The role of Bcl6 in mature cardiac myocytes

DEAD-box helicase 17 (DDX17) protects cardiac function by promoting mitochondrial homeostasis in heart failure

DEAD-box helicase 17 (DDX17) is a typical member of the DEAD-box family with transcriptional cofactor activity. Although DDX17 is abundantly expressed in the myocardium, its role in heart is not fully understood. We generated cardiomyocyte-specific Ddx17-knockout mice (Ddx17-cKO), cardiomyocyte-specific Ddx17 transgenic mice (Ddx17-Tg), and various models of cardiomyocyte injury and heart failure (HF). DDX17 is downregulated in the myocardium of mouse models of heart failure and cardiomyocyte injury. Cardiomyocyte-specific knockout of Ddx17 promotes autophagic flux blockage and cardiomyocyte apoptosis, leading to progressive cardiac dysfunction, maladaptive remodeling and progression to heart failure. Restoration of DDX17 expression in cardiomyocytes protects cardiac function under pathological conditions. Further studies showed that DDX17 can bind to the transcriptional repressor B-cell lymphoma 6 (BCL6) and inhibit the expression of dynamin-related protein 1 (DRP1). When DDX17 expression is reduced, transcriptional repression of BCL6 is attenuated, leading to increased DRP1 expression and mitochondrial fission, which in turn leads to impaired mitochondrial homeostasis and heart failure. We also investigated the correlation of DDX17 expression with cardiac function and DRP1 expression in myocardial biopsy samples from patients with heart failure. These findings suggest that DDX17 protects cardiac function by promoting mitochondrial homeostasis through the BCL6-DRP1 pathway in heart failure.

Delineating the mechanism of fragility at BCL6 breakpoint region associated with translocations in diffuse large B cell lymphoma

BCL6 translocation is one of the most common chromosomal translocations in cancer and results in its enhanced expression in germinal center B cells. It involves the fusion of BCL6 with any of its twenty-six Ig and non-Ig translocation partners associated with diffuse large B cell lymphoma (DLBCL). Despite being discovered long back, the mechanism of BCL6 fragility is largely unknown. Analysis of the translocation breakpoints in 5' UTR of BCL6 reveals the clustering of most of the breakpoints around a region termed Cluster II. In silico analysis of the breakpoint cluster sequence identified sequence motifs that could potentially fold into non-B DNA. Results revealed that the Cluster II sequence folded into overlapping hairpin structures and identified sequences that undergo base pairing at the stem region. Further, the formation of cruciform DNA blocked DNA replication. The sodium bisulfite modification assay revealed the single-strandedness of the region corresponding to hairpin DNA in both strands of the genome. Further, we report the formation of intramolecular parallel G4 and triplex DNA, at Cluster II. Taken together, our studies reveal that multiple non-canonical DNA structures exist at the BCL6 cluster II breakpoint region and contribute to the fragility leading to BCL6 translocation in DLBCL patients.

Uncovering the molecular mechanisms between heart failure and end-stage renal disease via a bioinformatics study

Background: Heart failure (HF) is not only a common complication in patients with end-stage renal disease (ESRD) but also a major cause of death. Although clinical studies have shown that there is a close relationship between them, the mechanism of its occurrence is unclear. The aim of this study is to explore the molecular mechanisms between HF and ESRD through comprehensive bioinformatics analysis, providing a new perspective on the crosstalk between these two diseases. Methods: The HF and ESRD datasets were downloaded from the Gene Expression Omnibus (GEO) database; we identified and analyzed common differentially expressed genes (DEGs). First, Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and gene set variation analyses (GSVA) were applied to explore the potential biological functions and construct protein-protein interaction (PPI) networks. Also, four algorithms, namely, random forest (RF), Boruta algorithm, logical regression of the selection operator (LASSO), and support vector machine-recursive feature elimination (SVM-RFE), were used to identify the candidate genes. Subsequently, the diagnostic efficacy of hub genes for HF and ESRD was evaluated using eXtreme Gradient Boosting (XGBoost) algorithm. CIBERSORT was used to analyze the infiltration of immune cells. Thereafter, we predicted target microRNAs (miRNAs) using databases (miRTarBase, TarBase, and ENOCRI), and transcription factors (TFs) were identified using the ChEA3 database. Cytoscape software was applied to construct mRNA-miRNA-TF regulatory networks. Finally, the Drug Signatures Database (DSigDB) was used to identify potential drug candidates. Results: A total of 68 common DEGs were identified. The enrichment analysis results suggest that immune response and inflammatory factors may be common features of the pathophysiology of HF and ESRD. A total of four hub genes (BCL6, CCL5, CNN1, and PCNT) were validated using RF, LASSO, Boruta, and SVM-RFE algorithms. Their AUC values were all greater than 0.8. Immune infiltration analysis showed that immune cells such as macrophages, neutrophils, and NK cells were altered in HF myocardial tissue, while neutrophils were significantly correlated with all four hub genes. Finally, 11 target miRNAs and 10 TFs were obtained, and miRNA-mRNA-TF regulatory network construction was performed. In addition, 10 gene-targeted drugs were discovered. Conclusion: Our study revealed important crosstalk between HF and ESRD. These common pathways and pivotal genes may provide new ideas for further clinical treatment and experimental studies.

Liver Injury and Cell Survival in Non-Alcoholic Steatohepatitis Regulated by Sex-Based Difference through B Cell Lymphoma 6

The liver is a crucial organ for maintaining homeostasis in living organisms and is the center of various metabolic functions. Therefore, abnormal metabolic activity, as in metabolic syndrome, leads to pathological conditions, such as abnormal accumulation of lipids in the liver. Inflammation and cell death are induced by several stresses in the fatty liver, namely steatohepatitis. In recent years, an increase in non-alcoholic steatohepatitis (NASH), which is not dependent on excessive alcohol intake, has become an issue as a major cause of liver cirrhosis and liver cancer. There are several recent findings on functional sex-based differences, NASH, and cell stress and death in the liver. In particular, NASH-induced liver injury and tumorigeneses were suppressed by B cell lymphoma 6, the transcriptional factor regulating sex-based liver functional gene expression. In this review, we discuss cell response to stress and lipotoxicity in NASH and its regulatory mechanisms.

B cell lymphoma 6A regulates immune development and function in zebrafish

BCL6A is a transcriptional repressor implicated in the development and survival of B and T lymphoctyes, which is also highly expressed in many non-Hodgkin’s lymphomas, such as diffuse large B cell lymphoma and follicular lymphoma. Roles in other cell types, including macrophages and non-hematopoietic cells, have also been suggested but require further investigation. This study sought to identify and characterize zebrafish BCL6A and investigate its role in immune cell development and function, with a focus on early macrophages. Bioinformatics analysis identified a homologue for BCL6A (bcl6aa), as well as an additional fish-specific duplicate (bcl6ab) and a homologue for the closely-related BCL6B (bcl6b). The human BCL6A and zebrafish Bcl6aa proteins were highly conserved across the constituent BTB/POZ, PEST and zinc finger domains. Expression of bcl6aa during early zebrafish embryogenesis was observed in the lateral plate mesoderm, a site of early myeloid cell development, with later expression seen in the brain, eye and thymus. Homozygous bcl6aa mutants developed normally until around 14 days post fertilization (dpf), after which their subsequent growth and maturation was severely impacted along with their relative survival, with heterozygous bcl6aa mutants showing an intermediate phenotype. Analysis of immune cell development revealed significantly decreased lymphoid and macrophage cells in both homozygous and heterozygous bcl6aa mutants, being exacerbated in homozygous mutants. In contrast, the number of neutrophils was unaffected. Only the homozygous bcl6aa mutants showed decreased macrophage mobility in response to wounding and reduced ability to contain bacterial infection. Collectively, this suggests strong conservation of BCL6A across evolution, including a role in macrophage biology.

Construction and Bioinformatics Analysis of circRNA-miRNA-mRNA Network in Acute Myocardial Infarction

Background: Acute myocardial infarction (AMI) is one of the main fatal diseases of cardiovascular diseases. Circular RNA (circRNA) is a non-coding RNA (ncRNA), which plays a role in cardiovascular disease as a competitive endogenous RNA (ceRNA). However, their role in AMI has not been fully clarified. This study aims to explore the mechanism of circRNA-related ceRNA network in AMI, and to identify the corresponding immune infiltration characteristics.

Materials and Methods: The circRNA (GSE160717), miRNA (GSE24548), and mRNA (GSE60993) microarray datasets of AMI were downloaded from the Gene Expression Omnibus (GEO) database. Differentially expressed circRNAs (DEcircRNAs), miRNAs (DEmiRNAs), and mRNAs (DEmRNAs) were identified by the “limma” package. After integrating the circRNA, miRNA and mRNA interaction, we constructed a circRNA-miRNA-mRNA network. The “clusterProfiler” package and String database were used for functional enrichment analysis and protein-protein interaction (PPI) analysis, respectively. After that, we constructed a circRNA-miRNA-hub gene network and validated the circRNAs and mRNAs using an independent dataset (GSE61144) as well as qRT-PCR. Finally, we used CIBERSORTx database to analyze the immune infiltration characteristics of AMI and the correlation between hub genes and immune cells.

Results: Using the “limma” package of the R, 83 DEcircRNAs, 54 DEmiRNAs, and 754 DEmRNAs were identified in the microarray datasets of AMI. Among 83 DEcircRNAs, there are 55 exonic DEcircRNAs. Then, a circRNA-miRNA-mRNA network consists of 21 DEcircRNAs, 11 DEmiRNAs, and 106 DEmRNAs were predicted by the database. After that, 10 hub genes from the PPI network were identified. Then, a new circRNA-miRNA-hub gene network consists of 14 DEcircRNAs, 7 DEmiRNAs, and 9 DEmRNAs was constructed. After that, three key circRNAs (hsa_circ_0009018, hsa_circ_0030569 and hsa_circ_0031017) and three hub genes (BCL6, PTGS2 and PTEN) were identified from the network by qRT-PCR. Finally, immune infiltration analysis showed that hub genes were significantly positively correlated with up-regulated immune cells (neutrophils, macrophages and plasma cells) in AMI.

Conclusion: Our study constructed a circRNA-related ceRNA networks in AMI, consists of hsa_circ_0031017/hsa-miR-142-5p/PTEN axis, hsa_circ_0030569/hsa-miR-545/PTGS2 axis and hsa_circ_0009018/hsa-miR-139-3p/BCL6 axis. These three hub genes were significantly positively correlated with up-regulated immune cells (neutrophils, macrophages and plasma cells) in AMI. It helps improve understanding of AMI mechanism and provides future potential therapeutic targets.

Exome sequencing of deer mice on two California Channel Islands identifies potential adaptation to strongly contrasting ecological conditions

Understanding the forces that drive genotypic and phenotypic change in wild populations is a central goal of evolutionary biology. We examined exome variation in populations of deer mice from two of the California Channel Islands: Peromyscus maniculatus elusus from Santa Barbara Island and P. m. santacruzae from Santa Cruz Island exhibit significant differences in olfactory predator recognition, activity timing, aggressive behavior, morphology, prevalence of Sin Nombre virus, and population densities. We characterized variation in protein-coding regions using exome capture and sequencing of 25 mice from Santa Barbara Island and 22 mice from Santa Cruz Island. We identified and examined 386,256 SNPs using three complementary methods (BayeScan, pcadapt, and LFMM). We found strong differences in molecular variation between the two populations and 710 outlier SNPs in protein-coding genes that were detected by all three methods. We identified 35 candidate genes from this outlier set that were related to differences in phenotypes between island populations. Enrichment analyses demonstrated that patterns of molecular variation were associated with biological processes related to response to chemical stimuli and regulation of immune processes. Candidate genes associated with olfaction (Gfy, Tlr2, Vmn13r2, numerous olfactory receptor genes), circadian activity (Cry1), anxiety (Brca1), immunity (Cd28, Eif2ak4, Il12a, Syne1), aggression (Cyp19a, Lama2), and body size (Bc16, Syne1) exhibited non-synonymous mutations predicted to have moderate to large effects. Variation in olfaction-related genes, including a stop codon in the Santa Barbara Island population, suggests loss of predator-recognition traits at the molecular level, consistent with a lack of behavioral aversion to fox feces. These findings also suggest that divergent pathogen prevalence and population density may have influenced adaptive immunity and behavioral phenotypes, such as reduced aggression. Overall, our study indicates that ecological differences between islands are associated with signatures of selection in protein-coding genes underlying phenotypes that promote success in those environments.

Bcl6-Independent In Vivo Development of Functional Type 1 Classical Dendritic Cells Supporting Tumor Rejection

The transcriptional repressor Bcl6 has been reported as required for development of a subset of classical dendritic cell (cDCs) called cDC1, which is responsible for cross-presentation. However, mechanisms and in vivo functional analysis have been lacking. We generated a system for conditional deletion of Bcl6 in mouse cDCs. We confirmed the reported in vitro requirement for Bcl6 in cDC1 development and the general role for Bcl6 in cDC development in competitive settings. However, deletion of Bcl6 did not abrogate the in vivo development of cDC1. Instead, Bcl6 deficiency caused only a selective reduction in CD8α expression by cDC1 without affecting XCR1 or CD24 expression. Normal cDC1 development was confirmed in Bcl6cKO mice by development of XCR1+ Zbtb46-GFP+ cDC1 by rejection of syngeneic tumors and by priming of tumor-specific CD8 T cells. In summary, Bcl6 regulates a subset of cDC1-specific markers and is required in vitro but not in vivo for cDC1 development.

BCL-6 promotes the methylation of miR-34a by recruiting EZH2 and upregulating CTRP9 to protect ischemic myocardial injury

Acute myocardial infarction (AMI) and the following heart failure are public health problems faced all over the globe. The current study set out to investigate the role of B-cell lymphoma 6 (BCL-6) in cardiac protection after AMI. Initially, AMI mouse models and H9c2 cell oxygen-glucose deprivation (OGD) models were established. The cell models were transfected with the vectors containing oe-BCL-6, oe-EZH2, sh-EZH2, miR-34a mimic, and miR-34a inhibitor. RT-qPCR and Western blot analysis were applied to detect the expression patterns of microRNA-34a (miR-34a), BCL-6, enhancer of zeste homolog 2 (EZH2), and C1q tumor necrosis factor-related protein 9 (CTRP9) in the treated cell models. ChIP-qPCR and co-immunoprecipitation assay were performed to detect EZH2 enrichment and H3K27me3 levels in the miR-34a promoter region and the interaction between BCL-2 and EZH2, respectively. EdU staining, TUNEL staining, and flow cytometry were performed to detect cell proliferation and apoptosis, while ELISA was conducted to detect the oxidative stress levels. It was found that miR-34a was highly expressed in heart tissues of AMI models, while BCL-6 and EZH2 were poorly expressed. BCL-2 overexpression increased the recruitment of EZH2, upregulated H3K27me3 level in the miR-34a promoter region, and inhibited the miR-34a expression. Ctrp9, the downstream negative-regulatory molecule of miR-34a, was upregulated. Besides, miR-34a/CTRP9 expression changes were found to affect cardiomyocyte apoptosis, oxidation stress, and proliferation, and prevent myocardial injury in AMI mice. Our findings indicate that BCL-6 increases the level of H3K27me3 in the promoter region of miR-34a via EZH2 recruitment and CTRP9 upregulation, which inhibits the apoptosis of myocardial cells.

Network medicine in Cardiovascular Research

The ability to generate multi-omics data coupled with deeply characterizing the clinical phenotype of individual patients promises to improve understanding of complex cardiovascular pathobiology. There remains an important disconnection between the magnitude and granularity of these data and our ability to improve phenotype-genotype correlations for complex cardiovascular diseases. This shortcoming may be due to limitations associated with traditional reductionist analytical methods, which tend to emphasize a single molecular event in the pathogenesis of diseases more aptly characterized by crosstalk between overlapping molecular pathways. Network medicine is a rapidly growing discipline that considers diseases as the consequences of perturbed interactions between multiple interconnected biological components. This powerful integrative approach has enabled a number of important discoveries in complex disease mechanisms. In this review, we introduce the basic concepts of network medicine and highlight specific examples by which this approach has accelerated cardiovascular research. We also review how network medicine is well-positioned to promote rational drug design for patients with cardiovascular diseases, with particular emphasis on advancing precision medicine.

Liver-specific knockout of B cell lymphoma 6 suppresses progression of non-alcoholic steatohepatitis in mice

The prevalence of non-alcoholic steatohepatitis (NASH) rapidly increases with metabolic disorders such as dyslipidaemia, high blood pressure, and hyperglycaemia. B cell lymphoma 6 (Bcl6), a transcriptional repressor, is essential for the formation of germinal centre B cells. In this study, we analysed the role of Bcl6 in NASH progression-associated pathological changes, such as hepatic lipid accumulation, liver fibrosis, and hepatocarcinogenesis. The roles of Bcl6 in NASH were analysed using liver-specific Bcl6 knockout (Bcl6-LKO) and control wild-type (WT) mice. The murine NASH model was established by feeding the mice with choline-deficient, L-amino-acid-defined, high-fat diet (CDAHFD). Feeding the WT mice with CDAHFD for 7 weeks induced the formation of histopathological features resembling human NASH, such as hepatic lipid accumulation, hepatocellular injury, and fibrosis. These histopathological changes were significantly attenuated in Bcl6-LKO mice. Additionally, feeding the male WT mice with CDAHFD for 38 weeks induced the formation of liver tumours, which was suppressed in Bcl6-LKO mice. These findings indicate that Bcl6 is involved in the progression of NASH and NASH-derived tumours.

Heart disease in a mutant mouse model of spontaneous eosinophilic myocarditis maps to three loci

Background

Heart disease (HD) is the major cause of morbidity and mortality in patients with hypereosinophilic diseases. Due to a lack of adequate animal models, our understanding of the pathophysiology of eosinophil-mediated diseases with heart complications is limited. We have discovered a mouse mutant, now maintained on an A/J inbred background, that spontaneously develops hypereosinophilia in multiple organs. Cellular infiltration into the heart causes an eosinophilic myocarditis, with affected mice of the mutant line (i.e., A/J^HD) demonstrating extensive myocardial damage and remodeling that leads to HD and premature death, usually by 15-weeks old.

Results

Maintaining the A/J^HD line for many generations established that the HD trait was heritable and implied the mode of inheritance was not too complex. Backcross and intercross populations generated from mating A/J^HD males with females from four different inbred strains produced recombinant populations with highly variable rates of affected offspring, ranging from none in C57BL/6 J intercrosses, to a few mice with HD using 129S1/SvImJ intercrosses and C57BL/6 J backcrosses, but nearly 8% of intercrosses and > 17% of backcrosses from SJL/J related populations developed HD. Linkage analyses of these SJL/J derived recombinants identified three highly significant loci: a recessive locus mapping to distal chromosome 5 (LOD = 4.88; named Emhd1 for eosinophilic myocarditis to heart disease-1); and two dominant variants mapping to chromosome 17, one (Emhd2; LOD = 7.51) proximal to the major histocompatibility complex, and a second (Emhd3; LOD = 6.89) that includes the major histocompatibility region. Haplotype analysis identified the specific crossovers that defined the Emhd1 (2.65 Mb), Emhd2 (8.46 Mb) and Emhd3 (14.59 Mb) intervals.

Conclusions

These results indicate the HD trait in this mutant mouse model of eosinophilic myocarditis is oligogenic with variable penetrance, due to multiple segregating variants and possibly additional genetic or nongenetic factors. The A/J^HD mouse model represents a unique and valuable resource to understand the interplay of causal factors that underlie the pathology of this newly discovered eosinophil-associated disease with cardiac complications.

Eicosanoids and HB-EGF/EGFR in cancer

Eicosanoids are bioactive lipids that play crucial roles in various pathophysiological conditions, including inflammation and cancer. They include both the COX-derived prostaglandins and the LOX-derived leukotrienes. Furthermore, the epidermal growth factor receptor (EGFR) pathways family of receptor tyrosine kinases also are known to play a central role in the tumorigenesis. Various antitumor modalities have been approved cancer treatments that target therapeutically the COX-2 and EGFR pathways; these include selective COX-2 inhibitors and EGFR monoclonal antibodies. Research has shown that the COX-2 and epidermal growth factor receptor pathways actively interact with each other in order to orchestrate carcinogenesis. This has been used to justify a targeted combinatorial approach aimed at these two pathways. Although combined therapies have been found to have a greater antitumor effect than the administration of single agent, this does not exempt them from the possible fatal cardiac effects that are associated with COX-2 inhibition. In this review, we delineate the contribution of HB-EGF, an important EGFR ligand, to the cardiac dysfunction related to decreased shedding of HB-EGF after COX-2/PGE2 inhibition. A better understanding of the molecular mechanisms underlying these cardiac side effects will make possible more effective regimens that use the dual-targeting approach.

Cutting Edge: T Cell-Dependent Plasmablasts Form in the Absence of Single Differentiated CD4+ T Cell Subsets

The T follicular helper (Tfh) cell subset of CD4+ Th cells promotes affinity maturation by B cells in germinal centers. The contribution of other Th cell subsets to B cell responses has not been fully explored in vivo. We addressed this issue by analyzing the T cell-dependent B cell response to the protein Ag PE in mice lacking specific Th cell subsets. As expected, PE-specific germinal center B cell production required Tfh cells. However, Tfh, Th1, or Th17 cell-deficient mice produced as many PE-specific, isotype-switched plasmablasts as wild-type mice. This response depended on Th cell expression of CD154 and Ag presentation by B cells. These results indicate that many Th cell subsets can promote plasmablast formation by providing CD40 signals to naive B cells.

Establishment and analysis of a mouse model that regulates sex-related differences in liver drug metabolism

The adult liver performs many metabolic functions for maintaining homeostasis. There are several sex differences in liver function and disease pathogenesis. One important function of the liver is drug metabolism, where cytochrome p450s (CYPs) in hepatocytes are the main enzymes involved. The toxicity of various drugs and chemicals differs with sex due to differences in hepatocytic CYP expression. However, the molecular mechanism regulating sex-related differences in drug metabolism remains unknown. In this study, we identified transcriptional regulator B-cell lymphoma 6 (Bcl6) as an important factor in sex-biased differential CYP expression. Microarray analysis of livers derived from liver-specific Bcl6-knockout mice showed that Bcl6 is required for sex-biased CYP expression patterns in the liver. Additionally, quantitative PCR analysis revealed that hepatocytic expression of male-biased genes, such as Cyp2d9, Cyp2u1, Cyp4a12a/12b, and Cyp7b1, in liver-specific Bcl6-knockout male mice significantly decreased to levels similar to those observed in wild-type female mice. Conversely, hepatocytic expression of female-biased genes, such as Cyp2a4/2a5, Cyp2b9, Cyp3a41, and Cyp17a1, significantly increased in liver-specific Bcl6-knockout male mice. Deletion of Bcl6 caused female-like expression of CYPs in male livers. These results suggest that Bcl6 is a key regulator of sex-related differential regulation of drug metabolism. Moreover, serum sex hormone levels and fertility did not change in liver-specific, Bcl6-knockout mice. Hepatocytic Bcl6 regulates sex-related differential CYP expression in the liver without changing the sex of the whole body. Thus, this mouse model is useful for analyzing liver-specific sex-dependent regulation of drug metabolism and pathogenesis.

Allergic TH2 Response Governed by B-Cell Lymphoma 6 Function in Naturally Occurring Memory Phenotype CD4+ T Cells

Transcriptional repressor B-cell lymphoma 6 (Bcl6) appears to regulate T_H2 immune responses in allergies, but its precise role is unclear. We previously reported that Bcl6 suppressed IL-4 production in naïve CD4⁺ T cell-derived memory T_H2 cells. To investigate Bcl6 function in allergic responses in naturally occurring memory phenotype CD4⁺ T (MPT) cells and their derived T_H2 (MPT_H2) cells, Bcl6-manipulated mice, highly conserved intron enhancer (hcIE)-deficient mice, and reporter mice for conserved noncoding sequence 2 (CNS2) 3′ distal enhancer region were used to elucidate Bcl6 function in MPT cells. The molecular mechanisms of Bcl6-mediated T_H2 cytokine gene regulation were elucidated using cellular and molecular approaches. Bcl6 function in MPT cells was determined using adoptive transfer to naïve mice, which were assessed for allergic airway inflammation. Bcl6 suppressed IL-4 production in MPT and MPT_H2 cells by suppressing CNS2 enhancer activity. Bcl6 downregulated Il4 expression in MPT_H2 cells, but not MPT cells, by suppressing hcIE activity. The inhibitory functions of Bcl6 in MPT and MPT_H2 cells attenuated allergic responses. Bcl6 is a critical regulator of IL-4 production by MPT and MPT_H2 cells in T_H2 immune responses related to the pathogenesis of allergies.

Internal deletion of BCOR reveals a tumor suppressor function for BCOR in T lymphocyte malignancies

Tanaka et al. show that BCL6 corepressor (BCOR) targets a significant portion of NOTCH1 targets in thymocytes to restrain their activation. Conditional deletion of the BCL6-binding domain of BCOR results in induction of Notch-dependent acute T-cell lymphoblastic leukemia in mice.

Recurrent inactivating mutations have been identified in various hematological malignancies in the X-linked BCOR gene encoding BCL6 corepressor (BCOR); however, its tumor suppressor function remains largely uncharacterized. We generated mice missing Bcor exon 4, expressing a variant BCOR lacking the BCL6-binding domain. Although the deletion of exon 4 in male mice (Bcor^ΔE4/y) compromised the repopulating capacity of hematopoietic stem cells, Bcor^ΔE4/y thymocytes had augmented proliferative capacity in culture and showed a strong propensity to induce acute T-cell lymphoblastic leukemia (T-ALL), mostly in a Notch-dependent manner. Myc, one of the critical NOTCH1 targets in T-ALL, was highly up-regulated in Bcor^ΔE4/y T-ALL cells. Chromatin immunoprecipitation/DNA sequencing analysis revealed that BCOR was recruited to the Myc promoter and restrained its activation in thymocytes. BCOR also targeted other NOTCH1 targets and potentially antagonized their transcriptional activation. Bcl6-deficient thymocytes behaved in a manner similar to Bcor^ΔE4/y thymocytes. Our results provide the first evidence of a tumor suppressor role for BCOR in the pathogenesis of T lymphocyte malignancies.

Eosinophil-derived IL-4 drives progression of myocarditis to inflammatory dilated cardiomyopathy

Diny et al. report a pathogenic role for eosinophils in autoimmune myocarditis and dilated cardiomyopathy. Eosinophils are required for progression of myocarditis to dilated cardiomyopathy and drive severe disease when present in large numbers. Activated cardiac eosinophils mediate this process through IL-4.

Inflammatory dilated cardiomyopathy (DCMi) is a major cause of heart failure in children and young adults. DCMi develops in up to 30% of myocarditis patients, but the mechanisms involved in disease progression are poorly understood. Patients with eosinophilia frequently develop cardiomyopathies. In this study, we used the experimental autoimmune myocarditis (EAM) model to determine the role of eosinophils in myocarditis and DCMi. Eosinophils were dispensable for myocarditis induction but were required for progression to DCMi. Eosinophil-deficient ΔdblGATA1 mice, in contrast to WT mice, showed no signs of heart failure by echocardiography. Induction of EAM in hypereosinophilic IL-5Tg mice resulted in eosinophilic myocarditis with severe ventricular and atrial inflammation, which progressed to severe DCMi. This was not a direct effect of IL-5, as IL-5TgΔdblGATA1 mice were protected from DCMi, whereas IL-5^−/− mice exhibited DCMi comparable with WT mice. Eosinophils drove progression to DCMi through their production of IL-4. Our experiments showed eosinophils were the major IL-4–expressing cell type in the heart during EAM, IL-4^−/− mice were protected from DCMi like ΔdblGATA1 mice, and eosinophil-specific IL-4 deletion resulted in improved heart function. In conclusion, eosinophils drive progression of myocarditis to DCMi, cause severe DCMi when present in large numbers, and mediate this process through IL-4.

The transcriptional repressor Bcl6 controls the stability of regulatory T cells by intrinsic and extrinsic pathways

Foxp3(+) regulatory T (Treg) cells are essential to maintain immune homeostasis, yet controversy exists about the stability of this cell population. Bcl6-deficient (Bcl6(-/-) ) mice develop severe and spontaneous T helper type 2 (Th2) inflammation and Bcl6-deficient Treg cells are ineffective at controlling Th2 responses. We used a lineage tracing approach to analyse the fate of Treg cells in these mice. In the periphery of Bcl6(-/-) mice, increased numbers of Foxp3-negative 'exTreg' cells were found, particularly in the CD25(+) population. ExTreg cells from Bcl6(-/-) mice expressed increased interleukin-17 (IL-17) and extremely elevated levels of Th2 cytokines compared with wild-type exTreg cells. Although Treg cells normally express only low levels of cytokines, Treg cells from Bcl6(-/-) mice secreted higher levels of IL-4, IL-5, IL-13 and IL-17 than wild-type conventional T cells. Next, Treg-specific conditional Bcl6-deficient (Bcl6(Foxp3-/-) ) mice were analysed. Bcl6(Foxp3-/-) mice do not develop inflammatory disease, indicating a requirement for non-Treg cells for inflammation in Bcl6(-/-) mice, and have normal numbers of exTreg cells. We induced Th2-type allergic airway inflammation in Bcl6(Foxp3-/-) mice, and found that while exTreg cytokine expression was normal, Bcl6-deficient Treg cells expressed higher levels of the Th2-specific regulator Gata3 than Bcl6(+) Treg cells. Bcl6(Foxp3-/-) mice had increased numbers of Th2 cells after induction of airway inflammation and increased T cells in the bronchoalveolar lavage fluid. These data show both Treg-intrinsic and Treg-extrinsic roles for Bcl6 in controlling Treg cell stability and Th2 inflammation, and support the idea that Bcl6 expression in Treg cells is critical for controlling Th2 responses.

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.

Therapeutic inhibition of the miR-34 family attenuates pathological cardiac remodeling and improves heart function

MicroRNAs are dysregulated in a setting of heart disease and have emerged as promising therapeutic targets. MicroRNA-34 family members (miR-34a, -34b, and -34c) are up-regulated in the heart in response to stress. In this study, we assessed whether inhibition of the miR-34 family using an s.c.-delivered seed-targeting 8-mer locked nucleic acid (LNA)-modified antimiR (LNA-antimiR-34) can provide therapeutic benefit in mice with preexisting pathological cardiac remodeling and dysfunction due to myocardial infarction (MI) or pressure overload via transverse aortic constriction (TAC). An additional cohort of mice subjected to MI was given LNA-antimiR-34a (15-mer) to inhibit miR-34a alone as a comparison for LNA-antimiR-34. LNA-antimiR-34 (8-mer) efficiently silenced all three miR-34 family members in both cardiac stress models and attenuated cardiac remodeling and atrial enlargement. In contrast, inhibition of miR-34a alone with LNA-antimiR-34a (15-mer) provided no benefit in the MI model. In mice subjected to pressure overload, LNA-antimiR-34 improved systolic function and attenuated lung congestion, associated with reduced cardiac fibrosis, increased angiogenesis, increased Akt activity, decreased atrial natriuretic peptide gene expression, and maintenance of sarcoplasmic reticulum Ca(2+) ATPase gene expression. Improved outcome in LNA-antimiR-34-treated MI and TAC mice was accompanied by up-regulation of several direct miR-34 targets, including vascular endothelial growth factors, vinculin, protein O-fucosyltranferase 1, Notch1, and semaphorin 4B. Our results provide evidence that silencing of the entire miR-34 family can protect the heart against pathological cardiac remodeling and improve function. Furthermore, these data underscore the utility of seed-targeting 8-mer LNA-antimiRs in the development of new therapeutic approaches for pharmacologic inhibition of disease-implicated miRNA seed families.

BAZF, a novel component of cullin3-based E3 ligase complex, mediates VEGFR and Notch cross-signaling in angiogenesis

Angiogenic homeostasis is maintained by a balance between vascular endothelial growth factor (VEGF) and Notch signaling in endothelial cells (ECs). We screened for molecules that might mediate the coupling of VEGF signal transduction with down-regulation of Notch signaling, and identified B-cell chronic lymphocytic leukemia/lymphoma6-associated zinc finger protein (BAZF). BAZF was induced by VEGF-A in ECs to bind to the Notch signaling factor C-promoter binding factor 1 (CBF1), and to promote the degradation of CBF1 through polyubiquitination in a CBF1-cullin3 (CUL3) E3 ligase complex. BAZF disruption in vivo decreased endothelial tip cell number and filopodia protrusion, and markedly abrogated vascular plexus formation in the mouse retina, overlapping the retinal phenotype seen in response to Notch activation. Further, impaired angiogenesis and capillary remodeling were observed in skin-wounded BAZF(-/-) mice. We therefore propose that BAZF supports angiogenic sprouting via BAZF-CUL3-based polyubiquitination-dependent degradation of CBF1 to down-regulate Notch signaling.

Ectodomain shedding and remnant peptide signalling of EGFRs and their ligands

Both receptor tyrosine kinases epidermal growth factor receptors (EGFRs) and their ligands are transmembrane proteins. It has been known that ligand binding activates cytoplasmic tyrosine kinase domains of EGFRs, resulting in the transduction of signals for cell proliferation, migration, differentiation or survival. In an EGFRs-ligands system, however, signal transduction occurs not only unidirectionally but also bidirectionally, which is regulated by cell-cell contact and proteolytic cleavage. Recent studies of proteolytic cleavage 'ectodomain shedding' of EGFRs and their ligands mediated by membrane-type metalloproteinases, a disintegrin and metalloproteinases have been unveiling novel functions and molecular mechanism of their remnant peptides. In addition, the study of the remnant peptide signalling would be essential for understanding the physiological and pathological relevance of anti-shedding therapeutic strategies for diseases such as cancer.

CXCR4 expression on activated B cells is downregulated by CD63 and IL-21

CXCR4 expression is critical for localization of centroblasts in the dark zone of germinal centers (GCs), and centrocytes downregulate CXCR4 and thus leave the dark zone to reside in the light zone. However, mechanisms governing CXCR4 downregulation on centrocytes are not known. In this study, we show that the amount of intracellular CXCR4 in centroblasts was similar to that in centrocytes, suggesting differential control of CXCR4 protein expression in these GC B cells. Restimulation of activated B cells with IL-21, which is a major cytokine produced by T follicular helper cells, accelerated CXCR4 internalization by inducing endocytosis-related GRK6 expression. Although CXCR4 expression was downregulated on GC B cells by IL-21 stimulation, CXCR4(low) centrocytes developed in the spleens of IL-21R-deficient mice, suggesting other mechanisms for downregulation. The level of CD63 (which recruits CXCR4 to late endosome in CD4 T cells) in centrocytes was more than that in centroblasts and was strikingly elevated in activated Bcl6-deficient B cells. Bcl6, a transcriptional repressor, was detected on the chromatin of the CD63 gene in resting B cells, therefore CD63 is a molecular target of Bcl6. Downregulation of CD63 mRNA in activated Bcl6-deficient B cells by small interfering RNA upregulated CXCR4 expression on the B cells. Furthermore, addition of Bcl6 inhibitor to activated B cell cultures increased CD63 mRNA expression in (and downregulated CXCR4 expression on) those activated B cells. Thus, CXCR4 can be downregulated on activated B cells by IL-21-induced endocytosis and CD63-mediated endosomal recruitment, and these mechanisms may contribute to downregulation of CXCR4 on centrocytes.

Bcl6 is required for the development of mouse CD4+ and CD8α+ dendritic cells

Th2-type inflammation spontaneously shown in Bcl6-knockout (KO) mice is mainly caused by bone marrow (BM)-derived nonlymphoid cells. However, the function of dendritic cells (DCs) in Bcl6-KO mice has not been reported. We show in this article that the numbers of CD4(+) conventional DCs (cDCs) and CD8α(+) cDCs, but not of plasmacytoid DCs, were markedly reduced in the spleen of Bcl6-KO mice. Generation of cDCs from DC progenitors in BM cells was perturbed in the spleen of irradiated wild-type (WT) mice transferred with Bcl6-KO BM cells, indicating an intrinsic effect of Bcl6 in cDC precursors. Although cDC precursors were developed in a Bcl6-KO BM culture with Fms-like tyrosine kinase 3 ligand, the cDC precursors were more apoptotic than WT ones. Also p53, one of the molecular targets of Bcl6, was overexpressed in the precursors. The addition of a p53 inhibitor to Bcl6-KO BM culture protected apoptosis, suggesting that Bcl6 is required by cDC precursors for survival by controlling p53 expression. Furthermore, large numbers of T1/ST2(+) Th2 cells were naturally developed in the spleen of Bcl6-KO mice. Th2 skewing was accelerated in the culture of WT CD4 T cells stimulated with Ags and LPS-activated Bcl6-KO BM-derived DCs, which produced more IL-6 and less IL-12 than did WT DCs; the addition of anti-IL-6 Abs to the culture partially abrogated the Th2 skewing. These results suggest that Bcl6 is required in cDC precursors for survival and in activated DCs for modulating the cytokine profile.

Far from solved: a perspective on what we know about early mechanisms of left-right asymmetry

Consistent laterality is a crucial aspect of embryonic development, physiology, and behavior. While strides have been made in understanding unilaterally expressed genes and the asymmetries of organogenesis, early mechanisms are still poorly understood. One popular model centers on the structure and function of motile cilia and subsequent chiral extracellular fluid flow during gastrulation. Alternative models focus on intracellular roles of the cytoskeleton in driving asymmetries of physiological signals or asymmetric chromatid segregation, at much earlier stages. All three models trace the origin of asymmetry back to the chirality of cytoskeletal organizing centers, but significant controversy exists about how this intracellular chirality is amplified onto cell fields. Analysis of specific predictions of each model and crucial recent data on new mutants suggest that ciliary function may not be a broadly conserved, initiating event in left-right patterning. Many questions about embryonic left-right asymmetry remain open, offering fascinating avenues for further research in cell, developmental, and evolutionary biology.

The proto-oncogene BCL6 promotes survival of olfactory sensory neurons

For the mammalian olfactory epithelium to continually detect odorant, neuronal survival, apoptosis, and regeneration must be coordinated. Here, we showed that the proto-oncogene BCL6, which encodes a transcriptional repressor required for lymphocyte terminal differentiation, contributes to the survival of olfactory sensory neurons (OSNs). In the olfactory epithelia of the BCL6 null mutant mice, many OSNs were positive for both OMP and GAP43. The epithelium was relatively thinner, showing many apoptotic signals. These characters were phenotypically similar to those of the wild-type mice treated with nasal lectin irrigation, which acutely induces apoptosis of OSNs. Odorant receptors were expressed normally in the epithelia of the mutant mice, and their overall expression profile based on DNA microarray analyses was roughly similar to that of the apoptosis-induced olfactory epithelia of the wild-type mice. Experimental increase of BCL6 together with green fluorescent protein in OSNs using adenovirus-mediated gene transfer made the epifluorescence last longer than the control fluorescence without exogenous BCL6 after the nasal lectin irrigation, indicating that BCL6 made the infected neurons survive longer. We conclude that BCL6 plays an active role in the survival of OSNs as an anti-apoptotic factor and confers immature OSNs enough time to fully differentiate into mature ones.

BCL6 canalizes Notch-dependent transcription, excluding Mastermind-like1 from selected target genes during left-right patterning

Although the Notch signaling pathway is one of the most intensely studied intracellular signaling pathways, the mechanisms by which Notch signaling regulates transcription remain incompletely understood. Here, we report that B cell leukemia/lymphoma 6 (BCL6), a transcriptional repressor, is a Notch-associated factor. BCL6 is necessary to maintain the expression of Pitx2 in the left lateral plate mesoderm during the patterning of left-right asymmetry in Xenopus embryos. For this process, BCL6 forms a complex with BCL6 corepressor (BCoR) on the promoters of selected Notch target genes such as enhancer of split related 1. BCL6 also inhibits the transcription of these genes by competing for the Notch1 intracellular domain, preventing the coactivator Mastermind-like1 (MAM1) from binding. These results define a mechanism restricting Notch-activated transcription to cell-type-appropriate subsets of target genes, and elucidate its relevance in vivo during left-right asymmetric development.

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.

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.

Membrane-anchored growth factors, the epidermal growth factor family: beyond receptor ligands

The epidermal growth factor (EGF) family and the EGF receptor (EGFR, ErbB) tyrosine kinase family have been spearheading the studies of signal transduction events that determine cell fate and behavior in vitro and in vivo. The EGFR family and their signaling pathways are giving us tremendous advantages in developing fascinating molecular target strategies for cancer therapy. Currently, two important types of EGFR inhibitors are in clinical use: neutralizing antibodies of EGFR or ErbB2, and synthetic small compounds of tyrosine kinase inhibitors designed for receptors. On the other hand, basic research of the EGF family ligands presents new challenges as membrane-anchored growth factors. All members of the EGF family have important roles in development and diseases and are shed from the plasma membrane by metalloproteases. The ectodomain shedding of the ligands has emerged as a critical component in the functional transactivation of EGFRs in interreceptor cross-talk in response to various shedding stimulants such as G-protein coupled receptor agonists, growth factors, cytokines, and various physicochemical stresses. Among the EGFR-ligands, heparin-binding EGF-like growth factor (HB-EGF) is a prominent ligand in our understanding of the pathophysiological roles of ectodomain shedding in cancer, wound healing, cardiac diseases, etc. Here we focus on ectodomain shedding of the EGF family ligands, especially HB-EGF by disintegrin and metalloproteases, which are not only key events of receptor cross talk, but also novel intercellular signaling by their carboxy-terminal fragments to regulate gene expression directly.

Inhibition of interleukin-1beta-induced group IIA secretory phospholipase A2 expression by peroxisome proliferator-activated receptors (PPARs) in rat vascular smooth muscle cells: cooperation between PPARbeta and the proto-oncogene BCL-6

The inflammation that occurs during atherosclerosis is characterized by the release of large amounts of group IIA secretory phospholipase A2 (sPLA2-IIA). This study was designed to define the function of the three peroxisome proliferator-activated receptors (PPARs) on sPLA2 expression in vascular smooth muscle cells (VSMCs). We found that PPAR ligands decreased sPLA2-IIA activity and inhibited mRNA accumulation under inflammatory conditions. Furthermore, interleukin-1beta-induced sPLA2-IIA promoter activity was inhibited by the three PPAR ligands and in a similar way when cells were cotransfected with PPARalpha, PPARbeta, or PPARgamma, plus retinoid X receptor alpha (RXRalpha). Our study revealed that the regulation of sPLA2-IIA gene transcription by PPARalpha/RXR and PPARgamma/RXR heterodimers requires an interaction with a PPAR response element (PPRE) of the sPLA2-IIA promoter. In contrast, PPARbeta operates through a PPRE-independent mechanism. In addition, we demonstrated that VSMCs expressed the transcriptional repressor BCL-6. Overexpression of BCL-6 markedly reduced sPLA2-IIA promoter activity in VSMCs, while a dominant negative form of BCL-6 abrogated sPLA2 repression by PPARbeta. The PPARbeta agonist induced a BCL-6 binding to the sPLA2 promoter in VSMCs under inflammatory conditions. The knockdown of BCL-6 by short interfering RNA abolished the inhibitory effect of the PPARbeta ligand on sPLA2 activity and prostaglandin E2 release. Thus, the inhibition of sPLA2-IIA activity by PPARbeta agonists may provide a promising approach to impacting the initiation and progression of atherosclerosis.

Bcl6 controls granzyme B expression in effector CD8+ T cells

Bcl6, a sequence-specific transcriptional repressor, is important for generation and maintenance of memory CD8(+) T cells. Although memory CD8(+) T cells are generated from effector CD8(+) T cells, a role for Bcl6 in effector CD8(+) T cells is largely unknown. We show here that Bcl6 expression was transiently induced in activated CD8(+) T cells and continuously up-regulated in effector CD8(+) T cells. The amount of granzyme B mRNA among effector molecules produced by effector CD8(+) T cells inversely correlated with the amount of Bcl6 mRNA in CD8(+) T cells. Overexpression of Bcl6 in CD8(+) T cells resulted in lower killing activity at their effector phase, supporting the reduction of granzyme B expression in effector CD8(+) T cells by Bcl6. We identified a putative Bcl6-binding DNA sequence in the promoter region of the granzyme B gene. Binding of Bcl6 to the Bcl6-binding sequence was detected in naive CD8(+) T cells but not in activated CD8(+) T cells by chromatin immunoprecipitation assay. Furthermore, the Bcl6-binding sequence was required for Bcl6 to repress the luciferase reporter gene expression controlled by the granzyme B promoter. Thus, the granzyme B gene is a molecular target of Bcl6 in effector CD8(+) T cells.

JunD/AP-1 and STAT3 are the major enhancer molecules for high Bcl6 expression in germinal center B cells

The Bcl6 proto-oncogene, which encodes a transcriptional repressor, is ubiquitously expressed and predominantly in germinal center (GC) B cells. Although the promoter region of the human Bcl6 gene has been reported, enhancer molecules for its high expression in GC B cells were largely unknown. Here we show that transcriptional start sites of the murine Bcl6 gene were different from the reported human one. DNA sequence around the new promoter region is highly conserved between mice and humans and has no canonical TATA or CCAAT box. Two AP-1-binding elements in the promoter region were the major enhancer elements in GC-derived B lymphoma cells, and JunD/AP-1 was detected in GC B cells. In addition, we identified the silencer region with three Bcl6-binding elements around the start site. Bcl6 bound to the silencer elements and its over-expression repressed the promoter activity through the elements. Activated STAT factors (STATs), especially activated STAT3, also bound to the silencer elements in GC B cells and competed with Bcl6 for the binding, suggesting that JunD/AP-1 and activated STATs drive high Bcl6 expression in GC B cells. Since stimulation of splenic B cells with IL-4 or IL-21 induced high Bcl6 expression with induction of junD and activation of STATs, these cytokines may be inducers for its high expression in GC B cells. However, IL-21 but not IL-4 stimulation activated STAT3 in splenic B cells. Thus, IL-21 may be a major inducer for high Bcl6 expression in GC B cells.

The proto-oncogene BCL-6 is expressed in olfactory sensory neurons

Mammalian olfactory sensory neurons harbor a large repertoire of odorant receptors. Yet, each cell chooses a single odorant receptor gene to express, which may then serve as the molecular identification of that cell. This process of differentiation or receptor gene regulation of olfactory sensory neurons has largely been enigmatic. Here, we showed that the important proto-oncogene in B lymphocyte terminal differentiation, B-Cell Leukemia/Lymphoma 6 (BCL-6), which codes for a sequence-specific transcription repressor, is expressed in olfactory sensory neurons in the mouse. We detected BCL-6 mRNA in most, if not all, mature olfactory sensory neurons by in situ hybridization histochemistry. Western blot analysis also revealed the presence of BCL-6 protein in the olfactory epithelium, but immunohistochemical analysis using anti-BCL-6 antibodies showed that only a subset of olfactory sensory neurons highly expressed BCL-6 protein. BCL-6 mRNA was detected as early as embryonic day 14 (E14) in most olfactory sensory neurons as in adults, and at E15, BCL-6 protein was detected in most cells that were likely to be differentiating into mature olfactory sensory neurons. Artificial induction of apoptosis of olfactory sensory neurons resulted in the decrease of BCL-6 mRNA, but during the subsequent phase of regeneration and differentiation, it markedly increased. Taken together, our results suggest the possibility that BCL-6 plays an important role in terminal differentiation not only in B lymphocytes but also in olfactory sensory neurons.

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.

Bcl6 regulates Th2 type cytokine productions by mast cells activated by FcepsilonRI/IgE cross-linking

Bcl6-deficient (Bcl6-/-) mice displayed Th2 type inflammation, which caused by abnormality of non-lymphoid cells. However, initiators for the Th2 type inflammation were not clear. In order to elucidate the initiators, we investigated property and function of mast cells derived from Bcl6-/- mice. Mast cells were developed from bone marrow cells cultured with IL-3 (BMMCs). Although the development of BMMCs from Bcl6-/- mice was similar to that from wild-type mice, proliferation of Bcl6-/- BMMCs stimulated with IL-3 was slightly lower than that of wild-type BMMCs. When these BMMCs were stimulated by FcepsilonRI/IgE cross-linking, Bcl6-/- BMMCs produced Th2 cytokines more than wild-type BMMCs did. Thus, Bcl6-/- mast cells are one of the initiators for Th2 type inflammation in Bcl6-/- mice, and Bcl6 may be a molecular target for Th2 type allergic diseases.

The ARF tumor suppressor inhibits BCL6-mediated transcriptional repression

The ARF tumor suppressor gene antagonizes generation of various tumors. ARF-mediated tumor suppression occurs in a p53-independent manner as well as in a p53-dependent manner. We here demonstrate that BCL6 is a target of the ARF tumor suppressor. Either mouse p19(ARF) or human p14(ARF) binds to BCL6 and downregulates BCL6-induced transcriptional repression. ARF-mediated downregulation of the BCL6 activity may account in part for ARF-mediated tumor suppression.

Abnormal erythroid differentiation in neonatal bcl-6-deficient mice

OBJECTIVE

The bcl-6 proto-oncogene is ubiquitously expressed in various tissues. Since we found out the smaller number of TER119(+) cells in the spleen of neonatal bcl-6-deficient (bcl-6(-/-)) mice compared with that of control (bcl-6(+/+)) littermates, we studied functions of bcl-6 in differentiation of erythroid lineage cells.

MATERIALS AND METHODS

Erythroblasts in the definitive erythropoiesis were separated into four subsets using anti-TER119 and anti-CD71 mAbs. The cell number and property of these four subsets in spleens of neonatal bcl-6(+/+) and bcl-6(-/-) mice were examined using a flow cytometry.

RESULTS

bcl-6 mRNA expression was detected in the TER119(high)CD71(high) subset, which is morphologically equivalent to basophilic erythroblasts, by reverse-transcribed polymerase chain reaction. High percentages of cells in the TER119(low)CD71(high) and TER119(high)CD71(high) subsets were in the cell cycle. The cell number of the TER119(high)CD71(high) subset in the spleen and the percentage of reticulocytes in the peripheral blood of neonatal bcl-6(-/-) mice were significantly lower than those of neonatal bcl-6(+/+) mice. However, the percentage of apoptotic cells and that of cells in the cell cycle in the TER119(high)CD71(high) subset of bcl-6(-/-) mice were similar to those of bcl-6(+/+) mice.

CONCLUSION

bcl-6 detected in the TER119(high)CD71(high) subset of erythroblasts in the spleen of neonatal mice may be required to retain the erythroblasts in the cell proliferation stage.

Bcl6 is a transcriptional repressor for the IL-18 gene

Bcl6 functions as a sequence-specific transcriptional repressor, and Bcl6-deficient (Bcl6(-/-)) mice have been reported to display Th2-type inflammatory diseases in multiple organs. Since IL-18 is a potent stimulator of Th2 cells, we examined the expression of IL-18 mRNA in bone marrow-derived macrophages from Bcl6(-/-) mice after LPS stimulation. Here we show that the expression was strikingly up-regulated after stimulation. The expression was also up-regulated in RAW264 cells, a murine macrophage cell line, by transfection with the dominant negative type of Bcl6 gene. We identified a putative Bcl6-binding DNA sequence (IL-18BS) upstream of exon 1 of the murine IL-18 gene and three IL-18BSs in the promoter region of human IL-18 gene. Binding of Bcl6 in nuclear protein from resting RAW264 cells to murine IL-18BS was detected by gel retardation assay and chromatin immunoprecipitation assay. The binding activity was diminished gradually in RAW264 cells after LPS stimulation. However, the amount of Bcl6 protein in these cells was constant over the period examined, suggesting the functional modification of Bcl6 protein after stimulation. Furthermore, murine IL-18BS was required for Bcl6 to repress the expression of the luciferase reporter gene under control of the IL-18 promoter. Taken together, Bcl6 is a key regulator of IL-18 production by macrophages.

Eosinophilic heart: marked left ventricular wall thickening and myocardial dysfunction improving with corticosteroid therapy

We report a case of a 34-year-old male with acute severe heart failure associated with marked concentric left ventricular wall thickening and biopsy evidence of eosinophilic myocardial infiltrate. This appears to be an unusual description of this degree of concentric myocardial thickening in eosinophilic myocarditis coupled with Doppler tissue echocardiography. Following high-dose corticosteroid treatment, wall thickness, systolic and diastolic left ventricular function normalized and the patient experienced a dramatic clinical improvement.

Ambivalent role of BCL6 in cell survival and transformation

The BCL6 gene is often structurally altered and probably 'misregulated' in two different types of human B-cell non-Hodgkin lymphomas (BNHL) thought to arise from germinal centre B cells. BCL6 encodes a BTB/POZ and zinc finger protein whose biochemical properties support a role as a DNA-binding transcriptional repressor and disclose, in part, the underlying mechanisms. In contrast, the study of the 'oncogenic' structural alterations of BCL6 in BNHL and of its cellular functions gives rise to much more heterogeneous data with no obvious unifying picture so that how and even whether BCL6 contributes to lymphomagenesis remains unclear. This review will summarize the current knowledge about the 'oncogenic' alterations and cellular functions of BCL6 and, based on some results, will propose the following hypotheses: (1) In various systems, including in memory T cells and also in germinal centre B cells and possibly in certain postmitotic cells, BCL6 may act by stabilizing a particular stage of differentiation. (2) Both its ambivalent effects on cell survival and the heterogeneous consequences of its alterations in BNHL suggest that BCL6 can be oncogenic not only upon overexpression or persistent expression, as often proposed, but also, similar to some of its relatives, upon 'accidental' downregulation.

Memory B cells without somatic hypermutation are generated from Bcl6-deficient B cells

After immunization with T cell-dependent antigens, the high-affinity B cells selected in germinal centers differentiate into memory B cells or long-lived antibody-forming cells. However, a role for germinal centers in development of these B lineage cells is still controversial. We show here that Bcl6-deficient B cells, which cannot develop germinal centers, differentiated into IgM and IgG1 memory B cells in the spleen but barely differentiated into long-lived IgG1 antibody-forming cells in the bone marrow. Mutation in the V-heavy gene was null in these memory B cells. Therefore, Bcl6 and germinal center formation are essential for somatic hypermutation, and generation of memory B cells can occur independently of germinal center formation, somatic hypermutation, and Ig class switching.

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.

A putative silencer element in the IL-5 gene recognized by Bcl6

The Bcl6 gene is ubiquitously expressed in adult murine tissues and its product functions as a sequence-specific transcriptional repressor. Bcl6-deficient mice displayed eosinophilic inflammation caused by overproduction of Th2 cytokines. The regulatory mechanism of those cytokine productions by Bcl6 is controversial. When CD4(+) T cells from Bcl6-deficient and lck-Bcl6-transgenic mice were stimulated with anti-CD3 Abs, production of IL-5 among Th2 type cytokines was preferentially affected by the amount of Bcl6 in the T cells. We found a putative Bcl6-binding sequence (IL5BS) on the 3' untranslated region in the murine and human IL-5 genes, and specific binding of Bcl6 protein to the sequence was confirmed by gel retardation assay and chromatin immunoprecipitation assay. The binding activity of endogenous Bcl6 was transiently diminished in Th2 but not in Th1 clones after anti-CD3 stimulation. The exogenous Bcl6 repressed expression of the reporter gene with the IL5BS in K562 cells and the repressor activity was lost by a point mutation of the IL5BS. Furthermore, the IL5BS was required for Bcl6 to repress expression of the IL-5 cDNA. Thus, the IL5BS may act as a silencer element for Bcl6 to repress expression of the IL-5 gene.

Role for Bcl-6 in the generation and maintenance of memory CD8+ T cells

Naïve T cells proliferate and differentiate into memory cells after antigenic stimulation or in a lymphopenic environment. We showed here transient increases in memory phenotype CD8+ T cell numbers in the lymphopenic environment of spleens of very young mice. The magnitude of the increase correlated with Bcl-6 expression in the T cells. Bcl-6 controlled the generation and maintenance of antigen-specific memory phenotype CD8+ T cells in the spleens of immunized mice. These data suggest that Bcl-6, which is essential for memory B cell development in germinal centers, is a key molecule for the establishment not only of memory T cells but also of the peripheral T cell compartment in infancy.

Cloning and characterization of the human BAZF gene, a homologue of the BCL6 oncogene

The BAZF gene has recently been identified as a novel homologue of the BCL6 oncogene. Here we cloned the human BAZF gene using murine BAZF as a probe. The predicted amino acid sequence was 91% identical to that of murine BAZF. The BTB/POZ and zinc finger domains were almost completely conserved between human and murine BAZF. Fluorescence in situ hybridization analysis revealed that the human BAZF gene is located on chromosome 17p13.1. Although expression of human BAZF mRNA was ubiquitously detected in human tissues, abundant expression was detected in heart and placenta. BAZF mRNA was expressed in some immature B cell lines and erythroleukemia cell lines. The expression in a human erythroleukemia cell line, HEL cells, was upregulated during megakaryocytic differentiation induced by 12-O-tetradecanoyl-phorbol-13-acetate. These expression patterns of BAZF mRNA suggest that BAZF may regulate differentiation in stages or lineages that are different from those regulated by BCL6.

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.