Microfilament assembly is involved in B-cell apoptosis

Temporal regulation of Lsp1 O-GlcNAcylation and phosphorylation during apoptosis of activated B cells

Crosslinking of B-cell receptor (BCR) sets off an apoptosis programme, but the underlying pathways remain obscure. Here we decipher the molecular mechanisms bridging B-cell activation and apoptosis mediated by post-translational modification (PTM). We find that O-GlcNAcase inhibition enhances B-cell activation and apoptosis induced by BCR crosslinking. This proteome-scale analysis of the functional interplay between protein O-GlcNAcylation and phosphorylation in stimulated mouse primary B cells identifies 313 O-GlcNAcylation-dependent phosphosites on 224 phosphoproteins. Among these phosphoproteins, temporal regulation of the O-GlcNAcylation and phosphorylation of lymphocyte-specific protein-1 (Lsp1) is a key switch that triggers apoptosis in activated B cells. O-GlcNAcylation at S209 of Lsp1 is a prerequisite for the recruitment of its kinase, PKC-β1, to induce S243 phosphorylation, leading to ERK activation and downregulation of BCL-2 and BCL-xL. Thus, we demonstrate the critical PTM interplay of Lsp1 that transmits signals for initiating apoptosis after BCR ligation.

B cell receptor (BCR) activation can trigger signalling causing apoptosis in order to eliminate auto-reactive B cells. Here the authors show that the O-GlcNAcylation and phosphorylation of lymphocyte-specific protein-1 are involved in a switch that regulates the initiation of apoptosis induced by BCR cross-linking.

SEPTIN2 and STATHMIN Regulate CD99-Mediated Cellular Differentiation in Hodgkin's Lymphoma

Hodgkin’s lymphoma (HL) is a lymphoid neoplasm characterized by Hodgkin’s and Reed-Sternberg (H/RS) cells, which is regulated by CD99. We previously reported that CD99 downregulation led to the transformation of murine B lymphoma cells (A20) into cells with an H/RS phenotype, while CD99 upregulation induced differentiation of classical Hodgkin’s lymphoma (cHL) cells (L428) into terminal B-cells. However, the molecular mechanism remains unclear. In this study, using fluorescence two-dimensional differential in-gel electrophoresis and matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS), we have analyzed the alteration of protein expression following CD99 upregulation in L428 cells as well as downregulation of mouse CD99 antigen-like 2 (mCD99L2) in A20 cells. Bioinformatics analysis showed that SEPTIN2 and STATHMIN, which are cytoskeleton proteins, were significantly differentially expressed, and chosen for further validation and functional analysis. Differential expression of SEPTIN2 was found in both models and was inversely correlated with CD99 expression. STATHMIN was identified in the A20 cell line model and its expression was positively correlated with that of CD99. Importantly, silencing of SEPTIN2 with siRNA substantially altered the cellular cytoskeleton in L428 cells. The downregulation of STATHMIN by siRNA promoted the differentiation of H/RS cells toward terminal B-cells. These results suggest that SEPTIN2-mediated cytoskeletal rearrangement and STATHMIN-mediated differentiation may contribute to changes in cell morphology and differentiation of H/RS cells with CD99 upregulation in HL.

Proteomic analysis of differentially expressed proteins in lung cancer in Wistar rats using NNK as an inducer

Lung cancer is one of the commonest cancers detected worldwide with a high mortality rate. The responsible factors affecting survival include delayed prognosis, and lack of effective treatments. To help improve the disease management, there is a need for better screening and development of specific markers that help in the early diagnosis. Analysis of differentially expressed proteins in cancer cells in comparison to their normal counterparts using proteome profiling revealed identification of new biomarkers as therapeutic targets. Therefore, an animal model for lung cancer was developed and monitored by histopathological evaluation. Lung tissue proteins were isolated, solubilized and resolved on 2D gel electrophoresis using broad pH range IPG strips (pH 3-10). Liquid chromatography and mass spectrometry (LC-MS/MS) revealed 66 proteins to be differentially expressed in cancer tissue as compared to normal. The study identified and characterized three of these proteins, namely peroxiredoxin-6, β-actin and collagen α-1 (VI) as potentially prospective biomarkers for early detection of lung cancer.

Tubulin Polymerization Modulates Interleukin-2 Receptor Signal Transduction in Human T Cells

Few data exist on the modulation of cytokine receptor signaling by the actin or tubulin cytoskeleton. Therefore, we studied interleukin-2 receptor (IL-2R) signaling in phytohemagglutinine (PHA)-pretreated human T cells in the context of alterations in the cytoskeletal system induced by cytochalasin D (CyD), jasplaklinolide (Jas), taxol (Tax), or colchicine (Col). We found that changes in cytoskeletal tubulin polymerization altered the strength of several IL-2-triggered signals. Moreover, Tax-induced tubulin hyperpolymerization augmented the surface expression of the IL-2R ss -chain and enhanced the association of the IL-2R beta -chain with cytoskeletal tubulin. The IL-2R beta-chain, in turn, was constitutively associated with tubulin and, more weakly, actin. To exclude the possibility that these associations are artifacts caused by PHA, we confirmed them in T cells from TCR-transgenic DO 11.10 mice stimulated with their nominal antigen. We conclude that altered polymerization of cytoskeletal components, especially tubulin, is accompanied by modulation of IL-2 signaling at the receptor level.

BNIP-Sα induces cell rounding and apoptosis by displacing p50RhoGAP and facilitating RhoA activation via its unique motifs in the BNIP-2 and Cdc42GAP homology domain

Changes in cell morphology are linked to many cellular events including cytokinesis, differentiation, migration and apoptosis. We recently showed that BNIP-Salpha induced cell rounding that leads to apoptosis via its BNIP-2 and Cdc42GAP Homology (BCH) domain, but the underlying mechanism has not been determined. Here, we have identified a unique region (amino acid 133-177) of the BNIP-Salpha BCH domain that targets RhoA, but not Cdc42 or Rac1 and only the dominant-negative form of RhoA could prevent the resultant cell rounding and apoptotic effect. The RhoA-binding region consists of two parts; one region (residues 133-147) that shows some homology to part of the RhoA switch I region and an adjacent sequence (residues 148-177) that resembles the REM class I RhoA-binding motif. The sequence 133-147 is also necessary for its heterophilic interaction with the BCH domain of the Rho GTPase-activating protein, p50RhoGAP/Cdc42GAP. These overlapping motifs allow tripartite competition such that overexpression of BNIP-Salpha could reduce p50RhoGAP binding to RhoA and restore RhoA activation. Furthermore, BNIP-Salpha mutants lacking the RhoA-binding motif completely failed to induce cell rounding and apoptosis. Therefore, via unique binding motifs within its BCH domain, BNIP-Salpha could interact and activate RhoA while preventing its inhibition by p50RhoGAP. This concerted mechanism could allow effective propagation of the RhoA pathway for cell rounding and apoptosis.

Beta-actin is required for mitochondria clustering and ROS generation in TNF-induced, caspase-independent cell death

Tumor necrosis factor (TNF)-alpha induces caspase-independent cell death in the fibrosarcoma cell line L929. This cell death has a necrotic phenotype and is dependent on production of reactive oxygen species (ROS) in the mitochondria. To identify genes involved in this TNF-induced, ROS-dependent cell death pathway, we utilized retrovirus insertion-mediated random mutagenesis to generate TNF-resistant L929 cell lines and we subsequently identified genes whose mutations are responsible for the TNF-resistant phenotype. In one such resistant line, beta-actin was disrupted by viral insertion, and subsequent reconstitution of beta-actin expression levels in the mutant line Actin(mut) restored its sensitivity to TNF. Resistance to TNF in Actin(mut) cells is signal specific since the sensitivity to other death stimuli is either unchanged or even increased. Comparable NF-kappaB activation and p38 phosphorylation in TNF-treated wild-type and Actin(mut) cells also indicates that reduced expression of actin only selectively blocked some of the TNF-induced cellular changes. Actin cleavage involved in apoptosis does not occur in TNF-treated L929 cell death, as in HeLa cells. Consistent over-expression of a caspase-cleaved product, a 15 kDa actin fragment, had no effect on TNF-induced necrosis of L929 cell. By contrast, TNF-induced mitochondria clustering and ROS production were dramatically reduced in Actin(mut) cells, indicating that actin-deficiency-mediated TNF resistance is most likely due to impaired mitochondrial responses to TNF stimulation. Our findings suggest that a full complement of actin is required for transduction of a cell death signal to mitochondria in TNF-treated L929 cells.

Involvement of Ras/extracellular signal-regulated kinase, but not Akt pathway in risedronate-induced apoptosis of U937 cells and its suppression by cytochalasin B

Although risedronate, a nitrogen containing bisphosphonate (BPs), strongly inhibits bone resorption by enhanced apoptosis of osteoclasts, its mechanism remained unclear. In this study, we investigated the molecular mechanism of risedronate-induced apoptosis of U937 cells, with a focus on extracellular signal-regulated kinase 1/2 (ERK 1/2) and protein kinase B (Akt) pathways, mitochondria-mediated apoptosis, and the effect of disruption of the actin cytoskeleton. Risedronate facilitated the relocation of Ras from membrane to cytosol through inhibited isoprenylation. Accordingly, risedronate suppressed the phosphorylation of ERK 1/2, a downstream survival signaling kinase of Ras, affected the intracellular distribution of Bcl-xL, and induced the mitochondrial membrane depolarization, cytochrome c release, activated caspase cascade and DNA fragmentation. The risedronate-induced apoptosis was effectively suppressed with cyclosporine A plus trifluoperazine, potent inhibitors of mitochondrial membrane permeability transition (MPT). The risedronate-induced apoptosis was independent of Akt, another cAMP-dependent survival signaling kinase. Risedronate facilitated dephosphorylation of Bad at Ser112, an ERK phosphorylation site, but not at Ser136, an Akt phosphorylation site. All of these apoptosis-related changes induced by risedronate were strongly suppressed by cytochalasin B, an inhibitor of actin filament polymerization. These results indicate that risedronate-induced apoptosis in U937 cells involves Ras/ERK, but not Akt signaling pathway, and is dependent on MPT, and that disruption of the actin cytoskeleton inhibits the risedronate-induced apoptosis at its early step.

APS, an adaptor molecule containing PH and SH2 domains, has a negative regulatory role in B cell proliferation

Adaptor molecule containing PH and SH2 domains (APS) is an intracellular adaptor protein that forms part of an adaptor family along with Lnk and SH2-B. APS transcripts are expressed in various tissues including brain, kidney, and muscle, as well as in splenic B cells but not in T cells. We investigated the functions of APS in B cell development and activation by generating APS-transgenic (APS-Tg) mice that overexpressed APS in lymphocytes. The number of B-1 cells in the peritoneal cavity was reduced in APS-Tg mice, as were B-2 cells in the spleen. B cell development in the bone marrow was partially impaired at the transition stage from proliferating large pre-B to small pre-B cells. B cell proliferation induced by B cell receptor (BCR) crosslinking but not by other B cell mitogens was also impaired in APS-Tg mice. APS co-localized with BCR complexes and filamentous actin in activated APS-Tg B cells. Thus, APS appears to play novel negative regulatory roles in BCR signaling, actin reorganization pathways, and control of compartment sizes of B-lineage cells.

Lysophosphatidic acid inhibits anti-Fas-mediated apoptosis enhanced by actin depolymerization in epithelial ovarian cancer

Conflicting reports exist on the effect of actin depolymerization in anti-Fas-induced apoptosis. Lysophosphatidic acid (LPA) has been found to inhibit apoptosis in variable cell types. In this study, we evaluated LPA's protective effects on anti-Fas-induced apoptosis enhanced by actin depolymerization and possible mechanisms in epithelial ovarian cancer. OVCAR3 cells were pretreated with vehicle or LPA, then treated with Cytochalasin D (Cyto D), followed with anti-Fas mAb to induce apoptosis. Cells were stained with apoptotic markers and analyzed by flow cytometry. We report that LPA inhibited anti-Fas-induced apoptosis enhanced by actin depolymerization. Immunoprecipition of Fas death-inducing signaling complex (DISC) and Western blot suggested that the actin depolymerization accelerated caspase-8 activation, while LPA inhibited the association and activation of caspase-8 at the DISC. LPA inhibited caspase-3 and 7 activation induced by anti-Fas and/or Cyto D in cytosols. Phosphorylation of ERK and Bad112 by LPA may play a role in preventing caspase-3 activation through mitochondrial pathway induced by Cyto D. Our investigation found that LPA inhibited anti-Fas-induced apoptosis enhanced by actin depolymerization, and LPA may protect epithelial ovarian cancer from immune cell attack and cytoskeleton disrupting reagents induced apoptosis through multiple pathways.

Binary bacterial toxins: biochemistry, biology, and applications of common Clostridium and Bacillus proteins

Certain pathogenic species of Bacillus and Clostridium have developed unique methods for intoxicating cells that employ the classic enzymatic "A-B" paradigm for protein toxins. The binary toxins produced by B. anthracis, B. cereus, C. botulinum, C. difficile, C. perfringens, and C. spiroforme consist of components not physically associated in solution that are linked to various diseases in humans, animals, or insects. The "B" components are synthesized as precursors that are subsequently activated by serine-type proteases on the targeted cell surface and/or in solution. Following release of a 20-kDa N-terminal peptide, the activated "B" components form homoheptameric rings that subsequently dock with an "A" component(s) on the cell surface. By following an acidified endosomal route and translocation into the cytosol, "A" molecules disable a cell (and host organism) via disruption of the actin cytoskeleton, increasing intracellular levels of cyclic AMP, or inactivation of signaling pathways linked to mitogen-activated protein kinase kinases. Recently, B. anthracis has gleaned much notoriety as a biowarfare/bioterrorism agent, and of primary interest has been the edema and lethal toxins, their role in anthrax, as well as the development of efficacious vaccines and therapeutics targeting these virulence factors and ultimately B. anthracis. This review comprehensively surveys the literature and discusses the similarities, as well as distinct differences, between each Clostridium and Bacillus binary toxin in terms of their biochemistry, biology, genetics, structure, and applications in science and medicine. The information may foster future studies that aid novel vaccine and drug development, as well as a better understanding of a conserved intoxication process utilized by various gram-positive, spore-forming bacteria.

Increased numbers of B-1 cells and enhanced responses against TI-2 antigen in mice lacking APS, an adaptor molecule containing PH and SH2 domains

APS (adaptor molecule containing PH and SH2 domains) is an intracellular adaptor protein that forms an adaptor family along with Lnk and SH2-B. While experiments using cultured cell lines have demonstrated that APS is phosphorylated in response to various stimuli, its in vivo functions remain unclear. We attempted to determine the physiological roles of APS by generating APS-deficient (APS(-/-)) mice. APS(-/-) mice were viable and fertile and showed no abnormalities or growth retardation. Immunologically, APS(-/-) mice showed normal development and distribution of lymphocytes and myeloid cells, except for increased numbers of B-1 cells in the peritoneal cavity. APS(-/-) mice exhibited an enhanced humoral immune response against trinitrophenol-Ficoll, a thymus-independent type 2 antigen, while APS(-/-) B-2 cells exhibited normal proliferative responses and tyrosine phosphorylation of intracellular proteins upon B-cell receptor (BCR) cross-linking. APS colocalized with filamentous actin (F-actin) accumulated during the capping of BCRs in APS-transgenic B cells. After BCR stimulation, F-actin contents were lower in APS(-/-) B-1 cells than in wild-type B-1 cells. Our results indicate that APS might have a novel regulatory role in actin reorganization and control of B-1 cell compartment size.

Actin cytoskeletal architecture regulates nitric oxide-induced apoptosis, dedifferentiation, and cyclooxygenase-2 expression in articular chondrocytes via mitogen-activated protein kinase and protein kinase C pathways

Nitric oxide (NO) in articular chondrocytes regulates differentiation, survival, and inflammatory responses by modulating ERK-1 and -2, p38 kinase, and protein kinase C (PKC) alpha and zeta. In this study, we investigated the effects of the actin cytoskeletal architecture on NO-induced dedifferentiation, apoptosis, cyclooxygenase (COX)-2 expression, and prostaglandin E2 production in articular chondrocytes, with a focus on ERK-1/-2, p38 kinase, and PKC signaling. Disruption of the actin cytoskeleton by cytochalasin D (CD) inhibited NO-induced apoptosis, dedifferentiation, COX-2 expression, and prostaglandin E2 production in chondrocytes cultured on plastic or during cartilage explants culture. CD treatment did not affect ERK-1/-2 activation but blocked the signaling events necessary for NO-induced dedifferentiation, apoptosis, and COX-2 expression such as activation of p38 kinase and inhibition of PKCalpha and -zeta. CD also suppressed activation of downstream signaling of p38 kinase and PKC, such as NF-kappaB activation, p53 accumulation, and caspase-3 activation, which are necessary for NO-induced apoptosis. NO production in articular chondrocytes caused down-regulation of phosphatidylinositol (PI) 3-kinase and Akt activities. The down-regulation of PI 3-kinase and Akt was blocked by CD treatment, and the CD effects on apoptosis, p38 kinase, and PKCalpha and -zeta were abolished by the inhibition of PI 3-kinase with LY294002. Our results collectively indicate that the actin cytoskeleton mediates NO-induced regulatory effects in chondrocytes by modulating down-regulation of PI 3-kinase and Akt, activation of p38 kinase, and inhibition of PKCalpha and -zeta

Inhibition of actin polymerization enhances commitment to and execution of apoptosis induced by withdrawal of trophic support

We have previously shown, using jasplakinolide, that stabilization of the actin cytoskeleton enhanced apoptosis induced upon cytokine withdrawal (Posey and Bierer [1999] J. Biol. Chem. 274:4259-4265). It remained possible, however, that a disruption in the regulation of actin dynamics, and not simply F-actin stabilization, was required to affect the transduction of an apoptotic signal. We have now tested the effects of cytochalasin D, a well-characterized agent that promoted actin depolymerization. Actin depolymerization did not affect CD95 (Fas)-induced death of Jurkat T cells in the time course studied but did enhance the commitment to cytokine withdrawal-induced apoptosis of factor-dependent cell lines. The induction of cell death was not the result of direct cytoskeletal collapse, since treatment of the cells with cytochalasin D in the presence of IL-2 did not promote death. As with jasplakinolide, the enhancement of commitment to apoptosis could be delayed by overexpression of the anti-apoptotic protein Bcl-x(L), but, unlike jasplakinolide, cytochalasin D modestly affected the "execution" stage of apoptosis as well. Taken together, these results suggest that changes in actin dynamics, i.e., the rate of actin polymerization and depolymerization, modulate the transduction of the apoptotic signal committing lymphocytes, withdrawn from required growth factors, to the death pathway.

B cell receptor signaling and autoimmunity

The immune receptors of lymphocytes are able to sense the nature of bound ligands. Through coupled signaling pathways the generated signals are appropriately delivered to the intracellular machinery, allowing specific functional responses. A central issue in contemporary immunology is how the fate of B lymphocytes is determined at the successive developmental stages and how the B cell receptor distinguishes between signals that induce immune response or tolerance. Experiments with mice expressing transgenes or lacking signal transduction molecules that lead to abnormal lymphocyte development and/or response are providing important clues to the mechanisms that regulate signaling thresholds at different developmental stages. The studies are also revealing novel potential mechanisms of induction of autoimmunity, which may have a bearing on the understanding of human diseases.

Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is an X-linked immunodeficiency characterized by thrombocytopenia with small platelets, eczema, recurrent infections, autoimmune disorders, IgA nephropathy, and an increased incidence of hematopoietic malignancies. The identification of the responsible gene, WASP (Wiskott-Aldrich Syndrome Protein), revealed clinical heterogeneity of the syndrome, and showed that X-linked thrombocytopenia without, or with only mild immunodeficiency and eczema, is also caused by mutations of WASP. The study of WASP and its mutations demonstrates how a single gene defect can cause multiple and complex clinical symptoms.