CtBP is an essential corepressor for BCL6 autoregulation

BCL6 is a context-dependent mediator of the glioblastoma response to irradiation therapy

Glioblastoma is a rapidly fatal brain cancer that does not respond to therapy. Previous research showed that the transcriptional repressor protein BCL6 is upregulated by chemo and radiotherapy in glioblastoma, and inhibition of BCL6 enhances the effectiveness of these therapies. Therefore, BCL6 is a promising target to improve the efficacy of current glioblastoma treatment. BCL6 acts as a transcriptional repressor in germinal centre B cells and as an oncogene in lymphoma and other cancers. However, in glioblastoma, BCL6 induced by therapy may not be able to repress transcription. Using a BCL6 inhibitor, the whole proteome response to irradiation was compared with and without BCL6 activity. Acute high dose irradiation caused BCL6 to switch from repressing the DNA damage response to promoting stress response signalling. Rapid immunoprecipitation mass spectrometry of endogenous proteins (RIME) enabled comparison of BCL6 partner proteins between untreated and irradiated glioblastoma cells. BCL6 was associated with transcriptional coregulators in untreated glioblastoma including the known partner NCOR2. However, this association was lost in response to acute irradiation, where BCL6 unexpectedly associated with synaptic and plasma membrane proteins. These results reveal the activity of BCL6 under therapy-induced stress is context-dependent, and potentially altered by the intensity of that stress.

Interleukin-4 downregulates transcription factor BCL6 to promote memory B cell selection in germinal centers

Germinal center (GC)-derived memory B cells (MBCs) are critical for humoral immunity as they differentiate into protective antibody-secreting cells during re-infection. GC formation and cellular interactions within the GC have been studied in detail, yet the exact signals that allow for the selection and exit of MBCs are not understood. Here, we showed that IL-4 cytokine signaling in GC B cells directly downregulated the transcription factor BCL6 via negative autoregulation to release cells from the GC program and to promote MBC formation. This selection event required additional survival cues and could therefore result in either GC exit or death. We demonstrate that both increasing IL-4 bioavailability or limiting IL-4 signaling disrupted MBC selection stringency. In this way, IL-4 control of BCL6 expression serves as a tunable switch within the GC to tightly regulate MBC selection and affinity maturation.

Key Functions of the Transcription Factor BCL6 During T-Cell Differentiation

T lymphocytes consist of several subtypes with distinct functions that help to coordinate an immune response. They are generated within the thymus through a sequential developmental pathway that produces subsets with diverse antigen specificities and functions. Naïve T cells populate peripheral lymphoid organs and are activated upon foreign antigen encounter. While most T cells die soon after activation, a memory population survives and is able to quickly respond to secondary challenges, thus providing long-term immunity to the host. Although cell identity is largely stable and is instructed by cell-specific transcriptional programs, cells may change their transcriptional profiles to be able to adapt to new functionalities. Central to these dynamic processes are transcription factors, which control cell fate decisions, through direct regulation of gene expression. In this book chapter, we review the functions of the transcription factor B-cell lymphoma 6 (BCL6), which directs the fate of several lymphocyte subsets, including helper, cytotoxic, and innate-like T cells, but can also be involved in lymphomagenesis in humans.

MKL/SRF and Bcl6 mutual transcriptional repression safeguards the fate and positioning of neocortical progenitor cells mediated by RhoA

During embryogenesis, multiple intricate and intertwined cellular signaling pathways coordinate cell behavior. Their slightest alterations can have dramatic consequences for the cells and the organs they form. The transcriptional repressor Bcl6 was recently found as important for brain development. However, its regulation and integration with other signals is unknown. Using in vivo functional approaches combined with molecular mechanistic analysis, we identified a reciprocal regulatory loop between B cell lymphoma 6 (Bcl6) and the RhoA-regulated transcriptional complex megakaryoblastic leukemia/serum response factor (MKL/SRF). We show that Bcl6 physically interacts with MKL/SRF, resulting in a down-regulation of the transcriptional activity of both Bcl6 and MKL/SRF. This molecular cross-talk is essential for the control of proliferation, neurogenesis, and spatial positioning of neural progenitors. Overall, our data highlight a regulatory mechanism that controls neuronal production and neocortical development and reveal an MKL/SRF and Bcl6 interaction that may have broader implications in other physiological functions and in diseases.

IL-4 downregulates BCL6 to promote memory B cell selection in germinal centers

Germinal center (GC)-derived memory B cells (MBCs) are critical for humoral immunity as they differentiate into protective antibody-secreting cells during re-infection. GC formation and cellular interactions within the GC have been studied in detail, yet the exact signals that allow for the selection and exit of MBCs are not understood. Here, we show that IL-4 signaling in GC B cells directly downregulates BCL6 via negative autoregulation to release cells from the GC program and promote MBC formation. This selection event requires additional survival cues and can therefore result in either GC exit or death. We demonstrate that both increasing IL-4 bioavailability or limiting IL-4 signaling disrupt MBC selection stringency. In this way, IL-4 control of BCL6 expression serves as a tunable switch within the GC to tightly regulate MBC selection and affinity maturation.

B-cell Lymphoma 6 (BCL6): From Master Regulator of Humoral Immunity to Oncogenic Driver in Pediatric Cancers

B-cell lymphoma 6 (BCL6) is a protooncogene in adult and pediatric cancers, first identified in diffuse large B-cell lymphoma (DLBCL) where it acts as a repressor of the tumor suppressor TP53, conferring survival, protection, and maintenance of lymphoma cells. BCL6 expression in normal B cells is fundamental in the regulation of humoral immunity, via initiation and maintenance of the germinal centers (GC). Its role in B cells during the production of high affinity immunoglobins (that recognize and bind specific antigens) is believed to underpin its function as an oncogene. BCL6 is known to drive the self-renewal capacity of leukemia-initiating cells (LIC), with high BCL6 expression in acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), and glioblastoma (GBM) associated with disease progression and treatment resistance. The mechanisms underpinning BCL6-driven therapy resistance are yet to be uncovered; however, high activity is considered to confer poor prognosis in the clinical setting. BCL6's key binding partner, BCL6 corepressor (BCOR), is frequently mutated in pediatric cancers and appears to act in concert with BCL6. Using publicly available data, here we show that BCL6 is ubiquitously overexpressed in pediatric brain tumors, inversely to BCOR, highlighting the potential for targeting BCL6 in these often lethal and untreatable cancers. In this review, we summarize what is known of BCL6 (role, effect, mechanisms) in pediatric cancers, highlighting the two sides of BCL6 function, humoral immunity, and tumorigenesis, as well as to review BCL6 inhibitors and highlight areas of opportunity to improve the outcomes of patients with pediatric cancer.

The role of the ZEB1–neuroinflammation axis in CNS disorders

Zinc finger E-box binding homeobox 1 (ZEB1) is a master modulator of the epithelial–mesenchymal transition (EMT), a process whereby epithelial cells undergo a series of molecular changes and express certain characteristics of mesenchymal cells. ZEB1, in association with other EMT transcription factors, promotes neuroinflammation through changes in the production of inflammatory mediators, the morphology and function of immune cells, and multiple signaling pathways that mediate the inflammatory response. The ZEB1–neuroinflammation axis plays a pivotal role in the pathogenesis of different CNS disorders, such as brain tumors, multiple sclerosis, cerebrovascular diseases, and neuropathic pain, by promoting tumor cell proliferation and invasiveness, formation of the hostile inflammatory micromilieu surrounding neuronal tissues, dysfunction of microglia and astrocytes, impairment of angiogenesis, and dysfunction of the blood–brain barrier. Future studies are needed to elucidate whether the ZEB1–neuroinflammation axis could serve as a diagnostic, prognostic, and/or therapeutic target for CNS disorders.

NuRD complex recruitment to Thpok mediates CD4+ T cell lineage differentiation

Although BTB-zinc finger (BTB-ZF) transcription factors control the differentiation of multiple hematopoietic and immune lineages, how they function is poorly understood. The BTB-ZF factor Thpok controls intrathymic CD4⁺ T cell development and the expression of most CD4⁺ and CD8⁺ lineage genes. Here, we identify the nucleosome remodeling and deacetylase (NuRD) complex as a critical Thpok cofactor. Using mass spectrometry and coimmunoprecipitation in primary T cells, we show that Thpok binds NuRD components independently of DNA association. We locate three amino acid residues within the Thpok BTB domain that are required for both NuRD binding and Thpok functions. Conversely, a chimeric protein merging the NuRD component Mta2 to a BTB-less version of Thpok supports CD4⁺ T cell development, indicating that NuRD recruitment recapitulates the functions of the Thpok BTB domain. We found that NuRD mediates Thpok repression of CD8⁺ lineage genes, including the transcription factor Runx3, but is dispensable for Cd4 expression. We show that these functions cannot be performed by the BTB domain of the Thpok-related factor Bcl6, which fails to bind NuRD. Thus, cofactor binding critically contributes to the functional specificity of BTB-ZF factors, which control the differentiation of most hematopoietic subsets.

BCL6 maintains survival and self-renewal of primary human acute myeloid leukemia cells

B-cell lymphoma 6 (BCL6) is a transcription repressor and proto-oncogene that plays a crucial role in the innate and adaptive immune system and lymphoid neoplasms. However, its role in myeloid malignancies remains unclear. Here, we explored the role of BCL6 in acute myeloid leukemia (AML). BCL6 was expressed at variable and often high levels in AML cell lines and primary AML samples. AMLs with higher levels of BCL6 were generally sensitive to treatment with BCL6 inhibitors, with the exception of those with monocytic differentiation. Gene expression profiling of AML cells treated with a BCL6 inhibitor revealed induction of BCL6-repressed target genes and transcriptional programs linked to DNA damage checkpoints and downregulation of stem cell genes. Ex vivo treatment of primary AML cells with BCL6 inhibitors induced apoptosis and decreased colony-forming capacity, which correlated with the levels of BCL6 expression. Importantly, inhibition or knockdown of BCL6 in primary AML cells resulted in a significant reduction of leukemia-initiating capacity in mice, suggesting ablation of leukemia repopulating cell functionality. In contrast, BCL6 knockout or inhibition did not suppress the function of normal hematopoietic stem cells. Treatment with cytarabine further induced BCL6 expression, and the levels of BCL6 induction were correlated with resistance to cytarabine. Treatment of AML patient-derived xenografts with BCL6 inhibitor plus cytarabine suggested enhanced antileukemia activity with this combination. Hence, pharmacologic inhibition of BCL6 might provide a novel therapeutic strategy for ablation of leukemia-repopulating cells and increased responsiveness to chemotherapy.

Bcl6-Mediated Transcriptional Regulation of Follicular Helper T cells (TFH)

Follicular helper T cells (T_FH) are essential B cell-help providers in the formation of germinal centers (GCs), affinity maturation of GC B cells, differentiation of high-affinity antibody-producing plasma cells, and production of memory B cells. The transcription factor (TF) B cell lymphoma 6 (Bcl6) is at the center of gene regulation in T_FH biology, including differentiation and function, but how Bcl6 does this, and what additional TFs contribute, remain complex questions. This review focuses on advances in our understanding of Bcl6-mediated gene regulation of T_FH functions, and the modulation of T_FH by other TFs. These advances may have important implications in deciphering how repressor TFs can regulate many immunological cell types. An improved understanding of T_FH biology will likely provide insights into biomedically relevant diseases.

Bcl-6 is the nexus transcription factor of T follicular helper cells via repressor-of-repressor circuits

T follicular helper (T_FH) cells are a distinct type of CD4⁺ T cells that are essential for most antibody and B lymphocyte responses. T_FH cell regulation and dysregulation is involved in a range of diseases. Bcl-6 is the lineage defining transcription factor of T_FH cells and its activity is essential for T_FH cell differentiation and function. However, how Bcl-6 controls T_FH biology has largely remained unclear, at least in part due to intrinsic challenges of connecting repressors to gene upregulation in complex cell types with multiple possible differentiation fates. Multiple competing models were tested here by a series of experimental approaches to determine that Bcl-6 exhibited negative autoregulation and controlled pleiotropic attributes of T_FH differentiation and function, including migration, costimulation, inhibitory receptors, and cytokines, via multiple repressor-of-repressor gene circuits.

NAC1 attenuates BCL6 negative autoregulation and functions as a BCL6 coactivator of FOXQ1 transcription in cancer cells

Background: Nucleus accumbens-associated protein 1 (NAC1) has multifaceted roles in cancer pathogenesis and progression, including the development of drug resistance, promotion of cytokinesis, and maintenance of “stem cell-like” phenotypes. NAC1 is a transcriptional co-regulator belonging to the bric-a-brac tramtrack broad (BTB) family of proteins, although it lacks the characteristic DNA binding motif of the BTB family. The formation of higher-order transcription complexes likely depends on its interaction with other DNA-binding co-factors.

Results: NAC1 interacts with BCL6 via its C-terminal BEN domain and forms a complex that binds the promoter region and activates transcription of the NAC1 target gene, FOXQ1. NAC1 and BCL6 were coordinately upregulated. Our analysis also identified a novel function of NAC1 in attenuating BCL6 auto-downregulation in ovarian cancer. Lastly, we found a significant overlap among NAC1- and BCL6-regulated genes in tumor cells, suggesting that NAC1 and BCL6 coordinately control transcription in cancer.

Conclusions: The results of this study provide a novel mechanistic insight into the oncogenic roles of NAC1 and underline the importance of developing the NAC1/BCL6-targeted cancer therapy.

Methods: Using the Cistrome database and Chromatin Immunoprecipitation (ChIP) analyses, we identified BCL6 as a potential NAC1- interacting molecule. Co-immunoprecipitation (Co-IP), luciferase reporter assay, immunohistochemistry and microarray analysis were performed to analyze the interaction between NAC1 and BCL6 and the mechanisms by which they regulate the downstream genes including FOXQ1.

Mechanisms of Antiphospholipid Syndrome Induction: Role of NKT Cells

The review discusses the mechanisms of participation of natural killer T cells (NKT cells) in the induction of antiphospholipid antibodies (APA) that play a major pathogenetic role in the formation of antiphospholipid syndrome (APS), summarizes the data on APS pathogenesis, and presents modern concepts on the antibody formation involving follicular helper type II NK cells.

The CtBP1-p300-FOXO3a transcriptional complex represses the expression of the apoptotic regulators Bax and Bim in human osteosarcoma cells

C-terminal binding protein 1 (CtBP1), a well-known transcriptional corepressor, functions as an oncogene in multiple cancer types, including osteosarcoma, by modulating the transcription of many tumor suppressors, such as cadherin 1 (CDH1), phosphatase and tensin homolog (PTEN), Bcl2-associated X (Bax), Bcl-2-interacting mediator (Bim), and cyclin-dependent kinase inhibitor 1A (CDKN1A). However, it is still unclear how CtBP1 regulates the expression of these downstream targets. Here, we identified that CtBP1 is overexpressed in osteosarcoma cells and found that CtBP1 directly interacts with the transcription factor forkhead box O3 (FOXO3a) and the histone acetyltransferase p300 in vivo and in vitro. Through microarray analysis, we found that CtBP1 negatively regulates FOXO3a levels. In contrast to the CtBP1 level, the FOXO3a expression level was found to be significantly reduced in osteosarcoma cells. Knockdown of CtBP1 or overexpression of FOXO3a in U2OS cells resulted in different gene expression patterns, and the former caused upregulation of CtBP1 downstream target genes such as CDH1, PTEN, Bax, Bim, and CDKN1A, whereas the latter caused upregulation of Bax and Bim, but not CDH1, PTEN, and CDKN1A. Further analysis indicated that the CtBP1-p300-FOXO3a transcriptional complex specifically binds to the promoters of Bax and Bim. Inhibition of CtBP1 by the constitutive expression of Pep1-E1A^WT peptide in U2OS and OSA cells reversed oncogenic phenotypes, including colony formation, cellular proliferation, and migration, and limited tumor growth in vivo. Together our results demonstrated that the CtBP1-p300-FOXO3a transcriptional complex represses the expression of the apoptotic regulators Bax and Bim in human osteosarcoma cells and that targeting CtBP1-mediated transcriptional events might be a potential therapeutic strategy for the osteosarcoma treatment.

PRMT5 interacts with the BCL6 oncoprotein and is required for germinal center formation and lymphoma cell survival

The germinal center (GC) reaction plays an important role in generating humoral immunity and is believed to give rise to most B-cell lymphomas. GC entry and exit are tightly regulated processes, controlled by the actions of transcription factors such as BCL6. Herein, we demonstrate that protein arginine methyltransferase 5 (PRMT5), a symmetric dimethyl arginine methyltransferase, is also necessary for GC formation and affinity maturation. PRMT5 contributes to GC formation and affinity maturation at least in part through its direct interaction with and methylation of BCL6 at arginine 305 (R305), a modification necessary for the full transcriptional repressive effects of BCL6. Inhibition of PRMT5 in B-cell lymphoma lines led to significant upregulation of BCL6 target genes, and the concomitant inhibition of both BCL6 and PRMT5 exhibited synergistic killing of BCL6-expressing lymphoma cells. Our studies identify PRMT5 as a novel regulator of the GC reaction and highlight the mechanistic rationale of cotargeting PRMT5 and BCL6 in lymphoma.

Inhibition of the transcriptional repressor complex Bcl-6/BCoR induces endothelial sprouting but does not promote tumor growth

The oncogenic potential of the transcriptional repressor Bcl-6 (B-cell lymphoma 6) was originally discovered in non-Hodgkin patients and the soluble Bcl-6 inhibitor 79-6 was developed to treat diffuse large B-cell lymphomas with aberrant Bcl-6 expression. Since we found Bcl-6 and its co-repressor BCoR (Bcl-6 interacting co-repressor) to be regulated in human microvascular endothelium by colorectal cancer cells, we investigated their function in sprouting angiogenesis which is central to tumor growth. Based on Bcl-6/BCoR gene silencing we found that the transcriptional repressor complex in fact constitutes an endogenous inhibitor of vascular sprouting by supporting the stalk cell phenotype: control of Notch target genes (HES1, HEY1, DLL4) and cell cycle regulators (cyclin A and B1). Thus, when endothelial cells were transiently transfected with Bcl-6 and/or BCoR siRNA, vascular sprouting was prominently induced. Comparably, when the soluble Bcl-6 inhibitor 79-6 was applied in the mouse retina model of physiological angiogenesis, endothelial sprouting and branching were significantly enhanced. To address the question whether clinical treatment with 79-6 might therefore have detrimental therapeutic effects by promoting tumor angiogenesis, mouse xenograft models of colorectal cancer and diffuse large B-cell lymphoma were tested. Despite a tendency to increased tumor vessel density, 79-6 therapy did not enhance tumor expansion. In contrast, growth of colorectal carcinomas was significantly reduced which is likely due to a combined 79-6 effect on cancer cells and tumor stroma. These findings may provide valuable information regarding the future clinical development of Bcl-6 inhibitors.

Identification of zinc finger protein Bcl6 as a novel regulator of early adipose commitment

Adipose tissue is a key determinant of whole-body metabolism and energy homeostasis. Unravelling the transcriptional regulatory process during adipogenesis is therefore highly relevant from a biomedical perspective. In these studies, zinc finger protein B-cell lymphoma 6 (Bcl6) was demonstrated to have a role in early adipogenesis of mesenchymal stem cells. Bcl6 is enriched in preadipose versus non-preadipose fibroblasts and shows upregulated expression in the early stage of adipogenesis. Gain- and loss-of-function studies revealed that Bcl6 acts as a key regulator of adipose commitment and differentiation both in vitro and ex vivo. RNAi-mediated knockdown of Bcl6 in C3H10T1/2 cells greatly inhibited adipogenic potential, whereas Bcl6 overexpression enhanced adipogenic differentiation. This transcription factor also directly or indirectly targets and controls the expression of some early and late adipogenic regulators (i.e. Zfp423, Zfp467, KLF15, C/EBPδ, C/EBPα and PPARγ). We further identified that Bcl6 transactivated the signal transducers and activators of transcription 1 (STAT1), which was determined as a required factor for adipogenesis. Moreover, overexpression of STAT1 rescued the impairment of adipogenic commitment and differentiation induced by Bcl6 knockdown in C3H10T1/2 cells, thereby confirming that STAT1 is a downstream direct target of Bcl6. This study identifies Bcl6 as a positive transcriptional regulator of early adipose commitment.

AMP kinase promotes Bcl6 expression in both mouse and human T cells

The transcription factor Bcl6 is a master regulator of follicular helper T (T_FH) cells, and understanding the signaling pathway that induces Bcl6 and T_FH cell differentiation is therefore critical. IL-2 produced during T cell activation inhibits Bcl6 expression but how T_FH cells evade IL-2 inhibition is not completely understood. Here we show that Bcl6 is highly up-regulated in activated CD4 T cells following glucose deprivation (GD), and this pathway is insensitive to inhibition by IL-2. Similar to GD, the glucose analog 2-deoxyglucose (2DG) inhibits glycolysis, and 2DG induced Bcl6 expression in activated CD4 T cells. The metabolic sensor AMP kinase (AMPK) is activated when glycolysis is decreased, and the induction of Bcl6 by GD was inhibited by the AMPK antagonist compound C. Additionally, activation of AMPK by the drug AICAR caused Bcl6 up-regulation in activated CD4 T cells. When mice were immunized with KLH using AICAR as an adjuvant, there was a strong T_FH-dependent enhancement of KLH-specific antibody (Ab) responses, and higher Bcl6 expression in T_FH cells in vivo. Activation of AMPK strongly induced BCL6 and the up-regulation of T_FH cell marker expression by human CD4 T cells. Our data reveal a major new pathway for T_FH cell differentiation, conserved by both mouse and human T cells. Mature T_FH cells are reported to have a lower metabolic state compared to T_H1 cells. Our data indicates that decreased metabolism may be deterministic for T_FH cell differentiation, and not simply a result of T_FH cell differentiation.

An oxidative stress-based mechanism of doxorubicin cytotoxicity suggests new therapeutic strategies in ABC-DLBCL

Diffuse large B-cell lymphomas (DLBCLs) contain 2 major molecular subtypes; namely, the germinal center B-cell-like (GCB) and the activated B-cell-like (ABC) DLBCLs. It is well documented that ABC-DLBCL cases have a significantly poorer survival response than GCB-DLBCLs in both the CHOP (cyclophosphamide, vincristine, doxorubicin, and prednisone) and the rituximab (R)-CHOP eras. However, the underlying cause of this subtype disparity is poorly understood. Nevertheless, these clinical observations raise the possibility for an ABC-DLBCL-specific resistance mechanism that is directed toward 1 of the CHOP components and is inadequately addressed by rituximab. Here, we report that the main cytotoxic ingredient in CHOP, doxorubicin (Dox), has subtype-specific mechanisms of cytotoxicity in DLBCLs resulting from differences in the subcellular distribution pattern. Specifically, in cell line models of ABC-DLBCL, Dox is often enriched in the cytoplasm away from the nuclear DNA. As a result, Dox-induced cytotoxicity in ABC-DLBCLs is often dependent on oxidative stress, rather than DNA damage response. These findings are corroborated by gene signature analysis, which demonstrates that basal oxidative stress status predicts treatment outcome among patients with ABC-DLBCL, but not patients with GCB-DLBCL. In terms of redox-related resistance mechanism, our results suggest that STAT3 confers Dox resistance in ABC-DLBCLs by reinforcing an antioxidant program featuring upregulation of the SOD2 gene. Furthermore, a small-molecule STAT3 inhibitor synergizes with CHOP to trigger oxidative stress and kill ABC-DLBCL cells in preclinical models. These results provide a mechanistic basis for development of novel therapies that target either STAT3 or redox homeostasis to improve treatment outcomes for ABC-DLBCLs.

Molecular mechanisms of OLIG2 transcription factor in brain cancer

Oligodendrocyte lineage transcription factor 2 (OLIG2) plays a pivotal role in glioma development. Here we conducted a comprehensive study of the critical gene regulatory networks involving OLIG2. These include the networks responsible for OLIG2 expression, its translocation to nucleus, cell cycle, epigenetic regulation, and Rho-pathway interactions. We described positive feedback loops including OLIG2: loops of epigenetic regulation and loops involving receptor tyrosine kinases. These loops may be responsible for the prolonged oncogenic activity of OLIG2. The proposed schemes for epigenetic regulation of the gene networks involving OLIG2 are confirmed by patient survival (Kaplan-Meier) curves based on the cancer genome atlas (TCGA) datasets. Finally, we elucidate the Coherent-Gene Modules (CGMs) networks-framework of OLIG2 involvement in cancer. We showed that genes interacting with OLIG2 formed eight CGMs having a set of intermodular connections. We showed also that among the genes involved in these modules the most connected hub is EGFR, then, on lower level, HSP90 and CALM1, followed by three lower levels including epigenetic genes KDM1A and NCOR1. The genes on the six upper levels of the hierarchy are involved in interconnections of all eight CGMs and organize functionally defined gene-signaling subnetworks having specific functions. For example, CGM1 is involved in epigenetic control. CGM2 is significantly related to cell proliferation and differentiation. CGM3 includes a number of interconnected helix-loop-helix transcription factors (bHLH) including OLIG2. Many of these TFs are partially controlled by OLIG2. The CGM4 is involved in PDGF-related: angiogenesis, tumor cell proliferation and differentiation. These analyses provide testable hypotheses and approaches to inhibit OLIG2 pathway and relevant feed-forward and feedback loops to be interrogated. This broad approach can be applied to other TFs.

Regulation of hematopoietic development by ZBTB transcription factors

Hematopoietic development is governed by the coordinated expression of lineage- and differentiation stage-specific genes. Transcription factors play major roles in this process and their perturbation may underlie hematologic and immunologic disorders. Nearly 1900 transcription factors are encoded in the human genome: of these, 49 BTB (for broad-complex, tram-track and bric à brac)-zinc finger transcription factors referred to as ZBTB or POK proteins have been identified. ZBTB proteins, including BCL6, PLZF, ThPOK and LRF, exhibit a broad spectrum of functions in normal and malignant hematopoiesis. This review summarizes developmental and molecular functions of ZBTB proteins relevant to hematology.

Mechanisms of action of BCL6 during germinal center B cell development

The transcriptional repressor B cell lymphoma 6 (BCL6) controls a large transcriptional network that is required for the formation and maintenance of germinal centers (GC). GC B cells represent the normal counterpart of most human B-cell lymphomas, which are often characterized by deregulated BCL6 expression or BCL6-mediated pathways. BCL6 suppresses gene transcription largely through recruitment of its co-repressors through its distinct repression domain. Understanding the precise biological roles of each repression domain in normal and malignant B cells is helpful for development of targeted inhibition of BCL6 functions that is emerging as the basis for design of anti-lymphoma therapies. This review focuses on recent progress in the molecular mechanisms of action of BCL6 in B cells and discusses remaining unresolved questions related to how these mechanisms are linked to normal and malignant B cell development.

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.

The BCL6 RD2 domain governs commitment of activated B cells to form germinal centers

To understand how the Bcl6 transcriptional repressor functions in the immune system, we disrupted its RD2 repression domain in mice. Bcl6RD2(MUT) mice exhibit a complete loss of germinal center (GC) formation but retain normal extrafollicular responses. Bcl6RD2(MUT) antigen-engaged B cells migrate to the interfollicular zone and interact with cognate T helper cells. However, these cells fail to complete early GC-commitment differentiation and coalesce as nascent GC aggregates. Bcl6 directly binds and represses trafficking receptors S1pr1 and Gpr183 by recruiting Hdac2 through the RD2 domain. Deregulation of these genes impairs B cell migration and may contribute to GC failure in Bcl6RD2(MUT) mice. The development of functional GC-TFH cells was partially impaired in Bcl6RD2(MUT) mice. In contrast to Bcl6(-/-) mice, Bcl6RD2(MUT) animals experience no inflammatory disease or macrophage deregulation. These results reveal an essential role for RD2 repression in early GC commitment and striking biochemical specificity in Bcl6 control of humoral and innate immune-cell phenotypes.

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.

Breaking bad in the germinal center: how deregulation of BCL6 contributes to lymphomagenesis

The B cell lymphoma 6 (BCL6) transcriptional repressor is a master regulator of the germinal center (GC) B cell program, required for their unique proliferative and stress tolerant phenotype. Most B cell lymphomas arise from GC B cells and are dependent on the continued or deregulated expression of BCL6 to maintain their survival. The actions of BCL6 in B cells involve formation of distinct chromatin modifying complexes that silence specific promoter and enhancer networks, respectively. The same biochemical mechanisms are maintained in malignant lymphoma cells. Targeted inhibition of these BCL6 functions has emerged as the basis for rational design of lymphoma therapies and combinatorial regimens. In this review, we summarize recent advances on BCL6 mechanisms of action and the deregulation of its target gene networks in lymphoma.

The transcriptional modulator BCL6 as a molecular target for breast cancer therapy

Inappropriate expression or activation of transcription factors can drive patterns of gene expression, leading to the malignant behavior of breast cancer cells. We have found that the transcriptional repressor BCL6 is highly expressed in breast cancer cell lines, and its locus is amplified in about half of primary breast cancers. To understand how BCL6 regulates gene expression in breast cancer cells, we used chromatin immunoprecipitation followed by deep sequencing to identify the BCL6 binding sites on a genomic scale. This revealed that BCL6 regulates a unique cohort of genes in breast cancer cell lines compared with B-cell lymphomas. Furthermore, BCL6 expression promotes the survival of breast cancer cells, and targeting BCL6 with a peptidomimetic inhibitor leads to apoptosis of these cells. Finally, combining a BCL6 inhibitor and a signal transducer and activator of transcription3 inhibitor provided enhanced cell killing in triple-negative breast cancer cell lines, suggesting that combination therapy may be particularly useful. Thus, targeting BCL6 alone or in conjunction with other signaling pathways may be a useful therapeutic strategy for treating breast cancer.

BACH2-BCL6 balance regulates selection at the pre-B cell receptor checkpoint

At the pre-B cell receptor (BCR) checkpoint, developing pre-B cells are selected for successful rearrangement of V(H)-DJ(H) gene segments and expression of a pre-BCR. Reduced stringency at this checkpoint may obstruct the B cell repertoire with nonfunctional B cell clones. Earlier studies have described that activation of B cell lymphoma/leukemia (BCL)6 by a functional pre-BCR mediates positive selection of pre-B cells that have passed the checkpoint. This concept is now further elaborated by the recent finding that the BTB and CNC homology 1 basic leucine zipper transcription factor 2 (BACH2) induces negative selection and opposes BCL6 function prior to the pre-BCR checkpoint. Here, we discuss the antagonism between BCL6 and BACH2 during early B cell development, as well as its implications in both repertoire selection and counter-selection of premalignant clones for leukemia suppression.

A hybrid mechanism of action for BCL6 in B cells defined by formation of functionally distinct complexes at enhancers and promoters

The BCL6 transcriptional repressor is required for the development of germinal center (GC) B cells and diffuse large B cell lymphomas (DLBCLs). Although BCL6 can recruit multiple corepressors, its transcriptional repression mechanism of action in normal and malignant B cells is unknown. We find that in B cells, BCL6 mostly functions through two independent mechanisms that are collectively essential to GC formation and DLBCL, both mediated through its N-terminal BTB domain. These are (1) the formation of a unique ternary BCOR-SMRT complex at promoters, with each corepressor binding to symmetrical sites on BCL6 homodimers linked to specific epigenetic chromatin features, and (2) the "toggling" of active enhancers to a poised but not erased conformation through SMRT-dependent H3K27 deacetylation, which is mediated by HDAC3 and opposed by p300 histone acetyltransferase. Dynamic toggling of enhancers provides a basis for B cells to undergo rapid transcriptional and phenotypic changes in response to signaling or environmental cues.

STAT5 outcompetes STAT3 to regulate the expression of the oncogenic transcriptional modulator BCL6

Inappropriate activation of the transcription factors STAT3 and STAT5 has been shown to drive cancer pathogenesis through dysregulation of genes involved in cell survival, growth, and differentiation. Although STAT3 and STAT5 are structurally related, they can have opposite effects on key genes, including BCL6. BCL6, a transcriptional repressor, has been shown to be oncogenic in diffuse large B cell lymphoma. BCL6 also plays an important role in breast cancer pathogenesis, a disease in which STAT3 and STAT5 can be activated individually or concomitantly. To determine the mechanism by which these oncogenic transcription factors regulate BCL6 transcription, we analyzed their effects at the levels of chromatin and gene expression. We found that STAT3 increases expression of BCL6 and enhances recruitment of RNA polymerase II phosphorylated at a site associated with transcriptional initiation. STAT5, in contrast, represses BCL6 expression below basal levels and decreases the association of RNA polymerase II at the gene. Furthermore, the repression mediated by STAT5 is dominant over STAT3-mediated induction. STAT5 exerts this effect by displacing STAT3 from one of the two regulatory regions to which it binds. These findings may underlie the divergent biology of breast cancers containing activated STAT3 alone or in conjunction with activated STAT5.

C-Terminal Binding Protein: A Molecular Link between Metabolic Imbalance and Epigenetic Regulation in Breast Cancer

The prevalence of obesity has given rise to significant global concerns as numerous population-based studies demonstrate an incontrovertible association between obesity and breast cancer. Mechanisms proposed to account for this linkage include exaggerated levels of carbohydrate substrates, elevated levels of circulating mitogenic hormones, and inflammatory cytokines that impinge on epithelial programming in many tissues. Moreover, recently many scientists have rediscovered the observation, first described by Otto Warburg nearly a century ago, that most cancer cells undergo a dramatic metabolic shift in energy utilization and expenditure that fuels and supports the cellular expansion associated with malignant proliferation. This shift in substrate oxidation comes at the cost of sharp changes in the levels of the high energy intermediate, nicotinamide adenine dinucleotide (NADH). In this review, we discuss a novel example of how shifts in the concentration and flux of substrates metabolized and generated during carbohydrate metabolism represent components of a signaling network that can influence epigenetic regulatory events in the nucleus. We refer to this regulatory process as "metabolic transduction" and describe how the C-terminal binding protein (CtBP) family of NADH-dependent nuclear regulators represents a primary example of how cellular metabolic status can influence epigenetic control of cellular function and fate.

Lineage-specific functions of Bcl-6 in immunity and inflammation are mediated by distinct biochemical mechanisms

The transcription factor Bcl-6 orchestrates germinal center (GC) reactions through its actions in B cells and T cells and regulates inflammatory signaling in macrophages. Here we found that genetic replacement with mutated Bcl6 encoding Bcl-6 that cannot bind corepressors to its BTB domain resulted in disruption of the formation of GCs and affinity maturation of immunoglobulins due to a defect in the proliferation and survival of B cells. In contrast, loss of function of the BTB domain had no effect on the differentiation and function of follicular helper T cells or that of other helper T cell subsets. Bcl6-null mice had a lethal inflammatory phenotype, whereas mice with a mutant BTB domain had normal healthy lives with no inflammation. The repression of inflammatory responses by Bcl-6 in macrophages was accordingly independent of the repressor function of the BTB domain. Bcl-6 thus mediates its actions through lineage-specific biochemical functions.

IL-21 and CD40L synergistically promote plasma cell differentiation through upregulation of Blimp-1 in human B cells

After undergoing Ig somatic hypermutation and Ag selection, germinal center (GC) B cells terminally differentiate into either memory or plasma cells (PCs). It is known that the CD40L and IL-21/STAT3 signaling pathways play critical roles in this process, yet it is unclear how the B cell transcription program interprets and integrates these two types of T cell-derived signals. In this study, we characterized the role of STAT3 in the GC-associated PC differentiation using purified human tonsillar GC B cells and a GC B cell-like cell line. When primary GC B cells were cultured under PC differentiation condition, STAT3 inhibition by AG490 prevented the transition from GC centrocytes to preplasmablast, suggesting that STAT3 is required for the initiation of PC development. In a GC B cell-like human B cell line, although IL-21 alone can induce low-level Blimp-1 expression, maximum Blimp-1 upregulation and optimal PC differentiation required both IL-21 and CD40L. CD40L, although having no effect on Blimp-1 as a single agent, greatly augmented the amplitude and duration of IL-21-triggered Jak-STAT3 signaling. In the human PRDM1 locus, CD40L treatment enhanced the ability of STAT3 to upregulate Blimp-1 by removing BCL6, a potent inhibitor of Blimp-1 expression, from a shared BCL6/STAT3 site in intron 3. Thus, IL-21 and CD40L collaborate through at least two distinct mechanisms to synergistically promote Blimp-1 activation and PC differentiation.

Roles of BCL6 in normal and transformed germinal center B cells

BCL6 is a transcriptional repressor required in mature B cells during the germinal center (GC) reaction. Multiple mechanisms act coordinately to timely modulate BCL6 expression at transcriptional and post-transcriptional levels. BCL6 prevents premature activation and differentiation of GC B cells and provides an environment tolerant of the DNA breaks associated with immunoglobulin gene remodeling mechanisms involved in the production of high-affinity antibodies of different isotypes. The critical functions exerted by BCL6 during normal B-cell development can be hijacked by the malignant transformation process. Indeed, BCL6 is targeted by genetic aberrations and acts as an oncogene in GC-derived lymphomas. The aberrations affecting BCL6 interfere with the multiple levels of regulation that grant a fine tuning of BCL6 expression and activity in physiologic conditions. This review summarizes the current knowledge on BCL6 function and its role in lymphomagenesis.

The 1-1-1 fallacy

T-cell help to B cells is a fundamental aspect of adaptive immunity and the generation of B-cell memory (memory B cells and plasma cells). Follicular helper CD4(+) T (Tfh) cells are the specialized providers of B-cell help, and Tfh cells depend on Bcl6 for their differentiation. This review discusses Tfh cell functions, transcription factors, and induction signals, with particular focus on the richness of the underlying biology and assessing the simplicity or complexity of each of these processes.

The Drosophila BCL6 homolog Ken and Barbie promotes somatic stem cell self-renewal in the testis niche

Stem cells sustain tissue regeneration by their remarkable ability to replenish the stem cell pool and to generate differentiating progeny. Signals from local microenvironments, or niches, control stem cell behavior. In the Drosophila testis, a group of somatic support cells called the hub creates a stem cell niche by locally activating the Janus Kinase-Signal Transducer and Activator of Transcription (JAK-STAT) pathway in two adjacent types of stem cells: germline stem cells (GSCs) and somatic cyst stem cells (CySCs). Here, we find that ken and barbie (ken) is autonomously required for the self-renewal of CySCs but not GSCs. Furthermore, Ken misexpression in the CySC lineage induces the cell-autonomous self-renewal of somatic cells as well as the nonautonomous self-renewal of germ cells outside the niche. Thus, Ken, like Stat92E and its targets ZFH1 (Leatherman and Dinardo, 2008) and Chinmo (Flaherty et al., 2010), is necessary and sufficient for CySC renewal. However, ken is not a JAK-STAT target in the testis, but instead acts in parallel to Stat92E to ensure CySC self-renewal. Ken represses a subset of Stat92E targets in the embryo (Arbouzova et al., 2006) suggesting that Ken maintains CySCs by repressing differentiation factors. In support of this hypothesis, we find that the global JAK-STAT inhibitor Protein tyrosine phosphatase 61F (Ptp61F) is a JAK-STAT target in the testis that is repressed by Ken. Together, our work demonstrates that Ken has an important role in the inhibition of CySC differentiation. Studies of ken may inform our understanding of its vertebrate orthologue B-Cell Lymphoma 6 (BCL6) and how misregulation of this oncogene leads to human lymphomas.

POZ-, AT-hook-, and zinc finger-containing protein (PATZ) interacts with human oncogene B cell lymphoma 6 (BCL6) and is required for its negative autoregulation

The PATZ1 gene encoding a POZ/AT-hook/Kruppel zinc finger (PATZ) transcription factor, is considered a cancer-related gene because of its loss or misexpression in human neoplasias. As for other POZ/domain and Kruppel zinc finger (POK) family members, the transcriptional activity of PATZ is due to the POZ-mediated oligomer formation, suggesting that it might be not a typical transactivator but an architectural transcription factor, thus functioning either as activator or as repressor depending on the presence of proteins able to interact with it. Therefore, to better elucidate PATZ function, we searched for its molecular partners. By yeast two-hybrid screenings, we found a specific interaction between PATZ and BCL6, a human oncogene that plays a key role in germinal center (GC) derived neoplasias. We demonstrate that PATZ and BCL6 interact in germinal center-derived B lymphoma cells, through the POZ domain of PATZ. Moreover, we show that PATZ is able to bind the BCL6 regulatory region, where BCL6 itself acts as a negative regulator, and to contribute to negatively modulate its activity. Consistently, disruption of one or both Patz1 alleles in mice causes focal expansion of thymus B cells, in which BCL6 is up-regulated. This phenotype was almost completely rescued by crossing Patz1(+/-) with Bcl6(+/-) mice, indicating a key role for Bcl6 expression in its development. Finally, a significant number of Patz1 knock-out mice (both heterozygous and homozygous) also develop BCL6-expressing lymphomas. Therefore, the disruption of one or both Patz1 alleles may favor lymphomagenesis by activating the BCL6 pathway.

Pathogenesis of human B cell lymphomas

The mechanisms that drive normal B cell differentiation and activation are frequently subverted by B cell lymphomas for their unlimited growth and survival. B cells are particularly prone to malignant transformation because the machinery used for antibody diversification can cause chromosomal translocations and oncogenic mutations. The advent of functional and structural genomics has greatly accelerated our understanding of oncogenic mechanisms in lymphomagenesis. The signaling pathways that normal B cells utilize to sense antigens are frequently derailed in B cell malignancies, leading to constitutive activation of prosurvival pathways. These malignancies co-opt transcriptional regulatory systems that characterize their normal B cell counterparts and frequently alter epigenetic regulators of chromatin structure and gene expression. These mechanistic insights are ushering in an era of targeted therapies for these cancers based on the principles of pathogenesis.

BCL6-mediated repression of p53 is critical for leukemia stem cell survival in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is induced by the oncogenic BCR-ABL1 tyrosine kinase and can be effectively treated for many years with tyrosine kinase inhibitors (TKIs). However, unless CML patients receive life-long TKI treatment, leukemia will eventually recur; this is attributed to the failure of TKI treatment to eradicate leukemia-initiating cells (LICs). Recent work demonstrated that FoxO factors are critical for maintenance of CML-initiating cells; however, the mechanism of FoxO-dependent leukemia initiation remained elusive. Here, we identified the BCL6 protooncogene as a critical effector downstream of FoxO in self-renewal signaling of CML-initiating cells. BCL6 represses Arf and p53 in CML cells and is required for colony formation and initiation of leukemia. Importantly, peptide inhibition of BCL6 in human CML cells compromises colony formation and leukemia initiation in transplant recipients and selectively eradicates CD34⁺ CD38⁻ LICs in patient-derived CML samples. These findings suggest that pharmacological inhibition of BCL6 may represent a novel strategy to eradicate LICs in CML. Clinical validation of this concept could limit the duration of TKI treatment in CML patients, which is currently life-long, and substantially decrease the risk of blast crisis transformation.

Follicular helper CD4 T cells (TFH)

T cell help to B cells is a fundamental aspect of adaptive immunity and the generation of immunological memory. Follicular helper CD4 T (T(FH)) cells are the specialized providers of B cell help. T(FH) cells depend on expression of the master regulator transcription factor Bcl6. Distinguishing features of T(FH) cells are the expression of CXCR5, PD-1, SAP (SH2D1A), IL-21, and ICOS, among other molecules, and the absence of Blimp-1 (prdm1). T(FH) cells are important for the formation of germinal centers. Once germinal centers are formed, T(FH) cells are needed to maintain them and to regulate germinal center B cell differentiation into plasma cells and memory B cells. This review covers T(FH) differentiation, T(FH) functions, and human T(FH) cells, discussing recent progress and areas of uncertainty or disagreement in the literature, and it debates the developmental relationship between T(FH) cells and other CD4 T cell subsets (Th1, Th2, Th17, iTreg).

Bcl-6 and NF-kappaB cistromes mediate opposing regulation of the innate immune response

In the macrophage, toll-like receptors (TLRs) are key sensors that trigger signaling cascades to activate inflammatory programs via the NF-κB gene network. However, the genomic network targeted by TLR/NF-κB activation and the molecular basis by which it is restrained to terminate activation and re-establish quiescence is poorly understood. Here, using chromatin immunoprecipitation sequencing (ChIP-seq), we define the NF-κB cistrome, which is comprised of 31,070 cis-acting binding sites responsive to lipopolysaccharide (LPS)-induced signaling. In addition, we demonstrate that the transcriptional repressor B-cell lymphoma 6 (Bcl-6) regulates nearly a third of the Tlr4-regulated transcriptome, and that 90% of the Bcl-6 cistrome is collapsed following Tlr4 activation. Bcl-6-deficient macrophages are acutely hypersensitive to LPS and, using comparative ChIP-seq analyses, we found that the Bcl-6 and NF-κB cistromes intersect, within nucleosomal distance, at nearly half of Bcl-6-binding sites in stimulated macrophages to promote opposing epigenetic modifications of the local chromatin. These results reveal a genomic strategy for controlling the innate immune response in which repressive and inductive cistromes establish a dynamic balance between macrophage quiescence and activation via epigenetically marked cis-regulatory elements.

Differential regulation of HIC1 target genes by CtBP and NuRD, via an acetylation/SUMOylation switch, in quiescent versus proliferating cells

The tumor suppressor gene HIC1 encodes a transcriptional repressor involved in regulatory loops modulating P53-dependent and E2F1-dependent cell survival, growth control, and stress responses. Despite its importance, few HIC1 corepressors and target genes have been characterized thus far. Using a yeast two-hybrid approach, we identify MTA1, a subunit of the NuRD complex, as a new HIC1 corepressor. This interaction is regulated by two competitive posttranslational modifications of HIC1 at lysine 314, promotion by SUMOylation, and inhibition by acetylation. Consistent with the role of HIC1 in growth control, we demonstrate that HIC1/MTA1 complexes bind on two new target genes, Cyclin D1 and p57KIP2 in quiescent but not in growing WI38 cells. In addition, HIC1/MTA1 and HIC1/CtBP complexes differentially bind on two mutually exclusive HIC1 binding sites (HiRE) on the SIRT1 promoter. SIRT1 transcriptional activation induced by short-term serum starvation coincides with loss of occupancy of the distal sites by HIC1/MTA1 and HIC1/CtBP. Upon longer starvation, both complexes are found but on a newly identified proximal HiRE that is evolutionarily conserved and specifically enriched with repressive histone marks. Our results decipher a mechanistic link between two competitive posttranslational modifications of HIC1 and corepressor recruitment to specific genes, leading to growth control.

BCL6 promoter interacts with far upstream sequences with greatly enhanced activating histone modifications in germinal center B cells

BCL6 encodes a transcriptional repressor that is essential for the germinal center (GC) reaction and important in lymphomagenesis. Although its promoter has been well studied, little is known concerning its possible regulation by more distal elements. To gain such information, we mapped critical histone modifications associated with active transcription within BCL6 as well as far upstream sequences at nucleosomal resolution in B-cell lines and in normal naive and GC B cells. Promoter-associated and intronic CpG islands (CGIs) in BCL6 showed a reciprocal pattern of histone modifications. Gene expression correlated with a paradoxical loss from the intronic CGI of histone H3 lysine-4 trimethylation, normally associated with transcription, suggesting that the intronic CGI may interfere with transcription. In an approximately 110-kb region extending 150-260 kb upstream of BCL6, highly active histone modifications were present only in normal GC B cells and a GC B-cell line; this region overlaps with an alternative breakpoint region for chromosomal translocations and contains a GC-specific noncoding RNA gene. By chromosome conformation capture, we determined that the BCL6 promoter interacts with this distant upstream region. It is likely that transcriptional enhancers in this region activate BCL6 and overcome strong autorepression in GC B cells.

Requirement for cyclin D3 in germinal center formation and function

Germinal centers (GC) of secondary lymphoid tissues are critical to mounting a high-affinity humoral immune response. B cells within the GC undergo rapid clonal expansion and selection while diversifying their antibody genes. Although it is generally believed that GC B cells employ a unique proliferative program to accommodate these processes, little is known about how the GC-associated cell cycle is orchestrated. The D-type cyclins constitute an important component of the cell cycle engine that enables the cells to respond to physiological changes. Cell type- and developmental stage-specific roles of D-type cyclins have been described but the cyclin D requirement during GC reaction has not been addressed. In this study, we report that cyclin D3 is largely dispensable for proliferation and Ig class switching of in vitro activated B cells. In contrast, GC development in Ccnd3(-/-) mice is markedly impaired, as is the T cell-dependent antibody response. Within the GC, although both switched and unswitched B cells are affected by cyclin D3 inactivation, the IgM(-) pool is more severely reduced. Interestingly, despite a compensatory increase in cyclin D2 expression, a significant number of Ccnd3(-/-) GC B cells accumulate in quiescent G0 state. Lastly, although cyclin D3 inactivation did not disrupt BCL6 expression in GC B cells, it completely blocked the GC promoting effect of BCL6 overexpression, suggesting that cyclin D3 acts downstream of BCL6 to regulate GC formation. This is the first demonstration that cyclin D3 plays an important and unique role at the GC stage of B cell development.

ZEB1 and CtBP form a repressive complex at a distal promoter element of the BCL6 locus

BCL6 is essential for normal antibody responses and is highly expressed in germinal centre B-cells. Constitutive expression due to chromosomal translocations or mutations of cis-acting regulatory elements contributes to diffuse large B-cell lymphoma. BCL6 expression is therefore tightly regulated in a lineage- and developmental-stage-specific manner, and disruption of normal controls can contribute to lymphomagenesis. In order to discover potential cis-acting control regions we carried out DNase I-hypersensitive site mapping. Gel-shift assays and chromatin immunoprecipitation of the core region of a hypersensitive site 4.4 kb upstream of BCL6 transcription initiation (HSS-4.4) showed an E-box element-binding ZEB1 (zinc finger E-boxbinding homeobox 1) and the co-repressor CtBP (C-terminal binding protein). As compared with peripheral blood B-cells, ZEB1, a two-handed zinc finger transcriptional repressor, is expressed at relatively low levels in germinal centre cells, whereas BCL6 has the opposite pattern of expression. Transfection of ZEB1 cDNA caused a reduction in BCL6 expression and a mutated ZEB1, incapable of binding CtBP, lacked this effect. siRNA (small interfering RNA)-mediated knockdown of ZEB1 or CtBP produced an increase in BCL6 mRNA. We propose that HSS-4.4 is a distal promoter element binding a repressive complex consisting of ZEB1 and CtBP. CtBP is ubiquitously expressed and the results of the present study suggest that regulation of ZEB1 is required for control of BCL6 expression.

A small-molecule inhibitor of BCL6 kills DLBCL cells in vitro and in vivo

The BCL6 transcriptional repressor is the most frequently involved oncogene in diffuse large B cell lymphoma (DLBCL). We combined computer-aided drug design with functional assays to identify low-molecular-weight compounds that bind to the corepressor binding groove of the BCL6 BTB domain. One such compound disrupted BCL6/corepressor complexes in vitro and in vivo, and was observed by X-ray crystallography and NMR to bind the critical site within the BTB groove. This compound could induce expression of BCL6 target genes and kill BCL6-positive DLBCL cell lines. In xenotransplantation experiments, the compound was nontoxic and potently suppressed DLBCL tumors in vivo. The compound also killed primary DLBCLs from human patients.

Effectors and memories: Bcl-6 and Blimp-1 in T and B lymphocyte differentiation

Bcl-6 and Blimp-1 have recently been identified as key transcriptional regulators of effector and memory differentiation in CD4(+) T cells and CD8(+) T cells. Bcl-6 and Blimp-1 were previously known to be critical regulators of effector and memory differentiation of B lymphocytes. The new findings unexpectedly point to the Bcl-6 and Blimp-1 regulatory axis as a ubiquitous mechanism for controlling effector and memory lymphocyte differentiation and function. Bcl-6 and Blimp-1 are antagonistic transcription factors and can function as a self-reinforcing genetic switch for cell-fate decisions. However, their influences in different lymphocytes are complex. Here we review and examine the commonalities and differences in the functions of these transcription factors in CD4(+) follicular helper T(FH) lymphocytes, effector CD8(+) T lymphocytes and B lymphocytes.

BCL6: master regulator of the germinal center reaction and key oncogene in B cell lymphomagenesis

BCL6 is a transcriptional repressor which has emerged as a critical regulator of germinal centers (GC), the sites where B cells are selected based on the production of antibodies with high affinity for the antigen. BCL6 is also a frequently activated oncogene in the pathogenesis of human B cell lymphomas, most of which derive from the GC B cells. A thorough understanding of the biological role of BCL6 in normal B cell development and lymphomagenesis depends upon the identification of the full set of genes that are targets of its transcriptional regulatory function. Recently, the identification of BCL6 targets has been implemented with the use of genome-wide chromatin immunoprecipitation and gene expression profiling approaches. A large set of promoters have been shown to be physically bound by BCL6, but only a fraction of them appears to be subjected to transcriptional repression in GC B cells. This set of BCL6 targets points to a number of cellular functions which are likely to be directly controlled by BCL6 during GC development, including activation, survival, DNA-damage response, cell cycle arrest, cytokine-, toll-like receptor-, TGFbeta-, WNT-signaling, and differentiation. Overall, BCL6 is revealing its dual role of "safe-keeper" in preventing centroblasts from responding to signals leading to a premature exit from the GC and of contributor to lymphomagenesis by allowing the instauration of conditions favorable to malignant transformation.

Computational and functional analysis of growth hormone (GH)-regulated genes identifies the transcriptional repressor B-cell lymphoma 6 (Bc16) as a participant in GH-regulated transcription

For insight into transcriptional mechanisms mediating physiological responses to GH, data mining was performed on a profile of GH-regulated genes induced or inhibited at different times in highly responsive 3T3-F442A adipocytes. Gene set enrichment analysis indicated that GH-regulated genes are enriched in pathways including phosphoinositide and insulin signaling and suggested that suppressor of cytokine signaling 2 (SOCS2) and phosphoinositide 3' kinase regulatory subunit p85alpha (Pik3r1) are important targets. Model-based Chinese restaurant clustering identified a group of genes highly regulated by GH at times consistent with its key physiological actions. This cluster included IGF-I, phosphoinositide 3' kinase p85alpha, SOCS2, and cytokine-inducible SH2-containing protein. It also contains the most strongly repressed gene in the profile, B cell lymphoma 6 (Bcl6), a transcriptional repressor. Quantitative real-time PCR verified the strong decrease in Bcl6 mRNA after GH treatment and induction of the other genes in the cluster. Transcriptional network analysis of the genes implicated signal transducer and activator of transcription (Stat) 5 as hub regulating the most responsive genes, Igf1, Socs2, Cish, and Bcl6. Transcriptional activation analysis demonstrated that Bcl6 inhibits SOCS2-luciferase and blunts its stimulation by GH. Occupancy of endogenous Bcl6 on SOCS2 DNA decreased after GH treatment, whereas occupancy of Stat5 increased concomitantly. Thus, GH-mediated inhibition of Bcl6 expression may reverse the repression of SOCS2 and facilitate SOCS2 activation by GH. Together these analyses identify Bcl6 as a participant in GH-regulated gene expression and suggest an interplay between the repressor Bcl6 and the activator Stat5 in regulating genes, which contribute to GH responses.