Lymphoma models for B cell activation and tolerance. X. Anti-mu-mediated growth arrest and apoptosis of murine B cell lymphomas is prevented by the stabilization of myc

IL-4 protects the B-cell lymphoma cell line CH31 from anti-IgM-induced growth arrest and apoptosis: contribution of the PI-3 kinase/AKT pathway

Interleukin-4 (IL-4) promotes lymphocyte survival and protects primary lymphomas from apoptosis. Previous studies reported differential requirements for the signal transducer and activator of transcription 6 (STAT6) and IRS2/phosphatidylinositol 3 kinase (PI-3K) signaling pathways in mediating the IL-4-induced protection from Fas-mediated apoptosis. In this study, we characterized IL-4-activated signals that suppress anti-IgM-mediated apoptosis and growth arrest of CH31, a model B-cell lymphoma line. In CH31, anti-IgM treatment leads to the loss of mitochondrial membrane potential, phospho-Akt, phospho-CDK2, and c-myc protein. These losses are followed by massive induction of p27(Kip1) protein expression, cell cycle arrest, and apoptosis. Strikingly, IL-4 treatment prevented or reversed these changes. Furthermore, IL-4 suppressed the activation of caspases 9 and 3, and, in contrast to previous reports, induced the phosphorylation (deactivation) of BAD. IL-4 treatment also induced expression of BclxL, a STAT6-dependent gene. Pharmacologic inhibitors and dominant inhibitory forms of PI-3K and Akt abrogated the anti-apoptotic function of IL-4. These results suggest that the IL-4 receptor activates several signaling pathways, with the Akt pathway playing a major role in suppression of the apoptotic program activated by anti-IgM.

Cutting Edge: TFII-I controls B cell proliferation via regulating NF-kappaB

The multifunctional transcription factor TFII-I physically and functionally interacts with Bruton's tyrosine kinase in murine B cells. However, the downstream functions of TFII-I in B cells are unknown. Toward achieving this goal, we established stable posttranscriptional silencing of TFII-I in WEHI-231 immature murine B cells, which undergoes growth arrest and apoptosis either upon anti-IgM or TGF-beta signaling. In this study, we show that TFII-I promotes growth arrest of cells in a signal-dependent manner. Unlike control cells, B cells exhibiting loss of TFII-I function fail to undergo arrest upon signaling due to up-regulation of c-Myc expression and concomitant down-regulation of both p21 and p27. Loss of TFII-I is also associated with simultaneous increase in nuclear c-rel and decrease in p50 homodimer binding. Thus, besides controlling c-myc transcription, TFII-I controls B cell proliferation by regulating both nuclear translocation of c-rel and DNA-binding activity of p50 NF-kappaB.

Stage-specific expression of DNaseγ during B-cell development and its role in B-cell receptor-mediated apoptosis in WEHI-231 cells

Here, we describe the non-redundant roles of caspase-activated DNase (CAD) and DNasegamma during apoptosis in the immature B-cell line WEHI-231. These cells induce DNA-ladder formation and nuclear fragmentation by activating CAD during cytotoxic drug-induced apoptosis. Moreover, these apoptotic manifestations are accompanied by inhibitor of CAD (ICAD) cleavage and are abrogated by the constitutive expression of a caspase-resistant ICAD mutant. No such nuclear changes occur during oxidative stress-induced necrosis, indicating that neither CAD nor DNasegamma functions under necrotic conditions. Interestingly, the DNA-ladder formation and nuclear fragmentation induced by B-cell receptor ligation occur in the absence of ICAD cleavage and are not significantly affected by the ICAD mutant. Both types of nuclear changes are preceded by the upregulation of DNasegamma expression and are strongly suppressed by 4-(4,6-dichloro-[1, 3, 5]-triazin-2-ylamino)-2-(6-hydroxy-3-oxo-3H-xanthen-9-yl)-benzoic acid (DR396), which is a specific inhibitor of DNasegamma. Our results suggest that DNasegamma provides an alternative mechanism for inducing nuclear changes when the working apoptotic cascade is unsuitable for CAD activation.

Fate decisions regulating bone marrow and peripheral B lymphocyte development

In adult mammals, bone marrow pluripotent hematopoietic stem cells generate B lymphoid-specified progeny that progress through a series of well-characterized stages before generating B-cell receptor expressing B lymphocytes. These functionally immature B lymphocytes then migrate to the spleen wherein they differentiate through transitional stages into follicular or marginal zone B lymphocytes capable of responding to T-dependent and -independent antigens, respectively. During the terminal stages of B lymphocyte development in the bone marrow, as well as immediately following egress into the peripheral compartments, B lymphocytes are counterselected to eliminate B lymphocytes with potentially dangerous self-reactivity. These developmental and selection events in the bone marrow and periphery are dependent on the integration of intrinsic genetic programs with extrinsic microenvironmental signals that drive progenitors toward increasing B lineage commitment and maturation. This chapter provides a comprehensive overview of the various stages of primary and secondary B lymphocyte development with an emphasis on the selection processes that affect decisions at critical checkpoints. Our intent is to stress the concept that at many steps in the developmental process leading to a mature immunocompetent B lymphocyte, B lineage cells are integrating multiple and different signaling inputs that are translated into specific and appropriate cell fate decisions.

Regulation of Epstein-Barr virus latency type by the chromatin boundary factor CTCF

Epstein Barr virus (EBV) can establish distinct latency types with different growth-transforming properties. Type I latency and type III latency can be distinguished by the expression of EBNA2, which has been shown to be regulated, in part, by the EBNA1-dependent enhancer activity of the origin of replication (OriP). Here, we report that CTCF, a chromatin boundary factor with well-established enhancer-blocking activity, binds to EBV sequences between the OriP and the RBP-Jkappa response elements of the C promoter (Cp) and regulates transcription levels of EBNA2 mRNA. Using DNA affinity, electrophoretic mobility shift assay, DNase I footprinting, and chromatin immunoprecipitation (ChIP), we found that CTCF binds both in vitro and in vivo to the EBV genome between OriP and Cp, with an approximately 50-bp footprint at EBV coordinates 10515 to 10560. Deletion of this CTCF binding site in a recombinant EBV bacterial artificial chromosome (BAC) increased EBNA2 transcription by 3.5-fold compared to a wild-type EBV BAC. DNA affinity and ChIP showed more CTCF binding at this site in type I latency cell lines (MutuI and KemI) than in type III latency cell lines (LCL3456 and Raji). CTCF protein and mRNA expression levels were higher in type I than type III cell lines. Short interfering RNA depletion of CTCF in type I MutuI cells stimulated EBNA2 mRNA levels, while overexpression of CTCF in type III Raji cells inhibited EBNA2 mRNA levels. These results indicate that increased CTCF can repress EBNA2 transcription. We also show that c-MYC, as well as EBNA2, can stimulate CTCF mRNA levels, suggesting that CTCF levels may contribute to B-cell differentiation as well as EBV latency type determination.

c-Jun N-terminal kinase (JNK) is required for survival and proliferation of B-lymphoma cells

Several primary murine and human B lymphomas and cell lines were found to constitutively express high levels of the activated form of c-jun N-terminal kinase (JNK), a member of the mitogen-activated protein (MAP) kinase family. Proliferation of murine B lymphomas CH31, CH12.Lx, BKS-2, and WEHI-231 and the human B lymphomas BJAB, RAMOS, RAJI, OCI-Ly7, and OCI-Ly10 was strongly inhibited by SP600125, an anthrapyrazolone inhibitor of JNK, in a dose-dependent manner. The lymphoma cells underwent apoptosis and arrested at the G2/M phase of cell cycle. Furthermore, JNK-specific small interfering RNA (siRNA) inhibited the growth of both murine and human B lymphomas. Thus in the B-lymphoma model, JNK appears to have a unique prosurvival role. Survival signals provided by CD40 and interleukin-10 (IL-10) together reversed the growth inhibition induced by the JNK inhibitor. c-Myc protein levels were reduced in the presence of both SP600125 and JNK-specific siRNA, and CD40 ligation restored c-Myc levels. Moreover, Bcl-xL rescued WEHI-231 cells from apoptosis induced by the JNK inhibitor. The JNK inhibitor also reduced levels of early growth response gene-1 (Egr-1) protein, and overexpressing Egr-1 partially rescued lymphoma cells from apoptosis. Thus, JNK may act via c-Myc and Egr-1, which were shown to be important for B-lymphoma survival and growth.

The irinotecan/5-fluorouracil combination induces apoptosis and enhances manganese superoxide dismutase activity in HT-29 human colon carcinoma cells

BACKGROUND

We examined whether induction of apoptosis and Mn-superoxide dismutase (Mn-SOD) and Cu,Zn-superoxide dismutase (Cu,Zn-SOD) activities were involved in the greater cytotoxicity of the irinotecan (CPT-11)/5-fluorouracil (5-FU) combination for human colon cancer cells when compared to both drugs alone.

METHODS

HT-29 and SNU-C4 human colon carcinoma cell lines were treated with 5-FU and CPT-11, then apoptosis was evaluated by flow cytometry and SOD activities were determined by polyacrylamide gel electrophoresis.

RESULTS

Enhanced apoptosis of HT-29 cells was observed with all treatments containing 5-FU in SNU-C4 cells; however, in HT-29 cells, apoptosis was enhanced only with the CPT-11/5-FU combination. In the SNU-C4 cell line, none of the treatments exerted a significant effect on Cu,Zn-SOD or Mn-SOD activity. However, in HT-29 cells, the CPT-11/5-FU combination enhanced Mn-SOD activity when compared to cells treated with CPT-11 alone. Nevertheless, the combined treatment did not interfere with Cu,Zn-SOD activity.

CONCLUSION

Treatment with the CPT-11/5-FU combination may promote in HT-29 cell apoptosis by enhancing Mn-SOD activity.

Differential c-Myc responsiveness to B cell receptor ligation in B cell-negative selection

Responsiveness of c-Myc oncogene to B cell receptor ligation has been implicated in the induction of apoptosis in transformed and normal immature B cells. These studies provided compelling evidence to link the c-Myc oncogene with the process of negative selection in B-lymphocytes. However, in addition to apoptosis, B cell-negative selection has been shown to occur by secondary Ig gene rearrangements, a mechanism called receptor editing. In this study, we assessed whether differential c-Myc responsiveness to B cell receptor (BCR) ligation is associated with the mechanism of negative selection in immature B cells. Using an in vitro bone marrow culture system and an Ig-transgenic mouse model (3-83) we show here that c-Myc is expressed at low levels throughout B cell development and that c-Myc responsiveness to BCR ligation is developmentally regulated and increased with maturation. Furthermore, we found that the competence to mount c-Myc responsiveness upon BCR ligation is important for the induction of apoptosis and had no effect on the process of receptor editing. Therefore, this study suggests an important role of c-Myc in promoting and/or maintaining B cell development and that compartmentalization of B cell tolerance may also be developmentally regulated by differential c-Myc responsiveness.

Post-transcriptional regulation of gene expression by degradation of messenger RNAs

Recent evidence suggests that gene expression may be regulated, at least in part, at post-transcriptional level by factors inducing the extremely rapid degradation of messenger RNAs. These factors include reactions between adenyl-uridyl-rich elements (AREs) of the relevant mRNA and either specific proteins that bind to these elements or exosomes. This review deals with examples of the proteins (AU-rich binding proteins, AUBPs) and exosomes, which have been shown to form complexes with AREs and bring about rapid degradation of the relevant mRNA, and with certain other factors, which protect the RNA from such degradation. The biochemical and physiological factors underlying the stability of messenger RNAs carrying the ARE motifs will be reviewed in the light of their emerging significance for cell physiology, human pathology, and molecular medicine. We also consider the possible application of the results of recent insights into the mechanisms to pharmacological interventions to prevent or cure disorders, especially developmental disorders, which the suppression of gene expression may bring about. Molecular targeting of specific steps in protein degradation by synthetic compounds has already been utilized for the development of pharmacological therapies.

IL-21 induces the apoptosis of resting and activated primary B cells

Cytokines play an important role in regulating the development and homeostasis of B cells by controlling their viability. In this study, we show that the recently described T cell-derived cytokine IL-21 induces the apoptosis of resting primary murine B cells. In addition, the activation of primary B cells with IL-4, LPS, or anti-CD40 Ab does not prevent IL-21-mediated apoptosis. The induction of apoptosis by IL-21 correlates with a down-regulation in the expression of Bcl-2 and Bcl-x(L), two antiapoptotic members of the Bcl-2 family. Furthermore, the reconstitution of Bcl-x(L) or Bcl-2 expression protects primary B cells from IL-21-induced apoptosis. In addition, a short-term preactivation of B cells with anti-CD40 Ab confers protection from IL-21-mediated apoptosis through the up-regulation of Bcl-x(L). These studies reveal a novel pathway that mediates B cell apoptosis via the IL-21R and suggest that IL-21 may play a role in regulating B cell homeostasis.

Daxx enhances Fas-mediated apoptosis in a murine pro-B cell line, BAF3

Daxx has been shown to play an essential in type I interferon (IFN-alpha/beta)-mediated suppression of B cell development and apoptosis. Recently, we demonstrated that Tyk2 is directly involved in IFN signaling for the induction and nuclear translocation of Daxx, which may result in growth arrest and/or apoptosis of B lymphocyte progenitors. To clarify the mechanism of Daxx-mediated apoptosis signaling in B lymphocyte progenitors, here we introduced an efficient suicide switch in a murine pro-B cell line, BAF3, by expressing FK506-binding protein-fused Fas intracellular domain (FKBP-Fas) and Daxx. It allows us to monitor Fas/Daxx-mediated signal by induction of Fas dimerization with the dimerizer drug AP20187. AP20187-mediated Fas dimerization induced not only apoptosis but also Jun N-terminal kinase (JNK) activation. However, AP20187 had no effect on cells expressing either Fas or Daxx only. Furthermore, expression of a JNK inhibitor, the JNK-binding domain of JIP-1, resulted in resistance to AP20187-mediated apoptosis in cells expressing FKBP-Fas and Daxx. These results imply that our novel suicide switch system may provide a powerful tool to delineate or identify the signaling molecules for Daxx-mediated apoptotic machinery in B lymphocyte progenitors through JNK activation.

CTCF functions as a critical regulator of cell-cycle arrest and death after ligation of the B cell receptor on immature B cells

The WEHI 231 B cell lymphoma is used as a model of self-tolerance by clonal deletion because B cell receptor (BCR) ligation results in apoptosis. Two critical events precede cell death: an early rise and fall in expression of MYC and cell-cycle arrest associated with enhanced expression of p21, p27, and p53. CTCF is a transcription factor identified as a repressor of MYC recently shown to cause cell growth inhibition. The present studies demonstrate that BCR ligation of WEHI 231 as well as of normal immature B cells greatly increased expression of CTCF in association with down-regulation of MYC followed by growth arrest and cell death. Conditional expression of CTCF in WEHI 231 mimicked BCR ligation with activated cells showing repressed expression of MYC, enhanced expression of p27, p21, p53, and p19(ARF), and inhibition of cell growth and induction of apoptosis. In keeping with a central role for CTCF in control of B cell death, conditional expression of a CTCF antisense construct in WEHI 231 resulted in inhibition of p27, p21, p53, and p19(ARF) in association with enhanced expression of MYC. Activation of the endogenous CTCF locus by BCR ligation was also mimicked by three other routes to apoptotic death in WEHI 231: inhibition of the phosphoinositide 3-kinase or mTORFRAP signaling cascades and treatment with transforming growth factor (TGF)-beta. Rapid activation of CTCF by BCR ligation or treatment with TGF-beta was suppressed by ligation of CD40. These results demonstrate that CTCF is a common determinant to different pathways of death signaling in immature B cells.

Cutting edge: tyk2 is required for the induction and nuclear translocation of Daxx which regulates IFN-alpha-induced suppression of B lymphocyte formation

IFN-alpha inhibits B lymphocyte development, and the nuclear protein Daxx has been reported to be essential for this biological activity. We show in this study that IFN-alpha inhibits the clonal proliferation of B lymphocyte progenitors in response to IL-7 in wild-type, but not in tyk2-deficient, mice. In addition, the IFN-alpha-induced up-regulation and nuclear translocation of Daxx are completely abrogated in the absence of tyk2. Therefore, tyk2 is directly involved in IFN-alpha signaling for the induction and translocation of Daxx, which may result in B lymphocyte growth arrest and/or apoptosis.

The protective effect of phorbol esters on Fas-mediated apoptosis in T cells. Transcriptional and postranscriptional regulation

Phorbol esters are tumor promoters that bind and activate both conventional and new Protein kinase C (PKC) isoforms. In various circumstances, PKC-dependent signaling pathways can promote cell survival and protect against cell death. This was first analysed in Jurkat T cells where Phorbol Myristate Acetate (PMA) was found to inhibit Fas-mediated apoptosis as judged by DiOC6(3) staining, caspase activation and DNA fragmentation, indicating that PMA exerts its protective effect upstream or at the mitochondrial level in these cells. PMA activated most of the main kinase pathways in T cells such as PKCs, p42/44MAPK, p38MAPK and p90Rsk but not JNK and Akt. A pharmacological approach allowed us to identify that nPKCs are both necessary and likely sufficient to promote T cell survival. Besides this post-transcriptional regulation, nPKCs may also regulate apoptosis at the transcriptional level. cDNA arrays were used to identify a set of genes whose expression was modulated in death versus survival conditions. Following PMA treatment, expression of Mcl-1 and Bcl-x increased while that of c-Myc was significantly reduced. Moreover, survivin expression decreased upon CH11 or PMA treatment. c-Myc, survivin and Bcl-x modulation seems to be regulated at the transcriptional level while decrease in Mcl-1 protein in CH11-treated cells resulted especially from a caspase-dependent proteolysis. Taken together, our data demonstrate that PMA-mediated inhibition of apoptosis is a complex process that is integrated at both the transcriptional and post-transcriptional level and point out to the potential role of Mcl-1, Bcl-x, c-Myc and survivin in this process.

ZnCl(2) prevents c-myc repression and apoptosis in serum-deprived Syrian hamster embryo cells

In order to understand the c-myc implication in the apoptotic process better, we investigated the influence of ZnCl(2) on its expression in normal and transformed Syrian hamster embryo (SHE) cells in relation to apoptosis induced by serum withdrawal. Normal primary SHE cells exposed to a serum-free medium undergo rapid apoptosis characterised by a dramatic down-regulation of c-myc transcription. In these normal cells treated with ZnCl(2), c-myc expression is maintained in serum-starved conditions while apoptosis is inhibited. The results shed light on the involvement of c-myc expression in the survival of normal cells in the absence of growth factors. The regulation of c-myc expression appears to be influenced by zinc treatment as an inhibitor of apoptosis, but mechanisms sustaining the level of c-myc transcription remain to be demonstrated. The hypothesis that maintenance of c-myc expression allows cells to escape apoptosis is in accordance with results in transformed SHE cells that underwent low apoptosis and poor down-regulation of c-myc in serum-deprived conditions.

TGF-beta-induced apoptosis is mediated by the adapter protein Daxx that facilitates JNK activation

Transforming growth factor-beta (TGF-beta) is a multifunctional growth factor that has a principal role in growth control through both its cytostatic effect on many different epithelial cell types and its ability to induce programmed cell death in a variety of other cell types. Here we have used a screen for proteins that interact physically with the cytoplasmic domain of the type II TGF-beta receptor to isolate the gene encoding Daxx - a protein associated with the Fas receptor that mediates activation of Jun amino-terminal kinase (JNK) and programmed cell death induced by Fas. The carboxy-terminal portion of Daxx functions as a dominant-negative inhibitor of TGF-beta-induced apoptosis in B-cell lymphomas, and antisense oligonucleotides to Daxx inhibit TGF-beta-induced apoptosis in mouse hepatocytes. Furthermore, Daxx is involved in mediating JNK activation by TGF-beta. Our findings associate Daxx directly with the TGF-beta apoptotic-signalling pathway, and make a biochemical connection between the receptors for TGF-beta and the apoptotic machinery.

The effect of cytokines on the activation-induced apoptosis of B cells in autoimmune NZB x NZW F1 mice

Programmed cell death (apoptosis) is an essential process in the development of various tissues and its involvement has been proposed for the elimination of self-reactive immature T and B lymphocytes when self antigens are first encountered. In order to further investigate the role of apoptosis in the pathogenesis of autoimmune disease, the apoptosis of lipopolysaccharide (LPS)-activated B cells, peritoneal cells from NZB x NZW F1 (NZB/W F1) mice and nonautoimmune BALB/c mice were assayed using an in vitro culture system. Splenic B cells were isolated and then stimulated with LPS before further activated with crosslinking antimu antibody. In addition, the apoptosis of peritoneal cells induced by crosslinking antimu antibody was also analyzed. The data revealed that the specific apoptosis of both activated B cells and peritoneal cells induced by crosslinking antimu antibody was very similar comparing NZB/W F1 and nonautoimmune BALB/c mice. This activation-induced B-cells apoptosis could be rescued, however, with the addition of cytokines such as interleukin (IL)-5 or IL-10, to the culture. The results suggest that there is no endogenous defect in the apoptosis of activated B cells for autoimmune NZB/W F1 comparing nonautoimmune BALB/c mice. Notably, however, abnormally high levels of the type 2 T helper (Th2)-related cytokines such as IL-5 or IL-10 may play an important role in the abnormal expansion of activated B cells in autoimmune NZB/W F1 mice.

Myc down-regulation induces apoptosis in M14 melanoma cells by increasing p27(kip1) levels

In recent years, increasing evidence indicated the importance of a deregulated c-myc gene in the melanoma pathogenesis. We have previously demonstrated that treatment of melanoma cells with c-myc antisense oligodeoxynucleotides can inhibit cell proliferation and activate apoptosis. To gain insight into the mechanisms activated by Myc down-regulation, we have now developed an experimental model that allows modulating Myc protein expression in melanoma cells. This was achieved by originating stable melanoma cell clones expressing ecdysone-inducible c-myc antisense RNA. We show that the induction of c-myc antisense RNA in M14 melanoma cells leads to an inhibition of cell proliferation characterized by accumulation of cells in the G(1) phase of the cell cycle (up to 80%) and activation of apoptosis (50%). These data are associated with an increase of p27(kip1) levels and a significant reduction of the cdk2-associated kinase activity. In addition, we show that an ectopic overexpression of p27(kip1) in this experimental model can enhance the apoptotic rate. Our results indicate that down-regulation of Myc protein induces a G(1) arrest and activates apoptosis by increasing p27(kip1) content in melanoma cells, that are known to be defective for the p16-cyclinD/cdk4-pRb G(1) checkpoint. This is particularly relevant for identifying new therapeutic strategies based on the re-establishment of the apoptotic pathways in cancer cells.

Life and death decisions in B1 lymphoma cells

Crosslinking of surface immunoglobulin (Ig) receptors with anti-IgM (anti-mu) but not anti-IgD (anti-delta) antibodies causes growth arrest and apoptosis in several extensively characterized B1-like lymphoma cell lines. While anti-mu stimulates a transient increase in c-myc mRNA and protein expression, followed by a rapid decline below the baseline level, anti-delta only causes a moderate increase in the expression of this oncogene, which returns to baseline levels within 24-48 hours. However, signals downstream from anti-delta can be converted into an apoptotic pathway by modulating PI3K activity, suggesting that PI3K is a critical rheostat controlling survival signals in B1 cell lines. Anti-mu-induced down-regulation of c-Myc is followed in time with an increase in the cyclin dependent kinase inhibitor, p27Kip1, in all anti-mu sensitive lymphoma lines. This increase correlates with growth arrest and apoptosis. The anti-mu-mediated decrease in c-Myc, increase in p27Kip1, growth arrest and apoptosis, can all be prevented via CD40/CD40L signaling. Inhibition of caspase activation, on the other hand, prevents anti-mu-induced apoptosis, but has no effect on c-Myc, p27Kip1, and G1 arrest. Interestingly, we also found that steroids and retinoids can mimic anti-mu-mediated signaling and lead to a loss of c-Myc, an increase in p27Kip1, G1 arrest, and apoptosis. Together, these data suggest that modulation of c-Myc and p27Kip1 protein levels is crucial for the life versus death decisions in murine immature B1-like lymphoma cells lines.

Difference in apoptosis induction between surface IgD and IgM

In the classic 'two-signal' model for B cell activation, signal 1 through the antigen receptor plus signal 2 through lymphokine receptors and CD40 leads to proliferation, but signal 1 alone leads to tolerance or anergy. In a protocol designed to deliver signal 1 in vitro with anti-delta without signal 2, purified small dense B cells from untreated mice exposed to any of three monoclonal anti-delta antibodies or to polyclonal anti-delta in vitro showed modest S phase entry at 50 microg/ml. In contrast, at low doses (0.1-0.5 microg/ml) of anti-delta, there was no cell cycle entry at 64 h, but apoptosis was accelerated at 16 h. Polyclonal anti-mu and three monoclonal anti-mus did not show this early apoptosis induction. Anti-CD40 and IL-4 inhibited apoptosis in B cells treated with 0.5 microg/ml anti-delta and increased S phase entry at 10 microg/ml anti-delta. Low-dose anti-delta (but not anti-mu) induced increased B7-2 and class II MHC expression on a subset of B cells, many of which were in apoptosis. Larger transient increases in c-Myc and Egr-1 expression were seen with low-dose anti-delta than with anti-mu, followed by an abrupt fall below baseline, a sequence previously linked to apoptosis. There was no change in Bcl-2, Bcl-x(L) or Bax. These data suggest a functional difference between delta and mu cross-linking on resting spleen B cells. A BCR stimulus sufficient for early activation events, but insufficient for full G1 entry, may lead to apoptosis.

Analysis of C-MYC function in normal cells via conditional gene-targeted mutation

Germline inactivation of c-myc in mice causes embryonic lethality. Therefore, we developed a LoxP/Cre-based conditional mutation approach to test the role of c-myc in mouse embryonic fibroblasts (MEFs) and mature B lymphocytes. Cre expression resulted in reduced proliferation of wild-type MEFs, but c-Myc-deficient MEFs showed a further reduction. In contrast to fibroblasts, Cre expression had no apparent affect on wild-type B cell proliferation. Deletion of both c-Myc genes in B cells led to severely impaired proliferation in response to anti-CD40 plus IL-4. However, treated cells did upregulate several early activation markers but not CD95 or CD95 ligand. We discuss these findings with respect to potential c-Myc functions in proliferation and apoptosis and also discuss potential limitations in the Cre-mediated gene inactivation approach.

Activation-induced cell death in B lymphocytes

Upon encountering the antigen (Ag), the immune system can either develop a specific immune response or enter a specific state of unresponsiveness, tolerance. The response of B cells to their specific Ag can be activation and proliferation, leading to the immune response, or anergy and activation-induced cell death (AICD), leading to tolerance. AICD in B lymphocytes is a highly regulated event initiated by crosslinking of the B cell receptor (BCR). BCR engagement initiates several signaling events such as activation of PLCgamma, Ras, and PI3K, which generally speaking, lead to survival. However, in the absence of survival signals (CD40 or IL-4R engagement), BCR crosslinking can also promote apoptotic signal transduction pathways such as activation of effector caspases, expression of pro-apoptotic genes, and inhibition of pro-survival genes. The complex interplay between survival and death signals determines the B cell fate and, consequently, the immune response.

Glucocorticoid mediated transcriptional repression of c-myc in apoptotic human leukemic CEM cells

Suppression of c-myc has been implicated as a critical event in some glucocorticoid-evoked apoptotic systems. It is therefore of interest to understand the mechanism of glucocorticoid-regulation of the c-myc gene. In the present study, a detailed analysis of dexamethasone (Dex)-evoked regulation of the human c-myc gene in human leukemic CEM-C7 cells has been performed. Dex suppresses c-myc mRNA and immunoreactive protein expression in clone CEM-C7 and subclone CEM-C7-14 cells. Nuclear run-on assays suggested that the regulation occurred at the level of transcription initiation. The half-life of c-myc mRNA was approximately 30 min and its stability was not affected by Dex treatment. In addition, Dex suppressed luciferase gene expression driven by -2052 to +34 bp c-myc promoter in transfected CEM-C7-14 cells. This result further supports that c-myc gene is suppressed by Dex at the transcriptional level in apoptotic human leukemic cells.

Protooncogenes as mediators of apoptosis

Apoptosis has been well established as a vital biological phenomenon that is important in the maintenance of cellular homeostasis. Three major protooncogene families and their encoded proteins function as mediators of apoptosis in various cell types and are the subject of this chapter. Protooncogenic proteins such as c-Myc/Max, c-Fos/c-Jun, and Bcl-2/Bax utilize a synergetic effect to enhance their roles in the pro- or antiapoptotic action. These family members activate and repress the expression of their target genes, control cell cycle progression, and execute programmed cell death. Repression or overproduction of these protooncogenic proteins induces apoptosis, which may vary as a result of either cell type specificity or the nature of the apoptotic stimuli. The proapoptotic and antiapoptotic proteins exert their effects in the membrane of cellular organelles. Here they generate cell-type-specific signals that activate the caspase family of proteases and their regulators for the execution of apoptosis.

Antigen receptor signalling in apoptosis-resistant mutants of WEHI 231 cells

Ligation of membrane immunoglobulin M (mIgM) induces cell cycle arrest and apoptosis in the WEHI 231 B-lymphoma cell line. The molecular mechanisms which link receptor ligation and the nuclear events that underlie this response, have yet to be fully elucidated. Here we have examined the signals induced following mIgM cross-linking in variants of WEHI 231 that no longer undergo apoptosis in response to this stimulus. Tyrosine phosphorylation of cellular substrates in two of the variants is identical to that seen in wild-type cells but in one of the mutants, VS2.12, a restricted set of substrates becomes tyrosine phosphorylated. In a second variant (E8), mIgM cross-linking does not induce elevation of intracellular Ca2+, although tyrosine phosphorylation of PLCgamma2 is induced to an equivalent extent to that seen in WEHI 231 cells. A third variant, 2E10.F9, is resistant to apoptosis despite the fact that all signals analysed appear to be similar to those induced in wild-type cells. Our findings show that resistance to apoptosis can arise as a result of mutations affecting discrete stages of the mIgM signalling pathway. The mutant lines reported here show defects that have not yet been identified in previous studies and are likely to be useful tools in dissecting the signalling of cell death in B lymphocytes.

Increased p27Kip1 Cyclin-Dependent Kinase Inhibitor Gene Expression Following Anti-IgM Treatment Promotes Apoptosis of WEHI 231 B Cells

Engagement of the B cell receptor of WEHI 231 immature B cells leads sequentially to a drop in c-Myc, to induction of the cyclin-dependent kinase inhibitor p27Kip1, and finally to apoptosis. Recently we demonstrated that the drop in c-Myc expression promotes cell death, whereas the induction of p27 has been shown to lead to growth arrest. In this paper, we demonstrate that increased p27 expression also promotes apoptosis of WEHI 231 B cells. The rescue of WEHI 231 cells by CD40 ligand engagement of its receptor prevented the increase in p27 induction. Inhibition of p27-ablated apoptosis induced upon expression of antisense c-myc RNA. Furthermore, specific induction of p27 gene expression resulted in apoptosis of WEHI 231 cells. Lastly, inhibition of expression of c-Myc, upon induction of an antisense c-myc RNA vector, was sufficient to induce increased p27 levels and apoptosis. Thus, these findings define a signaling pathway during B cell receptor engagement in which the drop in c-Myc levels leads to an increase in p27 levels that promotes apoptosis.

Mechanisms and applications of immune stimulatory CpG oligodeoxynucleotides

Immune stimulation has been widely recognized as an undesirable side effect of certain antisense oligodeoxynucleotides (ODN) which can interfere with their therapeutic application. It is now clear that these dose-dependent immune stimulatory effects primarily result from the presence of an unmethylated CpG dinucleotide in particular base contexts ('CpG motif). The sequence-specific immune activation is not just an experimental artifact, but is actually a highly evolved immune defense mechanism whose actual 'goal' is the detection of microbial nucleic acids. In contrast to vertebrate DNA, in which CpG dinucleotides are 'suppressed' and are highly methylated, microbial genomes do not generally feature CpG suppression or methylation [1]. Immune effector cells such as B cells, macrophages, dendritic cells, and natural killer cells appear to have evolved pattern recognition receptors (PRR) that by binding the microbe-restricted structure of CpG motifs, trigger protective immune responses. Although the specific immune activation appears to have a variety of potential therapeutic applications, it is generally undesirable in antisense ODN. Immune stimulation may be avoided in antisense oligos by the selection of CpG-free target sequences, by the use of ODN backbones that do not support immune stimulation, or by selective modifications of the cytosine in any CpG dinucleotides.

The role of TGF-beta in growth, differentiation, and maturation of B lymphocytes

Transforming growth factor-beta (TGF-beta) affects B cells at all stages in development. It appears to be involved in lymphopoiesis and is required for the development of plasma cells secreting all secondary isotypes. Its ability to inhibit proliferation and stimulate apoptosis suggest that it may be involved both in germinal center development and regulation of B-cell proliferation at sites of high antigen load such as the gastrointestinal tract. Although TGF-beta appears to be required for the generation of B cells secreting secondary isotypes, it inhibits secretion of IgM and IgA from cells expressing those isotypes. In this regard, TGF-beta may alter the level of RNA processing factors either directly or indirectly by inhibiting progression through the cell cycle. One of the best characterized effects of TGF-beta is its ability to stimulate isotype switching to IgA in both mouse and man. There is some controversy concerning its mechanism of action in this process, but its critical role is without question. The controversy may stem in part from an inability to separate the effects of endogenous and exogenous TGF-beta in the multiple models of isotype switching. The influence of endogenous TGF-beta is perhaps best exemplified by analysis of production of the different classes of IgG in mouse strains producing different levels of TGF-beta.

The MYC dualism in growth and death

Over-expression of the transcription factor c-Myc immortalizes primary cells and transforms in co-operation with activated ras. Therefore, c-myc is considered a proto-oncogene. Since its discovery c-Myc has been shown to render cells growth factor independent, accelerates passage through G1 of the cell cycle, inhibits differentiation and elicits apoptosis. Whereas the effects on immortalization, proliferation and inhibition of differentiation are in conceivable accordance with gain of function, as it is defined for a proto-oncogene, its pro-apoptotic activity disables a straight forward explanation of the physiological role of c-Myc and suggests a highly complex contribution during development. The recent accomplishments in c-Myc research shed some light on the difficile regulatory network which keeps check on c-Myc activity such as by binding to proteins some of which are transcription factors for non-c-Myc targets. Moreover, it was shown that genes are targeted by c-Myc depending on the sequence of flanking regions adjacent to the E-box or in dependence on the availability of binding partners which is most probably specific to the cellular context. Cdc25A and ornithine decarboxylase, both described to be c-Myc targets, have been brought forward as downstream effectors in the induction of proliferation under serum rich conditions, or in the induction of apoptosis when serum factors are limited. These genes seem to be regulated by c-Myc in a cell type-specific manner. H-ferritin, IRP2 and telomerase are the most recently discovered direct targets of c-Myc. The regulation of H-ferritin and IRP2 might explain the potential of c-Myc to promote proliferation and the regulation of telomerase could be responsible for the immortalizing properties of c-Myc. In the future, H-ferritin and telomerase have to be analyzed whether or not these genes are also Myc targets in other cell systems. Although the intense research efforts regarding the function of c-Myc last already two decades the role of this gene is still enigmatic.

Transforming growth factor beta induces caspase 3-independent cleavage of alphaII-spectrin (alpha-fodrin) coincident with apoptosis

Transforming growth factor beta (TGF-beta) is a potent growth inhibitor and inducer of cell death in B-lymphocytes and is essential for immune regulation and maintenance of self-tolerance. In this report the mouse immature B cell line, WEHI 231, was used to examine the mechanisms involved in TGF-beta-mediated apoptosis. Induction of apoptosis is detected as early as 8 h after TGF-beta administration. Coincident with the onset of apoptosis, the cytoskeletal actin-binding protein, alphaII-spectrin (alpha-fodrin) is cleaved into 150-, 115-, and 110-kDa fragments. The broad spectrum caspase inhibitor (Boc-D-fmk (BD-fmk)) completely abolished TGF-beta-induced apoptosis and alphaII-spectrin cleavage. Caspase 3, although present in WEH1 231 cells, was not activated by TGF-beta, nor was its substrate, poly(ADP-ribose) polymerase. These results identify alphaII-spectrin as a novel substrate that is cleaved during TGF-beta-induced apoptosis. Our data provide the first evidence of calpain and caspase 3-independent cleavage of alphaII-spectrin during apoptosis and suggests that TGF-beta induces apoptosis and alphaII-spectrin cleavage via a potentially novel caspase. This report also provides the first direct evidence of caspase 3 activation in WEH1 231 cells and indicates that at least two distinct apoptotic pathways exist.

Signaling through the antigen receptor of B lymphocytes activates a p53-independent pathway of c-Myc-induced apoptosis

Deregulated expression of c-Myc has been shown to induce or enhance apoptosis in various different cell types. c-Myc requires p53 for apoptosis in some but not all the cell types, indicating heterogeneous mechanisms for c-Myc-induced apoptosis. In B lymphoma line WEHI-231, stable expression of c-Myc has been demonstrated to protect cells from BCR-mediated apoptosis. However, stable expression of c-Myc carrying pro-apoptotic functions may generate variant cells resistant to apoptosis. By utilizing an inducible system for c-Myc, we demonstrated here that deregulated expression of c-Myc induced apoptosis of WEHI-231 by itself, indicating that c-Myc induces apoptosis in WEHI-231 as is the case for other cell types. When transactivation of p53 was inactivated, WEHI-231 cells overexpressing c-Myc no longer underwent apoptosis in the absence of other stimuli, but showed markedly enhanced apoptosis in the presence of BCR ligation. These results indicate that deregulated c-Myc expression enhances apoptosis by a p53-independent pathway in the presence of BCR signaling but requires p53 for apoptosis in the absence of BCR crosslinking in WEHI-231. BCR ligation may thus activate a p53-independent pathway of c-Myc-induced apoptosis.

The Importance of Antibody-Specificity in Determining Successful Radioimmunotherapy of B-Cell Lymphoma

We report the radioimmunotherapy of mouse B-cell lymphoma, BCL1, using a panel of anti–B-cell monoclonal antibodies (MoAb) (anti-CD19, anti-CD22, anti-major histocompatibility complex (MHC) II, and anti-idiotype (Id) radiolabeled with 131-iodine. When administered early in disease (day 4), the 131I–anti-MHCII MoAb cured tumors as a result of targeted irradiation alone, the unlabeled MoAb being nontherapeutic. In contrast,131I–anti-Id, despite targeting irradiation and having therapeutic activity as an unconjugated antibody, protected mice for only 30 days; 131I–anti-CD19 and anti-CD22 were therapeutically inactive. Binding and biodistribution studies showed that the anti-Id, unlike anti-MHCII, MoAb was cleared from target cells in vivo and delivered 4 times less irradiation to splenic tumor. Treating later in the disease (day 14) increased tumor load and produced the expected reduction in therapeutic activity with the anti-MHCII, but surprisingly, allowed 131I–anti-Id to cure most mice. This unexpected potency of 131I–anti-Id late in the disease appeared to result from the direct cytotoxicity of the anti-Id MoAb, which was more active in established disease, in combination with targeted irradiation. We believe the ability of targeted irradiation and certain cytotoxic MoAb to work cooperatively against tumor in this way has important implications for the selection of reagents in radioimmunotherapy of B-cell lymphoma.

Overexpression of RelA causes G1 arrest and apoptosis in a pro-B cell line

NF-kappaB/Rel family proteins form a network of post-translationally regulated transcription factors that respond to a variety of extracellular stimuli and mediate distinct cellular responses. These responses include cytokine gene expression, regulated cell cycle activation, and both the protection from and induction of the cell death program. To examine the function of individual Rel family proteins in B cell development and resolve their role in the signaling of apoptosis, we used a tetracycline-regulated gene expression system to overexpress either c-Rel or RelA in the transformed pro-B cell line 220-8. Elevated levels of RelA, but not c-Rel, induced a G1 cell cycle arrest followed by apoptosis. Both the DNA binding and transactivation domains of RelA were required for this effect. When RelA was overexpressed in the immature B cell line WEHI 231 or the mature B cell line M12, neither cell cycle arrest nor apoptosis was evident. The differential effects of elevated RelA levels in these cell lines suggests that susceptibility to NF-kappaB-induced apoptosis may reflect a relevant selection event during B cell development.

Transforming growth factor beta1 inhibits Fas ligand expression and subsequent activation-induced cell death in T cells via downregulation of c-Myc

Activation-induced cell death (AICD) is a process that regulates the size and the duration of the primary immune T cell response. In this report, we investigated the mechanisms involved in the regulation of AICD by transforming growth factor beta1 (TGF-beta1). We found that TGF-beta1 decreased apoptosis of human T cells or T cell hybridomas after activation by anti-CD3. This decrease was associated with inhibition of Fas (Apo-1/CD95) ligand (FasL) expression, whereas Fas signaling was not affected by TGF-beta1. In parallel, TGF-beta1 inhibited c-Myc expression in T cell hybridomas, and ectopic expression of a chimeric molecule composed of c-Myc and the steroid binding domain of the estrogen receptor (Myc-ER) blocked both the inhibition of FasL and the decrease of AICD induced by TGF-beta1, providing that 4-hydroxytamoxifen was present. These results identify one mechanism by which TGF-beta1 blocks AICD to allow the clonal expansion of effector T cells and the generation of memory T cells during immune responses.

Apoptosis induced by oxysterol in CEM cells is associated with negative regulation of c-myc

Previously we have demonstrated that treatment of the human lymphoblastic leukemic CEM cells with 25-hydroxycholesterol (25OHC) induces apoptosis. In the present study, we show that both c-myc mRNA and c-Myc protein levels are reduced only in oxysterol-sensitive and not in oxysterol-resistant cells after treatment with concentrations of 25OHC that kill the sensitive CEM cells. The repression of c-Myc protein precedes c-myc mRNA reduction, and both events occur before the onset of cell death. Our data suggest that 25OHC-induced suppression of c-myc gene expression in CEM cells results from posttranscriptional regulation. These results demonstrate the regulation by an oxysterol of a gene/gene product important for cell growth and viability and an association between oxysterol-induced apoptosis of CEM cells and the negative regulation of c-myc.

The CD40 TRAF family member interacting motif carries the information to rescue WEHI 231 cells from anti-IGM-induced growth arrest

Engagement of the antigen receptor on WEHI 231 murine B lymphoma cells leads to growth arrest and induction of apoptosis. Concomitant signaling through CD40 sustains proliferation and rescues the cells from apoptosis. At the molecular level, CD40 has been shown to activate nuclear factor kappaB (NF-kappaB) and stress-activated protein kinase (SAPK). The aim of our present study was to define the stretch of the CD40 cytoplasmic tail responsible for mediating these effects in WEHI 231 cells. Using recombinant retroviruses with the enhanced green fluorescent protein as selection marker we transduced WEHI 231 cells with chimeric molecules consisting of the extracellular and transmembrane region of human CD40 or rat CD4 and selected portions of the murine CD40 tail. Chimeric molecules with cytoplasmic fragments encompassing the "CD40 tumor necrosis factor-associated factor family member interacting motif" (TIM) were able to sustain growth and to uphold NF-kappaB activity as efficiently as the whole intracellular region of CD40. While the potential of the motif relative to the whole cytoplasmic tail was independent of the heterologous part of the chimeras it was strongly influenced by its distance to the membrane. Placing the 17-amino acid stretch of the motif too close to the membrane, i. e. only two or four amino acids apart, destroyed its capacity to mitigate the anti-IgM effect. Activation of SAPK through the chimeric molecules always correlated with their ability to activate NF-kappaB activity and to rescue the cells from apoptosis induced by antigen receptor ligation. Our data indicate that CD40-TIM carries most if not all of the information needed to deliver the signals responsible for sustaining growth in anti-IgM-stimulated WEHI 231 cells.

Characterization of immature B cells by a novel monoclonal antibody, by turnover and by mitogen reactivity

The transit of immature to mature sIgM+ B cells, the life span, maturation kinetics and response to polyclonal activators have been analyzed with the help of a new mAb (493), that distinguishes immature, 493+ from mature, 493 B cells in a variety of mouse strains tested. Analysis of the turnover of immature 493+ B cells by bromodeoxyuridine (BrdU) labeling kinetics indicate that only 10-20 % of the cells reach the spleen as immature 493+ cells. The life span of 493+ B cells in bone marrow and spleen is around 4 days. BrdU chase experiments show that most of the immature cells in spleen enter the pool of mature, 493+ B cells where they gain a longer life span of 15-20 weeks. Immature and mature B cells respond equally well to LPS stimulation; anti-CD40, however, stimulates mature B cells better than immature B cells. IgM cross-linking of mature B cells results in proliferation, while it induces apoptosis in immature B cells. This apoptosis of immature cells can be inhibited by costimulation with anti-CD40 or by overexpression of bcl-2. We speculate that Ig receptor ligand-mediated apoptosis (negative selection) plays a major role in the transit of immature B cells from bone marrow to spleen, but only a minor role in the transit from immature B cells to mature B cells in the spleen.

Involvement of c-myc in the resistance of non-obese diabetic mice to glucocorticoid-induced apoptosis

Non-obese diabetic (NOD) mice spontaneously develop insulin-dependent diabetes mellitus (IDDM) as a consequence of autoimmune aggression of beta cells of the endocrine pancreas by T cells. T lymphocytes of NOD mice are resistant to apoptosis induced by glucocorticoids, or by starving or DNA-damaging treatments, a feature that was interpreted as being linked to escape of autoreactive T cells from thymic negative selection. c-myc is one of the gene targets of glucocorticoids (GC), its expression being down-regulated by the activated GC-GC receptor complex. We investigated here whether expression of Myc protein, in response to dexamethasone stimulation, was the same in NOD mice and in non-autoimmune strains, namely NON, BALB/c and C57Bl.6. We found a consistent increase in the levels of Myc protein after GC-treatment of lymphocytes of NOD mice, a finding that was in contrast to the down-regulation of c-myc that we observed in lymphocytes from mice not prone to diabetes. We also report that, rather than a absolute resistance to GC-induced cell death, NOD mice display a delayed apoptotic response to GC. We propose that the resistance of NOD mice lymphocytes to GC-induced apoptosis is because of inhibition of the repressive action of GC-GR complexes at the level of c-myc transcription. This deficient action of GC-GR results in increased production of nuclear Myc protein, peculiar to NOD mice cells, following their treatment with GC.

Apoptosis may be either suppressed or enhanced with strategic combinations of antineoplastic drugs or anti-IgM

A variety of drugs have been used to treat B-lymphocyte neoplasms, including both cell cycle-specific (CCS) and non-cell-cycle-specific drugs. Although the therapy for such cancers is complex and can include both types of drugs, the efficacy of these drugs in inducing cell death remains unclear. In this paper we have concentrated on specific CCS drugs and have examined their ability to induce programmed cell death (apoptosis) in Burkitt's lymphoma cell lines derived from patients. The CCS drugs chosen were hydroxyurea and aphidicolin (active in late G1, early S phase), the topoisomerase poisons camptothecin and etoposide (S, early G2 phase) and vincristine and Taxol (late G2, M phase). These choices allow comparison of two drugs with differing modes of action for each of the various phases of the cell cycle. Our results indicate that the variation in apoptosis between drugs that act at the same phase of the cell cycle is negligible. Both S/G2 and G2/M blockers are very potent at inducing apoptosis whereas G1/S blockers are ineffective in the induction of apoptosis. In addition, marked kinetic variations in the rate of apoptosis induction were observed, etoposide and camptothecin being more rapid in their action than the other agents. The order of effectiveness in inducing apoptosis on a kinetic basis was S/G2 agents >> G2/M agents >> G1/S agents. In this study we have also found that growth inhibition was induced by all the CCS agents chosen and by anti-IgM in various Burkitt's lymphoma lines. Furthermore c-myc was down-regulated under similar conditions. Since apoptosis was only selectively induced by some of the CCS agents, it implies c-myc expression is associated with growth regulation and c-myc down-regulation is an insufficient condition for the induction of apoptosis. In addition, cotreatments using the CCS and other agents revealed the following: Cotreatment using two CCS drugs which act at the same stage in the cell cycle showed either no change or only additivity to the effects seen with either agent alone. However, cotreatment with CCS drugs showed that an inhibitory effect is found between G1/S and G2/M drugs or S/G2 and G2/M drugs. No effect was found between G1/S and S/G2 drugs. Anti-IgM, which by itself was capable of inducing apoptosis, was observed to augment apoptosis induced by very low concentrations of G2/M-acting drugs but it has little effect on G1/S or the S/G2 drugs. The inhibitory effect of anti-CD40 or TNF-alpha on anti-IgM-induced apoptosis did not carry over to an effect on apoptosis induction by the CCS agents. Thus specific combinations of agents may lead to either enhancement, inhibition, or no interactive effect on apoptosis.

Extracellular Signal-Regulated Kinase-2, But Not c-Jun NH2-Terminal Kinase, Activation Correlates with Surface IgM-Mediated Apoptosis in the WEHI 231 B Cell Line

Both extracellular signal-regulated kinase (ERK) and c-Jun NH2-terminal kinase (JNK) have been implicated in mediating the signaling events that precede apoptosis. We studied the activation of these kinases during apoptosis of WEHI 231 B cells. Surface IgM ligation induces apoptosis of WEHI 231 cells. This effect is augmented by simultaneous engagement of CD95 and is inhibited by costimulation with either CD40 or IL-4R. We determined that surface IgM ligation activates ERK2 to a much greater level than JNK, and that IgM-mediated ERK2 activation is enhanced by costimulation with anti-CD95. Costimulation with either IL-4 or anti-CD40 interferes with anti-IgM-stimulated ERK2 activation. Transient expression of mitogen-activated protein kinase phosphatase-1 (MKP-1) inhibits both ERK2 activation and cell death following stimulation with anti-IgM and the combination of anti-IgM plus anti-CD95. CD40 engagement alone activates JNK, but IL-4 stimulation does not. N-acetyl-l-cysteine pretreatment, which blocks CD40-mediated JNK activation, does not affect the ability of CD40 to inhibit anti-IgM-mediated ERK2 activation and apoptosis. Together, these data suggest that JNK activation is not required for CD40 inhibition of surface IgM-induced cell death and that ERK2 plays an active role in mediating anti-IgM-induced apoptosis of WEHI 231 B cells.

CpG Oligodeoxyribonucleotides Rescue Mature Spleen B Cells from Spontaneous Apoptosis and Promote Cell Cycle Entry

Isolated murine splenic B cells undergo spontaneous apoptosis. Motifs containing unmethylated CpG dinucleotides in bacterial DNA or in synthetic oligodeoxynucleotides (ODN) are known to activate murine B cells. Now we show that ODN that induce spleen B cell cycle entry also inhibit spontaneous apoptosis in a sequence-specific fashion. Reversal of the CG to GC abolished activity. Methylation of the central cytosine decreased activity. When CpG is preceded by a cytosine or followed by a guanine, activity was abolished. Other substitutions at the same positions had no effect. Dose-response curves for apoptosis protection and G1 entry suggested that a uniform population of ODN recognition sites controlled downstream ODN effects. A CpG ODN with a nuclease-resistant phosphorothioate backbone (S-ODN) was also active, and increased the levels of c-myc, egr-1,c-jun, bclXL, and bax mRNA and c-Myc, c-Jun, Bax, and BclXL protein in spleen B cells. Levels of c-myb, myn, c-Ki-ras, and bcl2 mRNA remained unchanged. When protein synthesis was inhibited, at 16 h ODN-induced cell cycle entry was abolished and apoptosis protection was partially preserved. Under these conditions, c-Myc was still present, but c-Jun and BclXL were not detected. Our results suggest that CpG containing ODN motifs provide signals for both survival and cell cycle entry. Single base changes determine whether this signal proceeds through a rate-limiting step governing at least two steps in apoptosis (plasma membrane transition, DNA cleavage) and two phases of the cell cycle (G1 and S phase entry). This biologic action is associated with increased c-Myc, c-Jun, and BclXL expression.

The many roles of c-Myc in apoptosis

The proto-oncogene c-myc encodes a transcription factor c-Myc, which is of great importance in controlling cell growth and vitality. The quantity of c-Myc is carefully controlled by many mechanisms, and its actions to induce and repress genes are modulated by interactions with other regulatory proteins. Understanding the kinetic and quantitative relationships that determine how and what genes c-Myc regulates is essential to understanding how Myc is involved in apoptosis. Reduction of c-myc expression and its inappropriate expression can be associated with cellular apoptosis. This review outlines the nature and regulation of the c-myc gene and of c-Myc and presents the systems and conditions in which Myc-related apoptotic events occur. Hypotheses of the mechanisms by which expression and repression of c-myc lead to apoptosis are discussed.

Modulation of E2F activity via signaling through surface IgM and CD40 receptors in WEHI-231 B lymphoma cells

Stimulation of the phenotypically immature B cell lymphoma WEHI-231 with anti-IgM induces G1 arrest followed by apoptotic cell death, which can be reversed by stimulation via the CD40 receptor. Here, we show that cells expressing bcl-xL (WEHI-bcl-xL) arrest at G0/G1 following culture with anti-IgM but do not undergo apoptosis. These arrested cells can be induced to reenter the cell cycle by ligation of CD40. We have therefore used these cells as a model to study the regulation of the transcription factor E2F, which is critically involved in transit through the cell cycle. We found that anti-IgM treatment induces the appearance of an inhibitory DNA binding complex containing the pRB-related pocket protein p130 together with E2F and a concomitant decrease in "free" E2F, consisting of E2F1 and its partner DP1; these effects were reversed following stimulation via CD40. These changes in free E2F levels were regulated by changes in E2F1 gene transcription, which is at least partly a result of control of E2F1 promoter activity through its E2F binding sites. Transient transfection experiments showed that either E2F1 or the viral oncoprotein E1A, which sequesters pocket proteins, including p130, overcame anti-IgM-induced cell cycle arrest in WEHI-bcl-xL. Taken together, these results indicate that in WEHI-231 sIgM ligation induces the accumulation of hypophosphorylated p130 with consequent inhibition of E2F1 gene transcription and cell cycle arrest. Conversely, ligation of CD40 causes hyperphosphorylation of p130, thereby releasing the repression of E2F1 and other E2F-regulated genes, enabling the cells to reenter the cycle. These results, therefore, provide novel insights into the mechanisms whereby antigen receptors on immature B cells deliver inhibitory signals (leading to negative selection of self-reactive B cells) and how these signals can be modulated by positive signals generated via CD40.

CpG DNA Rescue from Anti-IgM-Induced WEHI-231 B Lymphoma Apoptosis via Modulation of IκBα and IκBβ and Sustained Activation of Nuclear Factor-κB/c-Rel

Unmethylated CpG dinucleotides in particular base contexts in oligonucleotides (CpG DNA) rescue WEHI-231 cells from anti-IgM-induced cell cycle arrest and apoptosis. Anti-IgM rapidly elevated the levels of NFκB p50/c-Rel heterodimers followed by a decline of p50/c-Rel heterodimers by 3 h and a concomitant increase of p50/p50 homodimers. In contrast, CpG DNA induced and maintained the levels of p50/c-Rel heterodimers in the presence or absence of anti-IgM, while control non-CpG DNA failed to induce NFκB activation. Anti-IgM induced IκBα degradation followed by increased IκBα protein levels. The levels of IκBβ were increased after anti-IgM treatment. In contrast, CpG DNA, but not non-CpG DNA, induced sustained IκBα and IκBβ degradation in the presence or absence of anti-IgM. Inhibition of IκB degradation blocked CpG DNA-induced NFκB activation and expression of c-myc. Prevention of NFκB activation by inhibiting IκB degradation also suppressed the ability of CpG DNA to rescue WEHI-231 cells from anti-IgM-induced apoptosis. These results indicate that CpG DNA-mediated sustained activation of NFκB depends on the degradation of IκBα and IκBβ and is required for the CpG DNA-mediated anti-apoptosis gene expression and the protection against anti-IgM-induced apoptosis of WEHI-231 cells.

Role for CD40-mediated activation of c-Rel and maintenance of c-myc RNA levels in mitigating anti-IgM-induced growth arrest

CD40 crosslinking on B cells activates NF-kappaB and stress-activated protein kinase (SAPK) pathways. Since CD40 crosslinking rescues WEHI 231 B cells from anti-IgM-induced apoptosis, those pathways were likely candidates to be involved. Indeed, both signaling cascades predominated in anti-IgM-treated WEHI 231 cells, treated concurrently with anti-CD40 to rescue them from apoptosis. Crosslinking of CD40 activated the NF-kappaB proteins c-Rel and p50, but had no influence on their cytoplasmic steady state level. However, in contrast to-and even in the presence of-anti-IgM-mediated signals, engagement of CD40 resulted in a prolonged nuclear translocation of c-Rel, thereby allowing the formation of active NF-kappaB complexes. Consistent with this, the upstream regulatory element of the c-myc promoter, known to be regulated by NF-kappaB, was differently regulated after BCR ligation vs BCR plus CD40 crosslinking. The level of c-myc RNA was rapidly downregulated after BCR engagement, but persistent in the presence of CD40 signaling.