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

p53 inhibits CRISPR-Cas9 engineering in human pluripotent stem cells

CRISPR/Cas9 has revolutionized our ability to engineer genomes and conduct genome-wide screens in human cells1-3. Whereas some cell types are amenable to genome engineering, genomes of human pluripotent stem cells (hPSCs) have been difficult to engineer, with reduced efficiencies relative to tumour cell lines or mouse embryonic stem cells3-13. Here, using hPSC lines with stable integration of Cas9 or transient delivery of Cas9-ribonucleoproteins (RNPs), we achieved an average insertion or deletion (indel) efficiency greater than 80%. This high efficiency of indel generation revealed that double-strand breaks (DSBs) induced by Cas9 are toxic and kill most hPSCs. In previous studies, the toxicity of Cas9 in hPSCs was less apparent because of low transfection efficiency and subsequently low DSB induction3. The toxic response to DSBs was P53/TP53-dependent, such that the efficiency of precise genome engineering in hPSCs with a wild-type P53 gene was severely reduced. Our results indicate that Cas9 toxicity creates an obstacle to the high-throughput use of CRISPR/Cas9 for genome engineering and screening in hPSCs. Moreover, as hPSCs can acquire P53 mutations14, cell replacement therapies using CRISPR/Cas9-enginereed hPSCs should proceed with caution, and such engineered hPSCs should be monitored for P53 function.

MYC and the control of apoptosis

MYC expression is tightly correlated with cell-cycle progression in normal tissues, whereas unchecked MYC expression is among the most prominent hallmarks of the hyperproliferation associated with most forms of cancer. At first glance it might seem counterintuitive that MYC is also among the most robust agents of programmed cell death (apoptosis) in mammalian cells. However it is clearly beneficial for a multicellular organism to have a mechanism for triggering death in cells that express potentially oncogenic levels of MYC. Decades of intense study have begun to provide an understanding of the mechanisms that regulate MYC's seemingly split personality. Key features of MYC-induced apoptosis will be discussed here along with examples of how our understanding of this pathway might be exploited for the therapeutic benefit of cancer patients.

Maintenance of antigen-specific immunological memory through variable regions of heavy and light chains of anti-idiotypic antibody

Immunological memory is characterized by a quick and enhanced immune response after re-exposure to the same antigen. To explain the mechanism involved in generation and maintenance of immunological memory, we had earlier proposed a hypothesis involving the relay of memory by idiotypic and anti-idiotypic B cells. The peptidomimic present in the hypervariable region of anti-idiotypic antibody was hypothesized to carry forward immunological memory. In the present work, we provide evidence supporting a role for the anti-idiotypic antibody in eliciting antigen-specific B-cell and T-cell responses. Employing the idiotypic monoclonal antibody (Ab(1)) specific for haemagglutinin (H) protein of rinderpest virus, Ab(2beta) was generated, which possesses an internal image of the H protein in the region between amino acids 527 and 556. We demonstrate that antigen-specific memory is perpetuated by immunization with Ab(2), as shown by maintenance of antigen-specific T-cell responses upon restimulation in vitro of Ab(2) immune splenocytes by antigen-presenting cells expressing H protein or pulsed with H-protein-derived peptides. We have also shown that boosting with antigen-specific anti-idiotypic B cells generates a memory response in antigen-primed mice. Evidence has been provided for the existence of an antigen-specific B-cell idiotypic network in the body that supports the perpetuation of immunological memory as proposed in the relay hypothesis.

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.

Microarray analysis of Lyn-deficient B cells reveals germinal center-associated nuclear protein and other genes associated with the lymphoid germinal center

Lyn is the only member of the Src family expressed in DT40 B cells, which provide a unique model to study the singular contribution of this protein tyrosine kinase (PTK) family to cell signaling. In these cells, gene ablation of Lyn leads to defective B cell receptor signaling. Complementary DNA array analysis of Lyn-deficient DT40 cells shows that the absence of Lyn leads to down-regulation of numerous genes encoding proteins involved in B cell receptor signaling, proliferation, control of transcription, immunity/inflammation response, and cytoskeletal organization. Most of these expression changes have not been previously associated with Lyn PTK signaling. They include alterations in mRNA levels of germinal center-associated nuclear protein (germinal center-associated DNA primase) (GANP), CD74, CD22, NF-kappaB, elongation factor 1alpha, CD79b, octamer binding factor 1, Ig H chain, stathmin, and gamma-actin. Changes in GANP expression were also confirmed in Lyn-deficient mice, suggesting that Lyn PTK has a unique function not compensated for by other Src kinases. Because Lyn-deficient mice have impaired development of germinal centers in spleen, the decreased expression of GANP in the Lyn-deficient DT40 cell line and Lyn-deficient mice suggests that Lyn controls the formation and proliferation of germinal centers via GANP. GANP promoter activity was higher in wild-type vs Lyn-deficient cells. Mutation of the PU.1 binding site reduced activity in wild-type cells and had no effect in Lyn-deficient cells. The presence of Lyn enhanced PU.1 expression in a Northern blot. Thus, the following new signaling pathway has been described: Lyn-->PU.1-->GANP.

Involvement of cell cycle progression in survival signaling through CD40 in the B-lymphocyte line WEHI-231

The CD40 molecule transmits a signal that abrogates apoptosis induced by ligation of the antigen receptor (BCR) in both primary B cells and B-cell lines such as WEHI-231. Expression of Bcl-xL and A1, antiapoptotic members of the Bcl-2 family, is enhanced by CD40 ligation, and is suggested to mediate CD40-induced B-cell survival. CD40 ligation also promotes cell cycle progression by increasing the levels of cyclin-dependent kinases (CDKs) required for cell cycle progression, and reducing expression of the CDK inhibitor p27(kip1). Here we demonstrate that cell cycle inhibition by retrovirus-mediated p27(kip1) expression does not modulate the levels of Bcl-xL or A1, but significantly reduces the survival of BCR-ligated WEHI-231 cells by CD40 ligation. This indicates that cell cycle progression is crucial for CD40-mediated survival of B cells.

The p54 cleaved form of the tyrosine kinase Lyn generated by caspases during BCR-induced cell death in B lymphoma acts as a negative regulator of apoptosis

Engagement of the B cell receptor antigen (BCR) triggers apoptosis on immature B cell lines. We report here that BCR triggering leads to caspase activation followed by Lyn cleavage and induction of apoptosis. The cleavage process is mitochondrion-dependent and involves caspases 9 and 7. Stable expression of the cleaved form of Lyn (Lyn-Delta-N) in Ramos B cells impairs BCR-mediated apoptosis as judged by loss of Delta(psi)m, caspase activation and PARP cleavage. Activation of the main survival pathways upon BCR-triggering was unaltered in both cell variants. However, the PI3-K inhibitor Ly294002 resensitizes Lyn-Delta-N cells to apoptosis. Selected cDNA expression arrays revealed that anti-IgM modulates the expression of approximately 20 genes in both cell variants. Among them, only c-Myc was found to be differentially regulated, which suggests a role for c-Myc in the B cell apoptotic response. Interestingly, c-Myc expression decreased more rapidly in Lyn-Delta-N compared with Lyn-WT cells during the first hours of anti-IgM stimulation. Nevertheless, rapid down-regulation of c-Myc following BCR engagement seems to correlate with the resistance of B cells to apoptosis. Thus, the soluble form of Lyn generated by caspases following BCR triggering acts as an inhibitor of B lymphocyte death likely through the modulation of c-Myc expression.

CD27 and CD40 inhibit p53-independent mitochondrial pathways in apoptosis of B cells induced by B cell receptor ligation

B cells in the germinal center are known to undergo apoptosis after B cell receptor (BCR) ligation, a process relevant to immunological tolerance. Human CD27 is a B cell co-stimulatory molecule. The aim of this study was to compare the effects of CD27 and CD40 signals on BCR-mediated apoptosis of B cells. BCR ligation activated mitochondrial apoptotic pathways including down-regulation of Bcl-X(L), dissipation of mitochondrial transmembrane potential, release of cytochrome c, and activation of caspase-9. Each of these effects was significantly inhibited by CD27 and CD40. Bik expression was weakly but significantly down-regulated by CD27 but up-regulated by CD40. BCR ligation resulted in p53 activation including its phosphorylation at Ser(15), nuclear translocation, and target gene p53AIP1 induction. CD27 and CD40 clearly suppressed these processes. Analyses that used dominant-negative p53 variants revealed a low but still substantial level of BCR-mediated apoptosis and intact mitochondria-mediated apoptotic pathway. These pathways were further inhibited by CD27 and CD40, although the cells showed no p53 phosphorylation or p53AIP1 expression. Our results suggested that, at the mitochondrial level, CD27 and CD40 co-stimulatory signals regulated the p53-amplified apoptotic pathway in B cells through the inhibition of p53-independent apoptotic pathway primarily induced by BCR ligation.

Molecular mechanisms for apoptosis induced by signaling through the B cell antigen receptor

Although the B cell antigen receptor (BCR) transmits survival and activation signals, BCR ligation can induce apoptosis in both immature and mature B cells. BCR-mediated apoptosis is suggested to play a role in self-tolerance by deleting self-reactive B cells. Generation of an apoptotic signal through BCR appears to depend on the composition of the higher order BCR complex and is suggested to occur outside the plasma membrane microdomains, termed lipid rafts. During BCR-mediated apoptosis, mitochondrial dysfunction is induced and is essential for apoptosis, probably by activating both caspases, cysteine proteases that play a central role in apoptosis, and caspase-independent effectors for apoptosis. Although signaling pathways for apoptosis are not yet fully defined in BCR-mediated apoptosis, expression of the proto-oncogene product c-Myc is enhanced upon BCR ligation, and c-Myc appears to mediate BCR ligation-induced apoptosis by causing mitochondrial dysfunction, suggesting that BCR-mediated apoptosis is a form of Myc-induced apoptosis.

Idiotype and anti-idiotype specific T cell responses on transplantation with hybridomas reactive to viral hemagglutinin and human tumor antigen

B cell hybridomas expressing class I and II MHC molecules and producing antibodies directed against hemagglutinin protein of Rinderpest virus and human Mucin-1 have been used as surrogate B cells to study T cell responses against the antigens. The observed CTL and lymphoproliferative response indicates that anti-idiotypic B cells termed Jerne cells stimulate both T helper and T cytotoxic cells by virtue of their ability to present recycled or regurgitated peptido-mimics of antigen to T helper cells through class II MHC and de novo synthesized peptido-mimics of antigens to CTLs. Thus, T cell memory response can be perpetuated by anti-idiotypic Jerne B cells and these findings lend support to the earlier proposed relay hypothesis for perpetuation of immunological memory (IM).

SIV-associated lymphomas in rhesus monkeys (Macaca mulatta) in comparison with HIV-associated lymphomas

A retrospective study was performed to characterize malignant lymphomas of 16 Simian immunodeficiency virus (SIV)-infected rhesus monkeys (Macaca mulatta), 2-9 years of age, on the basis of clinical data, histologic and immunophenotypic results, and cell death indices compiled with the TdT-mediated X-duTP nick end labeling method. We particularly focused on providing immunohistochemical evidence of expression products of EBNA2, Bc12, c-Myc, P21, P53, and Bc16. Results were compared with data from the literature on human HIV-associated lymphomas. According to the updated Kiel classification, the lymphomas were classified as 11 centroblastic lymphomas, three immunoblastic lymphomas, one Burkitt-like lymphoma, and one immunocytoma. Using antibodies to CD20, the B-cell origin of tumor cells was demonstrated. SIV antigen was not demonstrated in the tumor cells. Infection with rhesus lymphocryptovirus was present in 94% of the monkeys. Lymphomas revealed expression of Bc12 in 15/16 (94%), c-Myc in 14/16 (88%), P21 in 10/ 16 (63%), P53 in 12/16 (75%), and Bc16 in 1/16 (6%) monkeys. This study provided evidence that the expression of these gene products, which are thought to play an important role in cell proliferation and apoptosis in HIV- and non-HIV-associated lymphomas, are also involved in the pathogenesis of lymphomas in SIV-infected rhesus monkeys. A tentative relationship between the described gene products and the cell death indices was established for the expression of Bc12. The present primate model represents a suitable animal model for studying the pathogenesis of AIDS-associated lymphomas.

The upregulation by peplomycin of signal transduction in human cells

To explore the mechanism of pulmonary fibrosis by bleomycin and its derivative, peplomycin (PLM), we examined the influence of PLM on signal transduction in human peripheral blood lymphocytes (HL), monocytes (HM) and fibroblasts (HF). Tyrosine phosphorylation of multiple proteins in HL and HM were induced by 0.001 to 0.05 microg/ml and by 0.01 to 0.5 microg/ml of PLM, respectively. In HF, 116-kDa protein was phosphorylated 0.2 to 5 microg/ml of PLM. When HL were treated with 0.01 microg/ml of PLM, phosphorylation of p56lck and activation of extracellular-signal related kinase-2 (ERK2) were induced. ERK2 was also activated in HM. Coordinately, the ratio of p21ras-binding GTP/GDP was increased by PLM. As well as interleukin-2, PLM induced tyrosine phosphorylation of JAK-3. In addition, PLM upregulated the nuclear translocation of nuclear factor-kappa B and the expression of c-myc-mRNA in HL, HM and HF. Furthermore, 0.01 to 0.001 microg/ml PLM enhanced the cytokine generation by HL and HM, and 1 to 5 microg/ml PLM increased cytokine generation and collagen synthesis by HF. These upregulatory effects of PLM were abrogated by pretreatment of the cells with a tyrosine kinase inhibitor. These results indicate that PLM upregulates signal transduction in a variety of cell types and the upregulation may induce pulmonary fibrosis.

Transcriptional activation of TRADD mediates p53-independent radiation-induced apoptosis of glioma cells

Survival of patients with Glioblastoma Multiforme (GM), a highly malignant brain tumor, remains poor despite concerted efforts to improve therapy. The median survival of patients with GM has remained approximately 1 year regardless of the therapeutic approach. Since radiation therapy is the most effective adjuvant therapy for GM and nearly half of GM tumors harbor p53 mutations, we sought to identify genes that mediate p53-independent apoptosis of GM cells in response to ionizing radiation. Using broad-scale gene expression analysis we found that following radiation treatment, TRADD expression was induced in a uniquely radiosensitive GM cell line but not in radioresistant GM cell lines. TRADD over-expression killed GM cells and activated NF-kappa B. We found that blocking the TRADD-mediated pathway using a dominant-negative mutant of FADD (FADD-DN) enhanced radiation resistance of GM cells, as reflected in both susceptibility to apoptosis and clonogenic survival following irradiation. Conversely, stable expression of exogenous TRADD enhanced radiation-induced apoptosis of GM cell lines, reflecting the biological significance of TRADD regulation in p53-independent apoptosis. These findings generate interest in utilizing TRADD in gene therapy for GM tumors, particularly in light of its dual function of directly inducing rapid apoptosis and sensitizing GM cells to standard anti-neoplastic therapy.

Idiotypic-anti-idiotypic B cell interactions generated against a protective antigen of a morbillivirus in mice

The idiotypic network theory (N. K. Jerne, Ann. Immunol. 125, 373-389, 1974) predicts that any antibody that can be made by an individual would have its preexisting specific complementary B cells in its germline repertoire. We transplanted syngeneic BALB/c mice with live hybridoma cells and demonstrated the simultaneous presence of interacting idiotypic and anti-idiotypic B cells in an individual animal by immuno-cytoadherence assays. Furthermore, we demonstrate that interacting B cells displaying idiotypic and anti-idiotypic antibodies are subjected to lysis by complement. It is therefore tempting to speculate that this complement-sensitive interaction between idiotypic and complementary anti-idiotypic B cells in vivo may provide a mechanism for the regulation of B cell populations.

Role of phosphatidylinositol 3-kinase in anti-IgM- and anti-IgD-induced apoptosis in B cell lymphomas

Cross-linking of surface Ig receptors with anti IgM (anti-mu heavy chain, anti-mu), but not anti-IgD (anti-delta heavy chain, anti-delta), Abs leads to growth arrest and apoptosis in several extensively characterized B cell lymphomas. By poorly understood mechanisms, both Igs transiently stimulate c-Myc protein expression. However, ultimately, only anti-mu causes a severe loss in c-Myc and a large induction of p27(Kip1) protein expression. Because phosphatidylinositol 3-kinase (PI3K) has been established as a major modulator of cellular growth and survival, we investigated its role in mediating anti-Ig-stimulated outcomes. Herein, we show that PI3K pathways regulate cell cycle progression and apoptosis in the ECH408 B cell lymphoma. Anti-mu and anti-delta driven c-Myc protein changes precisely follow their effects on the PI3K effector, p70(S6K). Upstream of p70(S6K), signaling through both Ig receptors depresses PI3K pathway phospholipids below control with time, which is followed by p27(Kip1) induction. Conversely, anti-delta, but not anti-mu stimulated PI3K-dependent phospholipid return to control levels by 4-8 h. Abrogation of the PI3K pathway with specific inhibitors mimics anti-mu action, potentiates anti-mu-induced cell death and, importantly, converts anti-delta to a death signal. Transfection with active PI3K kinase construct induces anti-mu resistance, whereas transfection with dominant negative PI3K augments anti-mu sensitivity. Our results show that prolonged disengagement of PI3K or down-regulation of its products by anti-mu (and not anti-delta) determines B cell fate.

Apoptotic proteins. p53 and c-myc related pathways

c-Myc and p53 are two proteins that have critical roles in the regulation of apoptosis and the cell cycle. The authors review how these two proteins are thought to control the opposing events of proliferation and apoptosis and examine whether their well-documented biological roles in tumorigenesis can be applied to the vascular system.

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.

c-Myc antagonizes the effect of p53 on apoptosis and p21WAF1 transactivation in K562 leukemia cells

c-myc protooncogene positively regulates cell proliferation and overexpression of c-myc is found in many solid tumors and leukemias. In the present study we used the K562 human myeloid leukemia cell line as a model to study the functional interaction between c-Myc and p53. Using two different methods, we generated K562 transfectant cell lines with conditional expression of either c-Myc or p53. The cells expressed the p53Vall35 mutant, which adopts a wild-type conformation at 32 degrees C, while c-Myc induction was achieved with a zinc-inducible expression vector. We found that p53 in wild-type conformation induces growth arrest and apoptosis of K562. Expression of c-Myc significantly attenuated apoptosis and impaired the transcriptional activity of p53 on p21WAF1, Bax and cytomegalovirus promoters. The impairment of p21WAF1 transactivation by c-Myc was confirmed by transfection of a c-Myc-estrogen receptor fusion protein and by induction of c-myc by zinc in transfected cells. Also, p53-mediated up-regulation of p21WAF1 mRNA protein were significantly reduced by c-Myc, while Bax levels were unaffected. Consistently, c-Myc increased cyclin-dependent kinase 2 activity in K562 cells expressing p53 in wild-type conformation. These results suggest that c-Myc overexpression may antagonize the pro-apoptotic function of p53, thus providing a molecular mechanism for the frequently observed deregulation of c-myc in human cancer.

Rapid B cell apoptosis induced by antigen receptor ligation does not require Fas (CD95/APO-1), the adaptor protein FADD/MORT1 or CrmA-sensitive caspases but is defective in both MRL-+/+ and MRL-lpr/lpr mice

Antigen receptor ligation-induced apoptosis is thought to play a role in self-tolerance by deleting autoreactive lymphocytes. Antigen receptor ligation-induced apoptosis of mature T cells and T cell lines requires autocrine or paracrine activation of Fas (CD95/APO-1). Whether B cell antigen receptor (BCR)-mediated apoptosis requires Fas or related molecules is unclear. Here we demonstrate that expression of either CrmA, the cowpox virus serpin, or an inhibitor of the adapter protein FADD/MORT1 blocks Fas-mediated apoptosis but has no effect on BCR ligation-induced apoptosis of the B cell line WEHI-231. In contrast, expression of Bcl-2 blocks BCR-mediated but not Fas-induced apoptosis in WEHI-231 cells. These results indicate that BCR ligation activates an apoptotic signaling pathway distinct from Fas-mediated apoptosis in WEHI-231 cells, and that BCR-mediated apoptosis of WEHI-231 cells does not require Fas or related molecules such as DR3, DR4 and DR5, as all of these death receptors require FADD/MORT1 and/or CrmA-sensitive caspases for induction of apoptosis. Moreover, extensive BCR ligation induces death of mature B cells from C57BL/6-lpr/lpr mice as efficiently as those from C57BL/6 mice, indicating that Fas is not essential for BCR-mediated apoptosis of mature B cells. In contrast, BCR ligation-induced apoptosis is reduced in mature B cells from MRL mice and this is not affected by the lpr mutation. Since MRL-lpr/lpr mice but not C57BL/6-lpr/lpr mice develop severe autoimmune disease, defects in BCR-mediated apoptosis in the MRL background, together with lpr mutation, may contribute to the development of severe autoimmune disease in MRL-lpr/lpr mice by allowing survival of self-reactive B cells.

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.