c-myc promotes survival of WEHI 231 B lymphoma cells from apoptosis

New Disubstituted Quindoline Derivatives Inhibiting Burkitt's Lymphoma Cell Proliferation by Impeding c-MYC Transcription

The c-MYC oncogene is overactivated during Burkitt's lymphoma pathogenesis. Targeting c-MYC to inhibit its transcriptional activity has emerged as an effective anticancer strategy. We synthesized four series of disubstituted quindoline derivatives by introducing the second cationic amino side chain and 5-N-methyl group based on a previous study of SYUIQ-5 (1) as c-MYC promoter G-quadruplex ligands. The in vitro evaluations showed that all new compounds exhibited higher stabilities and binding affinities, and most of them had better selectivity (over duplex DNA) for the c-MYC G-quadruplex compared to 1. Moreover, the new ligands prevented NM23-H2, a transcription factor, from effectively binding to the c-MYC G-quadruplex. Further studies showed that the selected ligand, 7a4, down-regulated c-MYC transcription by targeting promoter G-quadruplex and disrupting the NM23-H2/c-MYC interaction in RAJI cells. 7a4 could inhibit Burkitt's lymphoma cell proliferation through cell cycle arrest and apoptosis and suppress tumor growth in a human Burkitt's lymphoma xenograft.

RNAi-mediated c-Rel silencing leads to apoptosis of B cell tumor cells and suppresses antigenic immune response in vivo

c-Rel is a member of the Rel/NF-κB transcription factor family and is predominantly expressed in lymphoid and myeloid cells, playing a critical role in lymphocyte proliferation and survival. Persistent activation of the c-Rel signal transduction pathway is associated with allergies, inflammation, autoimmune diseases, and a variety of human malignancies. To explore the potential of targeting c-Rel as a therapeutic agent for these disorders, we designed a small interfering RNA (siRNA) to silence c-Rel expression in vitro and in vivo. C-Rel-siRNA expression via a retroviral vector in a B cell tumor cell line leads to growth arrest and apoptosis of the tumor cells. Silencing c-Rel in primary B cells in vitro compromises their proliferative and survival response to CD40 activation signals, similar to the impaired response of c-Rel knockout B cells. Most important, in vivo silencing of c-Rel results in significant impairment in T cell-mediated immune responses to antigenic stimulation. Our study thus validates the efficacy of c-Rel-siRNA, and suggests the development of siRNA-based therapy, as well as small molecular inhibitors for the treatment of B cell tumors as well as autoimmune diseases.

Bcl10 can promote survival of antigen-stimulated B lymphocytes

To understand the nature of negative responses through the B-cell antigen receptor (BCR), we have screened an expression cDNA library for the ability to block BCR-induced growth arrest and apoptosis in the immature B-cell line, WEHI-231. We isolated multiple copies of full-length, unmutated Bcl10, a signaling adaptor molecule encoded by a gene found to translocate to the immunoglobulin heavy chain (IgH) locus in some mucosa-associated lymphoid tissue (MALT) lymphomas. A conditionally active form of B-cell lymphoma 10 (Bcl10) protected WEHI-231 cells from BCR-induced apoptosis upon activation. Induction of Bcl10 activity caused rapid activation of nuclear factor-kappaB (NF-kappaB) and c-Jun N-terminal kinase (JNK), but not activation of extracellular signal-regulated kinase (ERK) or p38 mitogen-activated protein (MAP) kinases. These results support genetic and biochemical experiments that have implicated Bcl10 and its binding partners Carma1 and MALT1 in mediating the ability of the BCR to activate NF-kappaB. The ability of Bcl10 expression to prevent BCR-induced growth arrest and apoptosis of WEHI-231 cells was dependent on NF-kappaB activation. Finally, overexpression of Bcl10 in primary B cells activated ex vivo promoted the survival of these cells after removal of activating stimuli. Taken together these results support the hypothesis that enhanced BCL10 expression caused by translocation to the IGH locus can promote formation of MALT lymphomas.

Escherichia coli enterotoxin B subunit triggers apoptosis of CD8(+) T cells by activating transcription factor c-myc

Heat-labile enterotoxin from enterotoxinogenic Escherichia coli is not only an important cause of diarrhea in humans and domestic animals but also possesses potent immunomodulatory properties. Recently, the nontoxic, receptor-binding B subunit of heat-labile enterotoxin (EtxB) was found to induce the selective death of CD8(+) T cells, suggesting that EtxB may trigger activation of proapoptotic signaling pathways. Here we show that EtxB treatment of CD8(+) T cells but not of CD4(+) T cells triggers the specific up-regulation of the transcription factor c-myc, implicated in the control of cell proliferation, differentiation, and death. A concomitant elevation in Myc protein levels was also evident, with peak expression occurring 4 h posttreatment. Preincubation with c-myc antisense oligodeoxynucleotides demonstrated that Myc expression was necessary for EtxB-mediated apoptosis. Myc activation was also associated with an increase of IkappaBalpha turnover, suggesting that elevated Myc expression may be dependent on NF-kappaB. When CD8(+) T cells were pretreated with inhibitors of IkappaBalpha turnover and NF-kappaB translocation, this resulted in a marked reduction in both EtxB-induced apoptosis and Myc expression. Further, a non-receptor-binding mutant of EtxB, EtxB(G33D), was shown to lack the capacity to activate Myc transcription. These findings provide further evidence that EtxB is a signaling molecule that triggers activation of transcription factors involved in cell survival.

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.

Mink cell focus-forming murine leukemia virus infection induces apoptosis of thymic lymphocytes

In a previous study we identified the subpopulations of thymus cells that were infected by the lymphomagenic MCF13 murine leukemia virus (MLV) (F. K. Yoshimura, T. Wang, and M. Cankovic, J. Virol. 73:4890-4898, 1999) and observed an effect on thymus size by virus infection. In this report we describe our results which demonstrate that MCF13 MLV infection of thymuses reduced the number of T lymphocytes in this organ. Histological examination showed diffuse lymphocyte depletion, which was most striking in the CD4(+) CD8(+) lymphocyte-enriched cortical zone. Consistent with this, flow cytometric analysis showed that the lymphocytes which were depleted were predominantly the immature CD3(-) CD4(+) CD8(+) and CD3(+) CD4(+) CD8(+) cells. A comparison of the percentages of live, apoptotic, and dead cells of the gp70(+) and gp70(-) thymic lymphocytes suggested that this effect on thymus cellularity is a result of virus infection. Studies of the survival of thymic T lymphocytes in culture showed that cells from MCF13 MLV-inoculated mice underwent greater apoptosis and death than cells from control animals. Assays for apoptosis included 7-amino-actinomycin D staining, DNA fragmentation, and cleavage of caspase-3 and poly(ADP-ribose) polymerase proenzymes. Our results suggest that apoptosis of thymic lymphocytes by virus infection is an important step in the early stages of MCF13 MLV tumorigenesis.

A1 expression is stimulated by CD40 in B cells and rescues WEHI 231 cells from anti-IgM-induced cell death

Engagement of the antigen receptor on murine immature B cells leads to growth arrest followed by apoptosis. Concomitant signaling through CD40 sustains proliferation and rescues the cells from apoptosis. We show here that cross-linking CD40 stimulates the expression of A1, a member of the anti-apoptotic Bcl-2 family, in primary murine B lymphocytes. CD40-dependent stimulation of A1 was confirmed in WEHI 231 cells, an immature murine B cell lymphoma line. We transduced WEHI 231 cells with a bicistronic recombinant retroviral vector coding for A1 and a chimeric selection marker comprising the enhanced yellow fluorescent protein and the zeocin resistance protein. A1-transduced WEHI 231 cells showed a significant higher survival rate after engagement of the antigen receptor. In contrast, constitutive expression of A1 did not abrogate anti-IgM-induced c-myc down-regulation. Consistent with this, A1 did not release anti-IgM-induced cell cycle arrest. Our data indicate that CD40-stimulated A1 expression permits WEHI 231 cells to survive in the presence of anti-IgM antibodies and suggests a protective role for A1 in antigen receptor-mediated apoptosis in B cells.

Roles of the tumor suppressor p53 and the cyclin-dependent kinase inhibitor p21WAF1/CIP1 in receptor-mediated apoptosis of WEHI 231 B lymphoma cells

Treatment of WEHI 231 immature B lymphoma cells with an antibody against their surface immunoglobulin M (anti-IgM) induces apoptosis and has been studied extensively as a model of self-induced B cell tolerance. Since the tumor suppressor protein p53 has been implicated in apoptosis in a large number of cell types and has been found to be mutated in a variety of B cell tumors, here we sought to determine whether p53 and the p53 target gene cyclin-dependent kinase inhibitor p21(WAF1/CIP1) were involved in anti-IgM-induced cell death. Anti-IgM treatment of WEHI 231 cells increased expression of p53 and p21 protein levels. Ectopic expression of wild-type p53 in WEHI 231 cells induced both p21 expression and apoptosis. Ectopic expression of p21 similarly induced apoptosis. Rescue of WEHI 231 cells from apoptosis by costimulation with CD40 ligand ablated the increase in p21 expression. Lastly, a significant decrease in anti-IgM-mediated apoptosis was seen upon downregulation of endogenous p53 activity by expression of a dominant-negative p53 protein or upon microinjection of an antisense p21 expression vector or antibody. Taken together, the above data demonstrate important roles for p53 and p21 proteins in receptor-mediated apoptosis of WEHI 231 B cells.