Regulation of lymphocyte cell fate decisions and lymphomagenesis by BCL-6

Benzophenone-3 exposure alters composition of tumor infiltrating immune cells and increases lung seeding of 4T1 breast cancer cells

Environmental chemicals are a persistent and pervasive part of everyday life. A subset of environmental chemicals are xenoestrogens, compounds that bind to the estrogen receptor (ER) and drive estrogen-related processes. One such chemical, benzophenone-3 (BP3), is a common chemical in sunscreen. It is a potent UV protectant but also is quickly absorbed through the skin. While it has been approved by the FDA, there is a renewed interest in the safety of BP3, particularly in relation to breast cancer. The focus of this study was to examine the impact that BP3 has on triple negative breast cancer (TNBC) through alterations to cells in the immune microenvironment. In this study, we exposed female mice to one of two doses of BP3 before injecting them with a TNBC cell line. Several immune endpoints were examined both in the primary tissues and from in vitro studies of T cell behavior. Our studies revealed that in the lung tumor microenvironment, exposure to BP3 not only increased the number of metastases, but also the total area of tumor coverage. We also found that BP3 caused alterations in immune populations in a tissue-dependent manner, particularly in T cells. Taken together, our data suggest that while BP3 may not directly affect the proliferation of TNBC, growth and metastasis of TNBC-derived tumors can be altered by BP3 exposures via the alterations in the immune populations of the tumor microenvironment.

EBV microRNA-BHRF1-2-5p targets the 3'UTR of immune checkpoint ligands PD-L1 and PD-L2

Epstein-Barr virus-positive (EBV+) diffuse large B-cell lymphomas (DLBCLs) express high levels of programmed death ligand 1 (PD-L1) and PD-L2. MicroRNA (miR) regulation is an important mechanism for the fine-tuning of gene expression via 3'-untranslated region (3'UTR) targeting, and we have previously demonstrated strong EBV miR expression in EBV+ DLBCL. Whereas the EBV latent membrane protein-1 (LMP1) is known to induce PD-L1/L2, a potential counterregulatory role of EBV miR in the fine-tuning of PD-L1/L2 expression remains to be established. To examine this, a novel in vitro model of EBV+ DLBCL was developed, using the viral strain EBV WIL, which unlike common laboratory strains retains intact noncoding regions where several EBV miRs reside. This enabled interrogation of the relationship among EBV latency genes, cell of origin (COO), PD-L1, PD-L2, and EBV miRs. The model successfully recapitulated the full spectrum of B-cell differentiation, with 4 discrete COO phases: early and late germinal center B cells (GCBs) and early and late activated B cells (ABCs). Interestingly, PD-L1/L2 levels increased markedly during transition from late GCB to early ABC phase, after LMP1 upregulation. EBV miR-BamHI fragment H rightward open reading frame 1 (BHRF1)-2-5p clustered apart from other EBV miRs, rising during late GCB phase. Bioinformatic prediction, together with functional validation, confirmed EBV miR-BHRF1-2-5p bound to PD-L1 and PD-L2 3'UTRs to reduce PD-L1/L2 surface protein expression. Results indicate a novel mechanism by which EBV miR-BHRF1-2-5p plays a context-dependent counterregulatory role to fine-tune the expression of the LMP1-driven amplification of these inhibitory checkpoint ligands. Further identification of immune checkpoint-targeting miRs may enable potential novel RNA-based therapies to emerge.

Ectopic expression of transcription factor BATF3 induces B-cell lymphomas in a murine B-cell transplantation model

The mechanisms involved in malignant transformation of mature B and T lymphocytes are still poorly understood. In a previous study, we compared gene expression profiles of the tumor cells of Hodgkin lymphoma (HL) and anaplastic large cell lymphoma (ALCL) to their normal cellular counterparts and found the basic leucine zipper protein ATF-like 3 (BATF3) to be significantly upregulated in the tumor cells of both entities. To assess the oncogenic potential of BATF3 in lymphomagenesis and to dissect the molecular interactions of BATF3 in lymphoma cells, we retrovirally transduced murine mature T and B cells with a BATF3-encoding viral vector and transplanted each population into Rag1-deficient recipients. Intriguingly, BATF3-expressing B lymphocytes readily induced B-cell lymphomas after characteristic latencies, whereas T-cell transplanted animals remained healthy throughout the observation time. Further analyses revealed a germinal center B-cell-like phenotype of most BATF3-initiated lymphomas. In a multiple myeloma cell line, BATF3 inhibited BLIMP1 expression, potentially illuminating an oncogenic action of BATF3 in B-cell lymphomagenesis. In conclusion, BATF3 overexpression induces malignant transformation of mature B cells and might serve as a potential target in B-cell lymphoma treatment.

Bcl6 gene-silencing facilitates PMA-induced megakaryocyte differentiation in K562 cells

Targeted therapy via imatinib appears to be a promising approach for chronic myeloid leukemia (CML) therapy. However, refractory and resistance to imatinib therapy has encouraged many investigators to get involved in development of new therapeutic agents such as Phorbol 12-myrestrat 13-acetate (PMA) for patients with CML. In that line, we attempted to investigate the chemosensitizing effect of PMA on the imatinib-resistant cells. Based on our western blot analyses, resistant K562 cells (K562R) showed high levels of FoxO3a and Bcl6 expressions which were not modulated by imatinib treatment. However, upon PMA treatment, the levels of both FoxO3a and Bcl6 were up-regulated among both the sensitive and the resistant cells and this treatment was associated with initiation of megakaryocytic differentiation of the cells. SiRNA-silencing of FoxO3a led to augmentation of megakaryocytic differentiation of the cells. Similarly, siRNA gene silencing of Bcl6 enhanced the differentiation and induced cell apoptosis among both types of cells. Regarding these results, it might be concluded that Bcl6 knockdown combined with PMA therapy could present a new therapeutical strategy for refractory CML patients to imatinib.

Pannexin hemichannels: A novel promising therapy target for oxidative stress related diseases

Pannexins, which contain three subtypes: pannexin-1, -2, and -3, are vertebrate glycoproteins that form non-junctional plasma membrane intracellular hemichannels via oligomerization. Oxidative stress refers to an imbalance of the generation and elimination of reactive oxygen species (ROS). Studies have shown that elevated ROS levels are pivotal in the development of a variety of diseases. Recent studies indicate that the occurrence of these oxidative stress related diseases is associated with pannexin hemichannels. It is also reported that pannexins regulate the production of ROS which in turn may increase the opening of pannexin hemichannels. In this paper, we review recent researches about the important role of pannexin hemichannels in oxidative stress related diseases. Thus, pannexin hemichannels, novel therapeutic targets, hold promise in managing oxidative stress related diseases such as the tumor, inflammatory bowel diseases (IBD), pulmonary fibrosis, chronic obstructive pulmonary disease (COPD), cardiovascular disease, insulin resistance (IR), and neural degeneration diseases.

Regulation of CD8+ T-cell cytotoxicity in HIV-1 infection

Understanding the mechanisms involved in cellular immune responses against control of human immunodeficiency virus (HIV) infection is key to development of effective immunotherapeutic strategies against viral proliferation. Clear insights into the regulation of cytotoxic CD8+ T cells is crucial to development of effective immunotherapeutic strategies due to their unique ability to eliminate virus-infected cells during the course of infection. Here, we reviewed the roles of transcription factors, co-inhibitory molecules and regulatory cytokines following HIV infection and their potential significance in regulating the cytotoxic potentials of CD8+ T cells.

Suppression of human B cell activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin involves altered regulation of B cell lymphoma-6

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) produces marked suppression of the primary humoral immune response in virtually every animal species evaluated thus far. In addition, epidemiological studies performed in areas of dioxin contamination have identified an association between TCDD exposure and an increased incidence of non-Hodgkin's lymphoma (NHL). Recent studies using an in vitro CD40 ligand model of human B cell differentiation have shown that TCDD impairs both B cell activation and differentiation. The present study extends these findings by identifying B cell lymphoma-6 [BCL-6] as a putative cellular target for deregulation by TCDD, which may contribute to suppression of B cell function as well as NHL. BCL-6 is a multifunctional transcriptional repressor frequently mutated in NHLs and known to regulate critical events of B cell activation and differentiation. In the presence of TCDD, BCL-6 protein levels were elevated and concurrently the same population of cells with high BCL-6 levels showed decreased CD80 and CD69 expression indicative of impaired cellular activation. The elevated BCL-6 levels resulted in a concomitant increase in BCL-6 DNA binding activity at its cognate binding site within an enhancer region for CD80. Furthermore, a small molecule inhibitor of BCL-6 activity reversed TCDD-mediated suppression of CD80 expression in human B cells. In the presence of a low-affinity ligand of the aryl hydrocarbon receptor (AHR), suppression of B cell activation and altered BCL-6 regulation were not observed. These results provide new mechanistic insights into the role of BCL-6 in the suppression of human B cell activation by TCDD.

Reciprocal occupancy of BCL6 and STAT5 on Growth Hormone target genes: contrasting transcriptional outcomes and promoter-specific roles of p300 and HDAC3

Expression of the Growth Hormone (GH)-stimulated gene Socs2 (Suppressor of Cytokine Signaling 2) is mediated by the transcription activator STAT5 (Signal Transducer and Activator of Transcription 5) and the transcription repressor BCL6 (B-Cell Lymphoma 6). ChIP-Sequencing identified Cish (Cytokine-Inducible SH2-containing protein) and Bcl6 as having similar patterns of reciprocal occupancy by BCL6 and STAT5 in response to GH, though GH stimulates Cish and inhibits Bcl6 expression. The co-activator p300 occupied Socs2, Cish and Bcl6 promoters, and enhanced STAT5-mediated activation of Socs2 and Cish. In contrast, on Bcl6, p300 functioned as a repressor and inhibited in conjunction with STAT5 or BCL6. The co-repressor HDAC3 (Histone deacetylase 3) inhibited the Socs2, Cish and Bcl6 promoters in the presence of STAT5. Thus transcriptional outcomes on GH-regulated genes occupied by BCL6 and STAT5 are determined in a promoter-specific fashion by co-regulatory proteins which mediate the distinction between activating and repressive transcription factors.

Deficiency of the transcriptional repressor B cell lymphoma 6 (Bcl6) is accompanied by dysregulated lipid metabolism

The transcriptional repressor B-cell Lymphoma 6 (Bcl6) was recently identified in a profile of genes regulated in adipocytes, suggesting a relationship between Bcl6 and metabolic regulation. As a representative target gene repressed by Bcl6, Suppressor of Cytokine Signaling (Socs) 2 expression was elevated in Bcl6 deficient (KO) mice, including metabolic tissues liver, adipose tissue and muscle, as well as in spleen and thymus. Bcl6 occupied the Socs2 promoter in wild-type, but not Bcl6 KO mice, suggesting direct regulation of Socs2 by Bcl6 in vivo. Mice deficient in Bcl6 were found to exhibit multiple features of dysregulated lipid metabolism. Adipose tissue mass was dramatically reduced or absent in Bcl6 KO mice. Further, hepatic and serum triglycerides were low. Bcl6 deficiency was accompanied by decreased hepatic expression of Stearoyl-CoA desaturase 1 (Scd1) and Fatty acid synthase (Fasn) genes which encode lipogenic enzymes. Expression of the gene for the transcription factor Carbohydrate-Responsive Element Binding Protein (Chrebp), which regulates expression of lipogenic genes, was also reduced in liver of Bcl6 KO mice. Bcl6 deficiency disrupted fasting-induced increases in hepatic triglyceride deposition, but not decreases in lipogenic gene expression. Taken together, these findings suggest that in addition to its well-recognized roles in immune regulation, Bcl6 plays a role in regulatory events of lipid metabolism, and that in the absence of Bcl6, lipid metabolism in liver and adipose tissue is dysregulated.

Role of the Transcriptional Repressor BCL6 in Allergic Response and Inflammation

Various molecules participate in different phases of allergic reactions. This means that many genes are encoding molecules related to allergic reactions, such as cytokines, chemokines, and their receptors as effect or molecules. The transcriptional repressor BCL6 has emerged as a multifunctional regulator of lymphocyte differentiation and immune responses. BCL6-deficient (BCL6) mice display T helper type 2 (Th2)-type inflammation, which is caused by abnormality of both lymphoid cells and nonlymphoid cells. Thus, BCL6 apparently contributes to negative regulation of various central molecules such as cytokines, in particular Th2 cytokines, CC chemokines, and immunoglobulin E in allergic diseases. Therefore, BCL6 may be a molecular target for Th2-type allergic diseases.

Rac1 signalling modulates a STAT5/BCL-6 transcriptional switch on cell-cycle-associated target gene promoters

Gene expression depends on binding of transcriptional regulators to gene promoters, a process controlled by signalling pathways. The transcriptional repressor B-cell lymphoma (BCL)-6 downregulates genes involved in cell-cycle progression and becomes inactivated following phosphorylation by the Rac1 GTPase-activated protein kinase PAK1. Interestingly, the DNA motifs recognized by BCL-6 and signal transducers and activators of transcription 5 (STAT5) are similar. Because STAT5 stimulation in epithelial cells can also be triggered by Rac1 signalling, we asked whether both factors have opposing roles in transcriptional regulation and whether Rac1 signalling may coordinate a transcription factor switch. We used chromatin immunoprecipitation to show that active Rac1 promotes release of the repressor BCL-6 while increasing binding of STAT5A to a BCL-6-regulated reporter gene. We further show in colorectal cell lines that the endogenous activation status of the Rac1/PAK1 pathway correlated with the phosphorylation status of BCL-6 and STAT5A. Three cellular genes (cyclin D2, p15^INK4B, small ubiquitin-like modifier 1) were identified to be inversely regulated by BCL-6 and STAT5A and responded to Rac1 signalling with increased expression and corresponding changes in promoter occupancy. Together, our data show that Rac1 signalling controls a group of target genes that are repressed by BCL-6 and activated by STAT5A, providing novel insights into the modulation of gene transcription by GTPase signalling.

Emerging Therapeutic Targets in Diffuse Large B-Cell Lymphoma

OPINION STATEMENT: The standard front-line treatment of Diffuse Large B-Cell Lymphoma (DLBCL) remains Rituximab combined with multi-agent cytotoxic chemotherapy. In spite of high response rates to this therapy, relapsed/refractory disease is observed in up to 40% of patients. It is our opinion that additional chemoimmunotherapy, followed by high-dose therapy with autologous stem cell transplant (HDT-ASCT) for responsive disease, is the optimal therapy for these patients. However, many patients cannot tolerate HDT-ASCT, or have relapsed/refractory disease in spite of it. These patients have a poor overall prognosis, and there is no clear consensus as to how these patients should be treated. Over the past decade, significant advances have been made in the understanding of the molecular genesis and subtyping of DLBCL, leading to the identification of multiple pathways and molecules that can be targeted for clinical benefit. Examples include Bcl-2, Bcl-6, cell surface markers, and myriad molecules in both the B-Cell receptor and PI3K/Akt/mTOR pathways. As agents targeting these molecules and pathways progress from preclinical models to early clinical trials, more is learned about what might predict for response to these agents, such as cell of origin classification, and/or expression of relevant molecular markers, as measured by immunohistochemistry or gene expression profiling. Both the successes and failures of these novel targeted agents promise to dramatically refine, improve, and individualize the classification and treatment of DLBCL. Therefore, it is our opinion that patients with relapsed/refractory DLBCL are an ideal population for clinical trials due to both the lack of standardized treatment, and the recent advancements in pathobiology and early-phase treatment options.

Resveratrol alters microRNA expression profiles in A549 human non-small cell lung cancer cells

Resveratrol is a plant phenolic phytoalexin that has been reported to have antitumor properties in several types of cancers. In particular, several studies have suggested that resveratrol exerts antiproliferative effects against A549 human non-small cell lung cancer cells; however, its mechanism of action remains incompletely understood. Deregulation of microRNAs (miRNAs), a class of small, noncoding, regulatory RNA molecules involved in gene expression, is strongly correlated with lung cancer. In this study, we demonstrated that resveratrol treatment altered miRNA expression in A549 cells. Using microarray analysis, we identified 71 miRNAs exhibiting greater than 2-fold expression changes in resveratrol-treated cells relative to their expression levels in untreated cells. Furthermore, we identified target genes related to apoptosis, cell cycle regulation, cell proliferation, and differentiation using a miRNA target-prediction program. In conclusion, our data demonstrate that resveratrol induces considerable changes in the miRNA expression profiles of A549 cells, suggesting a novel approach for studying the anticancer mechanisms of resveratrol.

Transcriptional repressor BCL6 controls Th17 responses by controlling gene expression in both T cells and macrophages

The transcriptional repressor protein BCL6 regulates T cell differentiation by repressing Th2 responses and promoting follicular Th cell responses. However, little is known about the role of BCL6 in Th17 responses. We found that memory T cells from BCL6-deficient mice had increased IL-17 production. Additionally, BCL6 expression is upregulated in CD4 T cells cultured under Th17 conditions. T cells from BCL6-deficient mice showed defective Th17 differentiation and enhanced IL-4 production in vitro; however, normal Th17 differentiation was obtained with BCL6-deficient T cells under culture conditions when highly pure naive CD4 T cells were used, when IL-4 production was inhibited, or when TGF-beta levels were increased. Retrovirus-mediated expression of BCL6 in CD4 T cells repressed IL-4 and augmented basal IL-17 mRNA expression. These data support the idea that BCL6 promotes Th17 differentiation through suppression of Th2 differentiation. BCL6-deficient T cells transplanted into Rag1(-/-) mice produced wild-type levels of IL-17, indicating that, in vivo, BCL6-deficient T cells develop relatively normal Th17 responses. Macrophages from BCL6-deficient mice showed strikingly increased expression of the Th17-promoting cytokines IL-6, IL-23, and TGF-beta, and conditioned media from BCL6-deficient macrophages promoted augmented IL-17 expression by T cells. We propose that the increased Th17 activity in BCL6-deficient mice is due, in part, to BCL6-deficient macrophages promoting increased Th17 differentiation in vivo. T cells may require BCL6 for optimal Th17 differentiation; however, BCL6 function in macrophages critically regulates Th17 differentiation in vivo. We hypothesize that increased Th17 differentiation aggravates the severe Th2-type inflammatory disease in BCL6-deficient mice.

Dysregulation of germinal centres in autoimmune disease

In germinal centres, somatic hypermutation and B cell selection increase antibody affinity and specificity for the immunizing antigen, but the generation of autoreactive B cells is an inevitable by-product of this process. Here, we review the evidence that aberrant selection of these autoreactive B cells can arise from abnormalities in each of the germinal centre cellular constituents--B cells, T follicular helper cells, follicular dendritic cells and tingible body macrophages--or in the supply of antigen. As the progeny of germinal centre B cells includes long-lived plasma cells, selection of autoreactive B cells can propagate long-lived autoantibody responses and cause autoimmune diseases. Elucidation of crucial molecular signals in germinal centres has led to the identification of novel therapeutic targets.

Speckled-like pattern in the germinal center (SLIP-GC), a nuclear GTPase expressed in activation-induced deaminase-expressing lymphomas and germinal center B cells

We identified a novel GTPase, SLIP-GC, with expression limited to a few tissues, in particular germinal center B cells. It lacks homology to any known proteins, indicating that it may belong to a novel family of GTPases. SLIP-GC is expressed in germinal center B cells and in lymphomas derived from germinal center B cells such as large diffuse B cell lymphomas. In cell lines, SLIP-GC is expressed in lymphomas that express activation-induced deaminase (AID) and that likely undergo somatic hypermutation. SLIP-GC is a nuclear protein, and it localizes to replication factories. Reduction of SLIP-GC levels in the Burkitt lymphoma cell line Raji and in non-Hodgkin lymphoma cell lines resulted in an increase in DNA breaks and apoptosis that was AID-dependent, as simultaneous reduction of AID abrogated the deleterious effects of SLIP-GC reduction. These results strongly suggest that SLIP-GC is a replication-related protein in germinal center B cells whose reduction is toxic to cells through an AID-dependent mechanism.

BCL6 cooperates with CD40 stimulation and loss of p53 function to rapidly transform primary B cells

The BCL6 transcriptional repressor protein has been shown to promote B-cell lymphoma in transgenic mouse models. The mechanism by which BCL6 transforms primary B cells is unclear, although repression of the p53 tumor suppressor is thought to play a role. Here, we showed that BCL6 has critical oncogene functions that are independent of p53 repression. We found that BCL6 cooperates with constitutive CD40 signaling to rapidly transform p53-deficient primary mouse B cells in vitro. Constitutive CD40 signaling alone does not transform p53-deficient B cells, indicating that BCL6 acts specifically as an immortalizing oncogene in this system. The BCL6 transformed B cells are polyclonal and form polyclonal tumors. At the initiation of the cultures, BCL6 does not significantly alter cell cycle progression, but it does promote increased cell survival. Early cultures of BCL6-expressing B cells exhibited marked repression of ATR and p27kip1 but not other BCL6 target genes, suggesting that the ATR and p27kip1 genes have key early roles in mediating BCL6 transformation function. BCL6-transformed cell lines exhibited further decreases of ATR and p27kip1 expression plus strong decreases in Blimp1 and PDCD2 expression. Our study provides important clues about the critical target genes used by BCL6 to transform primary B cells and indicates that the CD40 signaling pathway can collaborate with BCL6 in the transformation of primary B cells. Thus, our study demonstrates a rapid in vitro system to analyze the transformation function of BCL6.

Rac1 signaling modulates BCL-6-mediated repression of gene transcription

Rac1 is a member of the Rho family of small GTPases that not only regulates signaling pathways involved in cell adhesion and migration but also regulates gene transcription. Here we show that the transcriptional repressor BCL-6 is regulated by Rac1 signaling. Transfection of active Rac1 mutants into colorectal DLD-1 cells led to increased expression of a BCL-6-controlled luciferase reporter construct. Conversely, inhibition of endogenous Rac1 activation by the Rac1 inhibitor NSC23766 decreased reporter activity. Moreover, BCL-6 lost its typical localization to nuclear dots upon activation of Rac1 and became predominantly soluble in a non-chromatin-bound cell fraction. Rac1 signaling also regulated the expression of endogenous BCL-6-regulated genes, including the p50 precursor NF-kappaB1/p105 and the cell adhesion molecule CD44. Interestingly, these effects were not stimulated by the alternative splice variant Rac1b. The mechanism of BCL-6 inhibition does not involve formation of a stable Rac1/BCL-6 complex and is independent of Rac-induced reactive oxygen species production or Jun NH(2)-terminal kinase activation. We show that PAK1 mediates inhibition downstream of Rac and can directly phosphorylate BCL-6. Together, these data provide substantial evidence that Rac1 signaling inhibits the transcriptional repressor BCL-6 in colorectal cells and reveal a novel pathway that links Rac1 signaling to the regulation of gene transcription.

Identification of new SLE-associated genes with a two-step Bayesian study design

In our earlier study, we utilized a Bayesian design to probe the association of approximately 1000 genes (approximately 10,000 single-nucleotide polymorphisms (SNPs)) with systemic lupus erythematosus (SLE) on a moderate number of trios of parents and children with SLE. Two genes associated with SLE, with a multitest-corrected false discovery rate (FDR) of <0.05, were identified, and a number of noteworthy genes with FDR of <0.8 were also found, pointing out a future direction for the study. In this report, using a large population of controls and adult- or childhood-onset SLE cases, we have extended the earlier investigation to explore the SLE association of 10 of these noteworthy genes (109 SNPs). We have found that seven of these genes exhibit a significant (FDR<0.05) association with SLE, both confirming some genes that have earlier been found to be associated with SLE (PTPN22 and IRF5) and presenting novel findings of genes (KLRG1, interleukin-16, protein tyrosine phosphatase receptor type T, toll-like receptor (TLR)8 and CASP10), which have not been reported earlier. The results signify that the two-step candidate pathway design is an efficient way to study the genetic foundations of complex diseases. Furthermore, the novel genes identified in this study point to new directions in both the diagnosis and the eventual treatment of this debilitating disease.

Role of the transcriptional corepressor Bcor in embryonic stem cell differentiation and early embryonic development

Bcor (BCL6 corepressor) is a widely expressed gene that is mutated in patients with X-linked Oculofaciocardiodental (OFCD) syndrome. BCOR regulates gene expression in association with a complex of proteins capable of epigenetic modification of chromatin. These include Polycomb group (PcG) proteins, Skp-Cullin-F-box (SCF) ubiquitin ligase components and a Jumonji C (Jmjc) domain containing histone demethylase. To model OFCD in mice and dissect the role of Bcor in development we have characterized two loss of function Bcor alleles. We find that Bcor loss of function results in a strong parent-of-origin effect, most likely indicating a requirement for Bcor in extraembryonic development. Using Bcor loss of function embryonic stem (ES) cells and in vitro differentiation assays, we demonstrate that Bcor plays a role in the regulation of gene expression very early in the differentiation of ES cells into ectoderm, mesoderm and downstream hematopoietic lineages. Normal expression of affected genes (Oct3/4, Nanog, Fgf5, Bmp4, Brachyury and Flk1) is restored upon re-expression of Bcor. Consistent with these ES cell results, chimeric animals generated with the same loss of function Bcor alleles show a low contribution to B and T cells and erythrocytes and have kinked and shortened tails, consistent with reduced Brachyury expression. Together these results suggest that Bcor plays a role in differentiation of multiple tissue lineages during early embryonic development.

Common mechanisms for the regulation of B cell differentiation and transformation by the transcriptional repressor protein BCL-6

The BCL-6 transcriptional repressor protein is a critical regulator of normal B cell differentiation and BCL-6 has recently been shown to act as an oncogene in several mouse model systems. The molecular pathways by which BCL-6 regulates B cell differentiation and also promotes the transformation of primary B cells are undoubtedly related; however, these pathways are poorly understood. The commonly accepted model for BCL-6 function in B cells is that BCL-6 inhibits the terminal differentiation of activated B cells into plasma cells and that deregulation of BCL-6 expression leads to an inhibition of terminal differentiation and continued proliferation. BCL-6 induces a germinal-center phenotype in primary B cells by unknown mechanisms, and can reverse the terminal differentiation of plasma cell tumor lines. BCL-6 can promote the immortalization of primary B cells and can augment telomerase activity. The role of the vast majority of BCL-6 target genes and interacting proteins in normal B cell differentiation and B cell transformation is essentially unresolved and is an important area for future investigation.

A library of gene expression signatures to illuminate normal and pathological lymphoid biology

Genomics has provided a lever to pry open lymphoid cells and examine their regulatory biology. The large body of available gene expression data has also allowed us to define the of coordinately expressed genes, termed gene expression signatures, which characterize the states of cellular physiology that reflect cellular differentiation, activation of signaling pathways, and the action of transcription factors. Gene expression signatures that reflect the action of individual transcription factors can be defined by perturbing transcription factor function using RNA interference (RNAi), small-molecule inhibition, and dominant-negative approaches. We have used this methodology to define gene expression signatures of various transcription factors controlling B-cell differentiation and activation, including BCL-6, B lymphocyte-induced maturation protein-1 (Blimp-1), X-box binding protein-1 (XBP1), nuclear factor-kappaB (NF-kappaB), and c-myc. We have also curated a wide variety of gene expression signatures from the literature and assembled these into a signature database. Statistical methods can define whether any signature in this database is differentially expressed in independent biological samples, an approach we have used to gain mechanistic insights into the origin and clinical behavior of B-cell lymphomas. We also discuss the use of genomic-scale RNAi libraries to identify genes and pathways that may serve as therapeutic targets in B-cell malignancies.

Aberrant regulation of hematopoiesis by T cells in BAZF-deficient mice

The BAZF (BCL-6b) protein is highly similar to the BCL-6 transcriptional repressor. While BCL-6 has been characterized extensively, relatively little is known about the normal function of BAZF. In order to understand the physiological role of BAZF, we created BAZF-deficient mice. Unlike BCL-6-deficient mice, BAZF-deficient mice are healthy and normal in size. However, BAZF-deficient mice have a hematopoietic progenitor phenotype that is almost identical to that of BCL-6-deficient mice. Compared to wild-type mice, both BAZF-deficient and BCL-6-deficient mice have greatly reduced numbers of cycling hematopoietic progenitor cells (HPC) in the BM and greatly increased numbers of cycling HPC in the spleen. In contrast to HPC from wild-type mice, HPC from BAZF-deficient and BCL-6-deficient mice are resistant to chemokine-induced myelosuppression and do not show a synergistic growth response to granulocyte-macrophage colony-stimulating factor plus stem cell factor. Depletion of CD8 T cells in BAZF-deficient mice reverses several of the hematopoietic defects in these mice. Since both BAZF- and BCL-6-deficient mice have defects in CD8 T-cell differentiation, we hypothesize that both BCL-6 and BAZF regulate HPC homeostasis by an indirect pathway involving CD8 T cells.

What is hidden in the pannexin treasure trove: the sneak peek and the guesswork

Connexins had been considered to be the only class of the vertebrate proteins capable of gap junction formation; however, new candidates for this function with no homology to connexins, termed pannexins were discovered. So far three pannexins were described in rodent and human genomes: Panx1, Panx2 and Panx3. Expressions of pannexins can be detected in numerous brain structures, and now found both in neuronal and glial cells. Hypothetical roles of pannexins in the nervous system include participating in sensory processing, hippocampal plasticity, synchronization between hippocampus and cortex, and propagation of the calcium waves supported by glial cells, which help maintain and modulate neuronal metabolism. Pannexin also may participate in pathological reactions of the neural cells, including their damage after ischemia and subsequent cell death. Recent study revealed non-gap junction function of Panx1 hemichannels in erythrocytes, where they serve as the conduits for the ATP release in response to the osmotic stress. High-throughput studies produced some evidences of the pannexin involvement in the process of tumorigenesis. According to brain cancer gene expression database REMBRANDT, PANX2 expression levels can predict post diagnosis survival for patients with glial tumors. Further investigations are needed to verify or reject hypotheses listed.

The role of BTB domain-containing zinc finger proteins in T cell development and function

Cell fate specifications during T lymphocyte differentiation result from the orchestrated expression of developmentally regulated genes. Furthermore, epigenetic processes that result in a heritable chromatin structure are required for the maintenance of gene expression programs within cells. More and more is known about the basic mechanisms of T cell development and their diversification into various peripheral T cell subsets. Recent research has begun to provide insight into the interactive network of transcription factors as critical regulators of T lymphocyte differentiation. In the past years several members of the BTB domain-containing family of zinc finger proteins (BTB-ZF) have been described to be important for the development and function of hematopoietic cells, and also to contribute to malignant hematopoiesis. This review will provide a brief overview about the role of BTB-ZF proteins during thymocyte development and T cell function.

A novel cervical cancer suppressor 3 (CCS-3) interacts with the BTB domain of PLZF and inhibits the cell growth by inducing apoptosis

Promyelocytic leukemia zinc finger protein (PLZF) is a sequence-specific, DNA binding, transcriptional repressor differentially expressed during embryogenesis and in adult tissues. PLZF is known to be a negative regulator of cell cycle progression. We used PLZF as bait in a yeast two-hybrid screen with a cDNA library from the human ovary tissue. A novel cervical cancer suppressor 3 (CCS-3) was identified as a PLZF interacting partner. Further characterization revealed the BTB domain as an interacting domain of PLZF. Interaction of CCS-3 with PLZF in mammalian cells was also confirmed by co-immunoprecipitation and in vitro binding assays. It was found that, although CCS-3 shares similar homology with eEF1A, the study determined CCS-3 to be an isoform. CCS-3 was observed to be downregulated in human cervical cell lines as well as in cervical tumors when compared to those from normal tissues. Overexpression of CCS-3 in human cervical cell lines inhibits cell growth by inducing apoptosis and suppressing human cyclin A2 promoter activity. These combined results suggest that the potential tumor suppressor activity of CCS-3 may be mediated by its interaction with PLZF.

A Putative Exonic Splicing Polymorphism in the BCL6 Gene and the Risk of Non-Hodgkin Lymphoma

Recent studies have shown that the B-cell lymphoma 6 gene (BCL6) is an oncogene that contributes to lymphomagenesis. Exon 6 of BCL6 contains a common single nucleotide polymorphism (SNP) (-195 C>T; dbSNP ID: rs1056932) that alters a potential binding site for an exonic splicing enhancer. We used unconditional logistic regression models to examine the association between this SNP and the risk of non-Hodgkin lymphoma (NHL) in a population-based case-control study of women residing in Connecticut (461 case patients and 535 control subjects). The risk of NHL among women with the CC genotype was more than double that of women with the TT genotype (odds ratio [OR] = 2.2, 95% confidence interval [CI] = 1.5 to 3.3). Higher risks were observed for two NHL subtypes, namely B-cell chronic lymphatic leukemia/prolymphocytic leukemia/small lymphocytic lymphoma (OR = 3.5, 95% CI = 1.6 to 7.8) and T-cell lymphoma (OR = 5.2, 95% CI = 2.0 to 13.3). Our results support the hypothesis that a genetic variant that could alter mRNA transcripts of BCL6 may contribute to the etiology of NHL and suggest that this variant warrants further investigation.

Molecular cloning of the BCL-6 gene, a transcriptional repressor for B-cell differentiation, in torafugu (Takifugu rubripes)

B-cell lymphoma-6 (BCL-6) is a transcriptional repressor that prevents the terminal differentiation of mature B-cells to plasma cells, and is essential for germinal center formation in the primary lymphoid organs of mammals. In this study, we identified the BCL-6 gene in torafugu (Takifugu rubripes) using the torafugu genome database, and analyzed the expression of BCL-6 mRNA in various tissues of torafugu, using RT-PCR. The BCL-6 gene consisted of eight exons and seven introns spanning a genome of ca. 3.3 kb. BCL-6 mRNA contained a 2112 bp open reading frame encoding 703 amino acids, with a predicted protein size of 78.8 kDa. The predicted torafugu BCL-6 primary structure contains two conserved specific motifs, the BTB/POZ domain at the N-terminus and the sixC2H2-type zinc finger motifs at the C-terminal region. The homology of torafugu BCL-6 to those of zebrafish (Danio rerio), Xenopus laevis, mouse (Mus musculus) and human (Homo sapiens) is 76, 59, 60 and 60%, respectively. RT-PCR analysis revealed that BCL-6 mRNA is highly expressed in pronephros, thymus, intestine, ovary, brain, nasal cavity and muscle. These results imply that torafugu BCL-6 is involved in regulation of B-cell differentiation in torafugu.

Expression of the bcl-6 and MUM1/IRF4 proteins correlate with overall and disease-specific survival in patients with primary cutaneous large B-cell lymphoma: a tissue microarray study

BACKGROUND

Systemic B-cell lymphomas have been studied using microarrays, which has led to a better understanding of their molecular characteristics. Initial microarray studies of these lymphomas have implicated several genes as important predictors of outcome. In this study, we used a tissue microarray (TMA) to characterize primary cutaneous large B-cell lymphomas (PCLBCL).

METHODS

We studied 14 patients for whom clinical follow up was available, including four patients whose lesions were limited to the leg on presentation. Immunohistochemical staining with CD20, CD44, CD21, CD5, CD10, bcl-2, bcl-6, Ki67, p53, and multiple myeloma 1 (MUM1) was examined.

RESULTS

Our results identify two subgroups of lymphomas. The first group showed staining with bcl-6 and had an overall survival of 176 months (p = 0.003). The majority of this group was negative for MUM1. The second group lacked staining with bcl-6 and had an overall survival of 26 months, with a majority of these cases staining with MUM1. Three of four patients with PCLBCL of the leg showed no staining with bcl-6.

CONCLUSIONS

Our study demonstrates the utility of TMAs in the analysis of PCLBCL and that expression of bcl-6 and MUM1 correlates with survival.

Predictive models for breast cancer susceptibility from multiple single nucleotide polymorphisms

Hereditary predisposition and causative environmental exposures have long been recognized in human malignancies. In most instances, cancer cases occur sporadically, suggesting that environmental influences are critical in determining cancer risk. To test the influence of genetic polymorphisms on breast cancer risk, we have measured 98 single nucleotide polymorphisms (SNPs) distributed over 45 genes of potential relevance to breast cancer etiology in 174 patients and have compared these with matched normal controls. Using machine learning techniques such as support vector machines (SVMs), decision trees, and naïve Bayes, we identified a subset of three SNPs as key discriminators between breast cancer and controls. The SVMs performed maximally among predictive models, achieving 69% predictive power in distinguishing between the two groups, compared with a 50% baseline predictive power obtained from the data after repeated random permutation of class labels (individuals with cancer or controls). However, the simpler naïve Bayes model as well as the decision tree model performed quite similarly to the SVM. The three SNP sites most useful in this model were (a) the +4536T/C site of the aldosterone synthase gene CYP11B2 at amino acid residue 386 Val/Ala (T/C) (rs4541); (b) the +4328C/G site of the aryl hydrocarbon hydroxylase CYP1B1 at amino acid residue 293 Leu/Val (C/G) (rs5292); and (c) the +4449C/T site of the transcription factor BCL6 at amino acid 387 Asp/Asp (rs1056932). No single SNP site on its own could achieve more than 60% in predictive accuracy. We have shown that multiple SNP sites from different genes over distant parts of the genome are better at identifying breast cancer patients than any one SNP alone. As high-throughput technology for SNPs improves and as more SNPs are identified, it is likely that much higher predictive accuracy will be achieved and a useful clinical tool developed.

Transcriptional control of B cell development and function

The generation, development, maturation and selection of mammalian B lymphocytes is a complex process that is initiated in the embryo and proceeds throughout life to provide the organism an essential part of the immune system it requires to cope with pathogens. Transcriptional regulation of this highly complex series of events is a major control mechanism, although control is also exerted on all other layers, including splicing, translation and protein stability. This review summarizes our current understanding of transcriptional control of the well-studied murine B cell development, which bears strong similarity to its human counterpart. Animal and cell models with loss of function (gene "knock outs") or gain of function (often transgenes) have significantly contributed to our knowledge about the role of specific transcription factors during B lymphopoiesis. In particular, a large number of different transcriptional regulators have been linked to distinct stages of the life of B lymphocytes such as: differentiation in the bone marrow, migration to the peripheral organs and antigen-induced activation.

Transcriptional repressor BCL-6 immortalizes germinal center-like B cells in the absence of p53 function

Chromosomal rearrangements in non-Hodgkin's B-cell lymphoma implicate BCL-6 as an oncogene, yet direct evidence for BCL-6 acting as an oncogene in B cells has been lacking. Here, we show that BCL-6 can immortalize primary B cells, but only in the absence of p53 tumor suppressor function. The expression of BCL-6 led to greatly increased B-cell proliferation, particularly in response to CD40 stimulation. Furthermore, BCL-6-infected p53-deficient B cells gave rise to immortalized cell lines that could be maintained by CD40 stimulation. We found that in primary mouse B cells, BCL-6 repressed expression of the Blimp-1, p27kip1, and cyclin D2 target genes. BCL-6 did not markedly repress the PDCD2 and BCL-XL target genes. The BCL-6 immortalized cell lines had a phenotype consistent with germinal center B cells, they expressed the germinal center-specific M17 gene, and a significant fraction of the cells stained positive with PNA. Our data indicate that BCL-6 may act to maintain B cells in a germinal center-like state, and repression of Blimp-1 by BCL-6 may be particularly crucial for stabilization of the germinal center phenotype. Our data also suggest that disruption of the p53 pathway may be crucial for the development of BCL-6-expressing B-cell lymphomas.

Inhibition of Th2 differentiation and GATA-3 expression by BCL-6

The B cell lymphoma (BCL)-6 transcriptional repressor protein is an important regulator of Th2 responses. Mice deficient in BCL-6 develop severe Th2-type inflammation that can develop even in the absence of IL-4 signaling. We have investigated the mechanism for how BCL-6 regulates Th2 cell differentiation and have found that IL-6 signaling can promote dramatically increased levels of Th2 differentiation in BCL-6(-/-) CD4 T cells compared with wild-type CD4 T cells. IL-6 can induce a low level of Th2 cytokine expression in BCL-6(-/-)STAT6(-/-) cells but not in STAT6(-/-) cells. Since the promoters for Th2 cytokines such as IL-4, IL-5, IL-10, and IL-13 do not contain consensus BCL-6 DNA binding sites, we investigated whether BCL-6 might regulate the GATA-3 transcription factor that activates the expression of multiple Th2 cytokines. Consistent with the idea that BCL-6 represses GATA-3 expression, we found that GATA-3 levels are up-regulated in BCL-6(-/-)STAT6(-/-) CD4 T cells compared with STAT6(-/-) CD4 T cells. Retrovirus-mediated expression of BCL-6 in BCL-6(-/-)STAT6(-/-) T cells as well as developing wild-type Th2 cells leads to a potent repression of IL-4 and IL-10 secretion. Retrovirus-mediated expression of BCL-6 in both BCL-6(-/-)STAT6(-/-) and wild-type T cells also leads to a significant decrease in GATA-3 protein levels. Surprisingly, BCL-6 does not appear to regulate GATA-3 mRNA levels and thus BCL-6 appears to regulate GATA-3 expression at a posttranscriptional level. Regulation of GATA-3 protein levels is likely a key mechanism for how BCL-6 regulates Th2 cytokine expression and Th2 differentiation independently of STAT6. These data also point to a novel regulatory mechanism for BCL-6 separate from transcriptional repression.

Frequent occurrence of BCL6 rearrangements in nodular lymphocyte predominance Hodgkin lymphoma but not in classical Hodgkin lymphoma

We studied the genomic status of BCL6 in 23 cases of nodular lymphocyte predominance Hodgkin lymphoma (NLPHL) and 40 cases of classical Hodgkin lymphoma (cHL), using dual-color interphase fluorescence in situ hybridization (FISH). The BCL6 rearrangement was identified in 48% of NLPHL cases and was not detected in cHL cases. As a confirmation, sequential or simultaneous immunohistochemistry (IHC) and FISH using CD20 or BCL6 antibodies and BCL6 DNA probes was performed in 8 NLPHL cases. The BCL6-associated translocations, t(3;22)(q27;q11), t(3;7)(q27;p12), and the most probable t(3;9)(q27;p13), were identified in 3 cases. A consistent expression of BCL6 protein in popcorn cells with the highest number of intensely stained cells in cases with a genomic BCL6 rearrangement was shown by IHC. These findings support the hypothesis of a germinal center B cell-derived origin of NLPHL, indicate a significant role of BCL6 in the pathogenesis of NLPHL, and provide further evidence of the genetic diversity underlying the pathogenesis of NLPHL and cHL.

Lymphoid malignancies: the dark side of B-cell differentiation

When the regulation of B-cell differentiation and activation is disrupted, lymphomas and leukaemias can occur. The processes that normally create immunoglobulin diversity might be misdirected, resulting in oncogenic chromosomal translocations that block differentiation, prevent apoptosis and/or promote proliferation. Prolonged or unregulated antigenic stimulation might contribute further to the development and progression of some malignancies. Lymphoid malignancies often resemble normal stages of B-cell differentiation, as shown by molecular techniques such as gene-expression profiling. The similarities and differences between malignant and normal B cells indicate strategies for the treatment of these cancers.

Gene expression data analysis of human lymphoma using support vector machines and output coding ensembles

The large amount of data generated by DNA microarrays was originally analysed using unsupervised methods, such as clustering or self-organizing maps. Recently supervised methods such as decision trees, dot-product support vector machines (SVM) and multi-layer perceptrons (MLP) have been applied in order to classify normal and tumoural tissues. We propose methods based on non-linear SVM with polynomial and Gaussian kernels, and output coding (OC) ensembles of learning machines to separate normal from malignant tissues, to classify different types of lymphoma and to analyse the role of sets of coordinately expressed genes in carcinogenic processes of lymphoid tissues. Using gene expression data from "Lymphochip", a specialised DNA microarray developed at Stanford University School of Medicine, we show that SVM can correctly separate normal from tumoural tissues, and OC ensembles can be successfully used to classify different types of lymphoma. Moreover, we identify a group of coordinately expressed genes related to the separation of two distinct subgroups inside diffuse large B-cell lymphoma (DLBCL), validating a previous Alizadeh's hypothesis about the existence of two distinct diseases inside DLBCL.

Follicular lymphoma without t(14;18) and with BCL-6 rearrangement: a lymphoma subtype with distinct pathological, molecular and clinical characteristics

Translocations involving the BCL-6 gene are frequently observed in diffuse large B cell lymphoma, but have rarely been reported in follicular lymphoma (FL). We studied a distinct cohort of FLs with a 3q27/BCL-6 gene rearrangement, but lacking the t(14;18) translocation. In 13/15 cases, translocations involved the 3q27 and the 14q32 regions. All cases displayed a marked follicular growth pattern and, in some instances, a monocytoid component. Tumor cells were CD5(-) CD20(+) CD23(-) CD43(-) BCL-6(+), and in the main CD10 negative (n = 10, 71%) and BCL-2 negative (n = 11, 78%). When compared to 20 typical t(14;18)(+) FLs, the presence of large follicles (P = 0.01) and a CD10(-)/BCL-2(-) phenotype were more frequently observed (P = 0.001) in our cohort. Clonal mutations arising in the BCL-6 first intron were observed in 5/7 cases with evidence of intraclonal heterogeneity, consistent with a germinal center origin. No significant difference was found in comparison to t(14;18)(+) FL regarding age, sex, performance status, bone marrow involvement or overall survival. However, in the 3q27(+) FL group, a stage III/IV disease and a bulky mass were less frequently observed. This study indicates that 3q27(+) FL without t(14;18) translocation have peculiar clinico-pathologic features and may correspond to a rare and distinct subtype of lymphoma originating from the germinal center.

Chromosomal translocations and their role in the pathogenesis of non-Hodgkin's lymphomas

The discovery that non-Hodgkin's lymphomas are monoclonal and that recurrent chromosomal translocations are involved in their pathogenesis has greatly revolutionised their diagnosis and improved our understanding of these diseases. In the last decades, many genes deregulated by such recurrent chromosomal translocations have been identified. However, we have also learned that these genetic alterations are apparently insufficient, in themselves, to cause neoplastic cell transformation and that more complex genetic events must be involved. This review examines the involved genes in chromosomal translocations and current evidence and postulated mechanisms for their role in the pathogenesis of non-Hodgkin's lymphomas.

Repression of AP-1 function: a mechanism for the regulation of Blimp-1 expression and B lymphocyte differentiation by the B cell lymphoma-6 protooncogene

The B cell lymphoma-6 (BCL-6) transcriptional repressor protein is an important regulator of B cell differentiation and is strongly implicated in the development of B cell lymphoma. Expression of the Blimp-1 transcription factor, which is critical for promoting B cell differentiation into plasma cells, is repressed by BCL-6. We have investigated the mechanism for how BCL-6 represses Blimp-1 transcription, and have found that BCL-6 regulates the Blimp-1 promoter through a novel mechanism involving AP-1 elements. Specifically, BCL-6 is a potent repressor of transcriptional activity mediated by AP-1 factors. We found that the zinc-finger region of BCL-6 interacts with c-Jun, JunB, and JunD proteins but does not bind c-Fos or Fra-2 proteins. An estrogen receptor ligand binding domain fusion with the BCL-6 zinc finger domain can act as a estrogen-inducible dominant negative protein and increase AP-1 activity in BCL-6(+) cells but not in BCL-6(-) cells, indicating that endogenous BCL-6 represses AP-1 activity. Additionally, we have confirmed a specific interaction between c-Jun and the zinc finger domain of BCL-6 in vivo using a mammalian two-hybrid assay. Repression of AP-1 function by BCL-6 may be a key mechanism for how BCL-6 regulates gene expression to control inflammation, lymphocyte differentiation, and lymphomagenesis.

Blimp-1 orchestrates plasma cell differentiation by extinguishing the mature B cell gene expression program

Blimp-1, a transcriptional repressor, drives the terminal differentiation of B cells to plasma cells. Using DNA microarrays, we found that introduction of Blimp-1 into B cells blocked expression of a remarkably large set of genes, while a much smaller number was induced. Blimp-1 initiated this cascade of gene expression changes by directly repressing genes encoding several transcription factors, including Spi-B and Id3, that regulate signaling by the B cell receptor. Blimp-1 also inhibited immunoglobulin class switching by blocking expression of AID, Ku70, Ku86, DNA-PKcs, and STAT6. These findings suggest that Blimp-1 promotes plasmacytic differentiation by extinguishing gene expression important for B cell receptor signaling, germinal center B cell function, and proliferation while allowing expression of important plasma cell genes such as XBP-1.

Gene expression profiling of lymphoid malignancies

Comprehensive gene expression profiling using DNA microarrays is providing a molecular classification of cancer into disease categories that are homogeneous with respect to pathogenesis and clinical behavior. Gene expression profiling revealed that diffuse large B cell lymphoma (DLBCL) consists of at least two molecularly distinct diseases that are derived from distinct stages of B cell differentiation and have strikingly different clinical outcomes. By contrast, chronic lymphocytic leukemia (CLL) was found to be a single disease defined by a characteristic gene expression signature. Nonetheless, gene expression profiling distinguished two clinically divergent CLL subtypes and provided evidence that signaling through the B cell antigen receptor may play a role in the clinically aggressive subtype. Gene expression analysis also illuminated the mechanism of lymphomagenesis caused by BCL-6 translocations and provided evidence that the NF-kappa B signaling pathway is a new molecular therapeutic target in DLBCL.

The forkhead transcription factor AFX activates apoptosis by induction of the BCL-6 transcriptional repressor

The activation of the AKT/protein kinase B kinases by mutation of the PTEN lipid phosphatase results in enhanced survival of a diversity of tumors. This resistance to apoptosis is partly accomplished by the inhibition of genetic programs induced by a subfamily of forkhead transcription factors including AFX. Here we describe an AFX-regulated pathway that appears to account for at least part of this apoptotic regulatory system. Cells induced to synthesize an active form of AFX die by activating the apoptotic death pathway. An analysis of genes regulated by AFX demonstrated that BCL-6, a transcriptional repressor, is up-regulated approximately 4-7-fold. An examination of the BCL-6 promoter demonstrated that AFX bound to specific target sites that could activate transcription. BCL-X(L), an anti-apoptotic protein, contains potential BCL-6 target sites in its promoter. An analysis of endogenous BCL-X(L) levels in AFX-expressing cells revealed enhanced down-regulation of the transcript ( approximately 1.3-1.7-fold) and protein, and BCL-6 directly binds to and suppresses the BCL-X(L) promoter. Finally, macrophages isolated from BCL-6-/- mice show enhanced survival in vitro. These results suggest that AFX regulates apoptosis in part by suppressing the levels of anti-apoptotic BCL-XL through the transcriptional repressor BCL-6.

Transcription: tantalizing times for T cells

The T helper lymphocyte is responsible for orchestrating an appropriate immune response to pathogens. To do so, it has evolved into two specialized subsets that direct type 1 and type 2 immunity. Here, we discuss the genetic programs that control lineage commitment of progenitor T helper cells along each of these pathways.

The human programmed cell death-2 (PDCD2) gene is a target of BCL6 repression: implications for a role of BCL6 in the down-regulation of apoptosis

BCL6, a gene on chromosome 3 band q27, encodes a Kruppel-type zinc finger transcriptional repressor. Rearrangements of this gene are frequent in various kinds of lymphomas, particularly of the large-cell B-cell type. The BCL6 nuclear phosphoprotein is expressed in a variety of tissues and is up-regulated particularly in lymph node germinal centers. The zinc fingers of BCL6 bind DNA in a sequence-specific manner. To identify targets of the BCL6 repressive effects, we used a VP16-BCL6 fusion protein containing the zinc fingers but devoid of the repressor domains to compete with the binding of endogenous BCL6 in a transiently transfected B-cell line and then performed subtractive hybridization by using a method to selectively amplify sequences that are differentially expressed. We found that the programmed cell death-2 (PDCD2) gene is a target of BCL6 repression. This gene is the human homolog of Rp8, a rat gene associated with programmed cell death in thymocytes. Immunohistochemistry reveals the anticipated inverse relationship between BCL6 and PDCD2 expression in human tonsil. PDCD2 is detectable in cells of the germinal center in areas where there is less BCL6 expression as well as in the mantle zone, where there is little or no BCL6 expression. These results raise the possibility that BCL6 may regulate apoptosis by means of its repressive effects on PDCD2. BCL6 deregulation may lead to persistent down-regulation of PDCD2, reduced apoptosis, and, as a consequence, accumulation of BCL6-containing lymphoma cells.

Critical residues within the BTB domain of PLZF and Bcl-6 modulate interaction with corepressors

The PLZF (promyelocytic leukemia zinc finger) transcriptional repressor, when fused to retinoic acid receptor alpha (RARalpha), causes a refractory form of acute promyelocytic leukemia. The highly conserved N-terminal BTB (bric a brac, tramtrack, broad complex)/POZ domain of PLZF plays a critical role in this disease, since it is required for transcriptional repression by the PLZF-RARalpha fusion protein. The crystal structure of the PLZF BTB domain revealed an obligate homodimer with a highly conserved charged pocket formed by apposition of the two monomers. An extensive structure-function analysis showed that the charged pocket motif plays a major role in transcriptional repression by PLZF. We found that mutations of the BTB domain that neutralize key charged pocket residues did not disrupt dimerization, yet abrogated the ability of PLZF to repress transcription and led to the loss of interaction with N-CoR, SMRT, and histone deacetylases (HDACs). We extended these studies to the Bcl-6 protein, which is linked to the pathogenesis of non-Hodgkin's lymphomas. In this case, neutralizing the charged pocket also resulted in loss of repression and corepressor binding. Experiments with purified protein showed that corepressor-BTB interactions were direct. A comparison of the PLZF, Bcl-6, and the FAZF (Fanconi anemia zinc finger)/ROG protein shows that variations in the BTB pocket result in differential affinity for corepressors, which predicts the potency of transcriptional repression. Thus, the BTB pocket represents a molecular structure involved in recruitment of transcriptional repression complexes to target promoters.

Regulation of gene expression by the proto-oncogene BCL-6

The proto-oncogene BCL-6 encodes a transcriptional repressor protein that is expressed at high levels in germinal center B cells and lymphomas with a germinal center B cell phenotype. The BCL-6 gene is a frequent target of chromosomal translocations, micro-deletions, and point mutations in non-Hodgkin's lymphoma. Studies of BCL-6-deficient mice have revealed that BCL-6 is critical for normal lymphocyte differentiation and also that BCL-6 is a negative regulator of inflammation. Recent studies have shed light on how BCL-6 controls these processes by showing that BCL-6 regulates a broad spectrum of target genes. BCL-6 represses transcription of genes involved in lymphocyte activation, differentiation, proliferation, and migration. Although much progress has been made in understanding gene regulation by BCL-6, many important questions are unresolved.