class I major histocompatibility complex (MHC) molecule compared with controls. The Thy28-overexpressing cells were considerably resistant to loss of mitochondrial membrane potential (ΔΨm), caspase-3 activation, and increase in annexin-positive cells upon mIg engagement. These changes were concomitant with an increase in G1 phase associated with upregulation of p27^Kip1. The anti-IgM-induced sustained activation of c-Jun N-terminal kinase (JNK), which was associated with late-phase hydrogen peroxide (H₂O₂) production, was partially reduced in the Thy28-expressing cells relative to controls. Taken together, the data suggest that in WEHI-231 B lymphoma cells, Thy28 regulates mIg-mediated apoptotic events through the JNK-H₂O₂ activation pathway, concomitant with an accumulation of cells in G1 phase associated with upregulation of p27^Kip1 in WEHI-231 B lymphoma cells.

Enhanced T cell-independent antibody responses in c-Jun N-terminal kinase 2 (JNK2)-deficient B cells following stimulation with CpG-1826 and anti-IgM

Although c-Jun NH2-terminal kinase (JNK) 1 and JNK2 have been demonstrated to modulate T cell activation, role of JNKs in B cell activation remains largely unclear. Phosphorylation of JNK2 was increased in murine B cells following stimulation with either anti-IgM or CpG-1826 oligonucleotide (ODN) alone, with a further increase by a combined stimulation with anti-IgM and CpG-1826 ODN. In this study, we examined whether antibody production induced by CpG ODN and/or anti-IgM is affected in B cells from JNK2-deficient (JNK2-/-) mice. After stimulation with CpG ODN or both CpG ODN and anti-IgM, JNK2-/- B cells displayed an enhanced antibody production of IgG1 and IgG2a, with less pronounced in IgG2b production, as assessed by enzyme-linked immunoassay (ELISA). However, IgM production in JNK2-/- B cells by CpG ODN was comparable to that in WT B cells. TLR9 expression was increased in JNK2-/- B cells after stimulation with anti-IgM or both CpG ODN and anti-IgM, suggesting that the anti-IgM/CpG ODN-induced enhancement of antibody production is partly due to the increased expression of TLR9. The enhanced antibody production in JNK2-/- B cells by the combined stimulation does not appear to involve either increased class switch recombination or cell proliferation. Our results provide useful information on the role of JNK2 in antibody responses mediated by T cell-independent antigens.

Feedback regulation of mitochondria by caspase-9 in the B cell receptor-mediated apoptosis

During the germinal centre reaction (GC), B cells with non-functional or self-reactive antigen receptors are negatively selected by apoptosis to generate B cell repertoire with appropriate antigen specificities. We studied the molecular mechanism of Fas/CD95- and B cell receptor (BCR)-induced apoptosis to shed light on the signalling events involved in the negative selection of GC B cells. As an experimental model, we used human follicular lymphoma (FL) cell line HF1A3, which originates from a GC B cell, and transfected HF1A3 cell lines overexpressing Bcl-x(L), c-FLIP(long) or dominant negative (DN) caspase-9. Fas-induced apoptosis was dependent on the caspase-8 activation, since the overexpression of c-FLIP(long), a natural inhibitor of caspase-8 activation, blocked apoptosis induced by Fas. In contrast, caspase-9 activation was not involved in Fas-induced apoptosis. BCR-induced apoptosis showed the typical characteristics of mitochondria-dependent (intrinsic) apoptosis. Firstly, the activation of caspase-9 was involved in BCR-induced DNA fragmentation, while caspase-8 showed only marginal role. Secondly, overexpression of Bcl-x(L) could block all apoptotic changes induced by BCR. As a novel finding, we demonstrate that caspase-9 can enhance the cytochrome-c release and collapse of mitochondrial membrane potential (DeltaPsi(m)) during BCR-induced apoptosis. The requirement of different signalling pathways in apoptosis induced by BCR and Fas may be relevant, since Fas- and BCR-induced apoptosis can thus be regulated independently, and targeted to different subsets of GC B cells.

Apoptosis signal-regulating kinase (ASK)-1 mediates apoptosis through activation of JNK1 following engagement of membrane immunoglobulin

Engagement of membrane immunoglobulin (mIg) on WEHI-231 mouse B lymphoma cells results in growth arrest at the G1 phase of the cell cycle, followed by a reduction of mitochondrial membrane potential (DeltaPsim) and apoptosis. WEHI-231 cells resemble immature B cells in terms of the cell surface phenotype and sensitivity to mIg engagement. However, the molecular mechanisms underlying mIg-induced loss of DeltaPsim and apoptosis have not yet been established. In this study, we show that apoptosis signal-regulating kinase 1 (ASK1)-c-Jun N-terminal kinase 1 (JNK1) signaling pathway participates in mIg-induced apoptosis through the generation of reactive oxygen species (ROS). Stimulation of WEHI-231 cells with anti-IgM induces phosphorylation and subsequent activation of ASK1, leading to JNK activation. Anti-IgM stimulation immediately (5 min) induces hydrogen peroxide (H2O2) production with a substantial increase during later time points (36-48 h), accompanied by loss of DeltaPsim and an increase in cells with sub-G1 DNA content. The anti-IgM-induced late-phase H2O2 production, loss of DeltaPsim, and increase in the sub-G1 fraction were all reduced substantially in WEHI-231 cells overexpressing a dominant-negative form of ASK1, compared with control vector alone, but enhanced substantially in cells overexpressing a constitutively active form of ASK1. These mIg-mediated events were also partially abrogated by ROS scavenger N-acetyl-L-cysteine (NAC). Taken together, these results suggest that mIg engagement induces H2O2 production leading to activation of ASK1-JNK1 pathway, creating a feedback amplification loop of ROS-ASK/JNK that leads to loss of DeltaPsim and finally apoptosis.

T cell leukemia/lymphoma 1 and galectin-1 regulate survival/cell death pathways in human naive and IgM+ memory B cells through altering balances in Bcl-2 family proteins

BCR signaling plays a critical role in purging the self-reactive repertoire, or in rendering it anergic to establish self-tolerance in the periphery. Differences in self-reactivity between human naive and IgM(+) memory B cells may reflect distinct mechanisms by which BCR signaling dictates their survival and death. Here we demonstrate that BCR stimulation protected naive B cells from apoptosis with induction of prosurvival Bcl-2 family proteins, Bcl-x(L) and Mcl-1, whereas it rather accelerated apoptosis of IgM(+) memory B cells by inducing proapoptotic BH3-only protein Bim. We found that BCR-mediated PI3K activation induced the expression of Mcl-1, whereas it inhibited Bim expression in B cells. Phosphorylation of Akt, a downstream molecule of PI3K, was more sustained in naive than IgM(+) memory B cells. Abundant expression of T cell leukemia/lymphoma 1 (Tcl1), an Akt coactivator, was found in naive B cells, and enforced expression of Tcl1 induced a high level of Mcl-1 expression, resulting in prolonged B cell survival. In contrast, Galectin-1 (Gal-1) was abundantly expressed in IgM(+) memory B cells, and inhibited Akt phosphorylation, leading to Bim up-regulation. Enforced expression of Gal-1 induced accelerated apoptosis in B cells. These results suggest that a unique set of molecules, Tcl1 and Gal-1, defines distinct BCR signaling cascades, dictating survival and death of human naive and IgM(+) memory B cells.

BCR-mediated apoptosis associated with negative selection of immature B cells is selectively dependent on Pten

The molecular basis of B cell receptor (BCR)-induced apoptosis during the negative selection of immature B cells is largely unknown. We use transitional immature B cells that are highly susceptible to BCR-induced apoptosis to show that Pten is selectively required for BCR-mediated initiation of the mitochondrial death pathway. Specifically, deleting Pten, but not other pro-apoptotic molecules, abrogates BCR-elicited apoptosis and improves viability in wild-type immature B cells. We further identify a physiologically and significantly higher intracellular Pten level in immature B cells, as compared to mature B cells, which is responsible for low AKT activity and the propensity towards death in immature B cells. Restoration of AKT activity using a constitutive form of AKT or reduction of Pten to a level comparable with that seen in mature B cells rescues immature B cells from BCR-induced apoptosis. Thus, we provide evidence that Pten is an essential mediator of BCR-induced cell death, and that differential regulation of intracellular Pten levels determines whether BCR ligation promotes cell death or survival. Our findings provide a valuable insight into the mechanisms underlying negative selection and clonal deletion of immature B cells.

c-Jun NH2-terminal kinase (JNK)-dependent nuclear translocation of apoptosis-inducing factor (AIF) following engagement of membrane immunoglobulin on WEHI-231 B lymphoma cells

WEHI-231 B lymphoma cells have been employed for analysis of antigen-induced B cell unresponsiveness because these cells undergo cell cycle arrest in G1, accompanied by induction of apoptosis. In the present study, we examined the requirement for toxic small molecules apoptosis-inducing factor (AIF) and cytochrome c, and subsequent caspase activation in apoptotic cell death in WEHI-231 and CH31 B lymphoma cells following engagement of membrane immunoglobulin (mIg). Pan-caspase inhibitor BD-fmk blocked mIg-mediated increase in cells with sub-G1 DNA content, whereas it did not affect mIg-mediated loss of mitochondrial membrane potential and phosphatidylserine exposure on B cell membrane. Dominant-negative form of c-Jun NH2-terminal kinase1 (JNK1) blocked the translocation of AIF into the nuclei and cytosol from the mitochondria in the WEHI-231 and CH31 cells following mIg engagement, whereas constitutively active form of JNK1 enhanced it. This AIF translocation was also blocked by Bcl-xL, but not by BD-fmk. Moreover, AIF-deficient clones via small interfering RNA (siRNA)-mediated method showed small increase in loss of mitochondrial membrane potential. After mIg engagement, the AIF-deficient clones displayed an enhanced sensitivity to mIg-mediated apoptosis, concomitant with translocation of a residual AIF into the nuclei, compared with control clone. Our findings are compatible with the notion that AIF has dual role, with a proapoptotic function in the nuclei and a distinct anti-apoptotic function in the mitochondria. These observations would be valuable for analysis of B cell unresponsiveness and hopefully for treatment of diseases involving B cell dysfunction.