An evolutionary trade-off between host immunity and metabolism drives fatty liver in male mice.

Adaptations to infectious and dietary pressures shape mammalian physiology and disease risk. How such adaptations affect sex-biased diseases remains insufficiently studied. In this study, we show that sex-dependent hepatic gene programs confer a robust (~300%) survival advantage for male mice during lethal bacterial infection. The transcription factor B cell lymphoma 6 (BCL6), which masculinizes hepatic gene expression at puberty, is essential for this advantage. However, protection by BCL6 protein comes at a cost during conditions of dietary excess, which result in overt fatty liver and glucose intolerance in males. Deleting hepatic BCL6 reverses these phenotypes but markedly lowers male survival during infection, thus establishing a sex-dependent trade-off between host defense and metabolic systems. Our findings offer strong evidence that some current sex-biased diseases are rooted in ancient evolutionary trade-offs between immunity and metabolism.

Bcl6 controls meningeal Th17-B cell interaction in murine neuroinflammation.

Ectopic lymphoid tissue containing B cells forms in the meninges at late stages of human multiple sclerosis (MS) and when neuroinflammation is induced by interleukin (IL)-17 producing T helper (Th17) cells in rodents. B cell differentiation and the subsequent release of class-switched immunoglobulins have been speculated to occur in the meninges, but the exact cellular composition and underlying mechanisms of meningeal-dominated inflammation remain unknown. Here, we performed in-depth characterization of meningeal versus parenchymal Th17-induced rodent neuroinflammation. The most pronounced cellular and transcriptional differences between these compartments was the localization of B cells exhibiting a follicular phenotype exclusively to the meninges. Correspondingly, meningeal but not parenchymal Th17 cells acquired a B cell-supporting phenotype and resided in close contact with B cells. This preferential B cell tropism for the meninges and the formation of meningeal ectopic lymphoid tissue was partially dependent on the expression of the transcription factor Bcl6 in Th17 cells that is required in other T cell lineages to induce isotype class switching in B cells. A function of Bcl6 in Th17 cells was only detected in vivo and was reflected by the induction of B cell-supporting cytokines, the appearance of follicular B cells in the meninges, and of immunoglobulin class switching in the cerebrospinal fluid. We thus identify the induction of a B cell-supporting meningeal microenvironment by Bcl6 in Th17 cells as a mechanism controlling compartment specificity in neuroinflammation.

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.

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.

Multiscale Modeling of Germinal Center Recapitulates the Temporal Transition From Memory B Cells to Plasma Cells Differentiation as Regulated by Antigen Affinity-Based Tfh Cell Help.

Germinal centers play a key role in the adaptive immune system since they are able to produce memory B cells and plasma cells that produce high affinity antibodies for an effective immune protection. The mechanisms underlying cell-fate decisions are not well understood but asymmetric division of antigen, B-cell receptor affinity, interactions between B-cells and T follicular helper cells (triggering CD40 signaling), and regulatory interactions of transcription factors have all been proposed to play a role. In addition, a temporal switch from memory B-cell to plasma cell differentiation during the germinal center reaction has been shown. To investigate if antigen affinity-based Tfh cell help recapitulates the temporal switch we implemented a multiscale model that integrates cellular interactions with a core gene regulatory network comprising BCL6, IRF4, and BLIMP1. Using this model we show that affinity-based CD40 signaling in combination with asymmetric division of B-cells result in switch from memory B-cell to plasma cell generation during the course of the germinal center reaction. We also show that cell fate division is unlikely to be (solely) based on asymmetric division of Ag but that BLIMP1 is a more important factor. Altogether, our model enables to test the influence of molecular modulations of the CD40 signaling pathway on the production of germinal center output cells.

OX40 stimulation and PD-L1 blockade synergistically augment HBV-specific CD4 T cells in patients with HBeAg-negative infection.

BACKGROUND & AIMS: Current antiviral therapies lack the potential to eliminate persistent hepatitis B virus (HBV) infection. HBV-specific T cells are crucial for HBV control and have recently been shown to be protective in patients following discontinuation of antiviral therapy. Thus, T cell-based approaches may greatly improve the therapeutic landscape of HBV infection. We aimed to augment HBV-specific CD4 T cells from chronically infected patients by targeting different immunological pathways. METHODS: Expression of various co-stimulatory and inhibitory receptors on HBV- and influenza-specific CD4 T cells was analyzed directly ex vivo by MHC class II-tetramers. Patients infected with HBV genotype D were screened for CD4 T cell responses by IFN-γ ELISpot and intracellular cytokine staining following stimulation with overlapping peptides (OLPs) spanning the HBV-polyprotein. Stimulation with recombinant IL-7, an agonistic OX40-antibody or blockade of PD-L1 was performed in antigen-specific in vitro cultures. Cytokine secretion and expression of transcription factors were analyzed by flow cytometry. Responses targeting influenza, Epstein-Barr virus and tetanus toxoid served as controls. RESULTS: Tetramer-staining revealed that the IL-7 receptor-alpha (CD127), OX40 and PD-1 constitute possible therapeutic targets as they were all strongly expressed on HBV-specific CD4 T cells ex vivo. The HBV-specific CD4 T cell responses identified by OLP screening targeted predominantly the HBV-polymerase and core proteins. Combined OX40 stimulation and PD-L1 blockade significantly augmented IFN-γ and IL-21 producing HBV-specific CD4 T cells in vitro, suggesting active T helper type 1 cell and follicular T helper cell programs. Indeed, transcription factors T-bet and Bcl6 were strongly expressed in cytokine-producing cells. CONCLUSIONS: Combined OX40 stimulation and PD-L1 blockade augmented secretion of the helper T cell signature cytokines IFN-γ and IL-21, suggesting that immunotherapeutic approaches can improve HBV-specific CD4 T cell responses. LAY SUMMARY: CD4 T cells are important in controlling viral infections but are impaired in the context of chronic hepatitis B virus (HBV) infection. Therapeutic approaches to cure chronic HBV infection are highly likely to require an immune-stimulatory component. This study demonstrates that HBV-specific CD4 T cells can be functionally augmented by combined stimulation of the co-stimulatory molecule OX40 and blockade of the inhibitory PD-1 pathway.

Small-molecule BCL6 inhibitor effectively treats mice with nonsclerodermatous chronic graft-versus-host disease.

Patient outcomes for steroid-dependent or -refractory chronic graft-versus-host diesease (cGVHD) are poor, and only ibrutinib has been US Food and Drug Administration (FDA) approved for this indication. cGVHD is often driven by the germinal center (GC) reaction, in which T follicular helper cells interact with GC B cells to produce antibodies that are associated with disease pathogenesis. The transcriptional corepressor B-cell lymphoma 6 (BCL6) is a member of the Broad-complex, Tramtrack, and Bric-abrac/poxvirus and zinc finger (BTB/POZ) transcription factor family and master regulator of the immune cells in the GC reaction. We demonstrate that BCL6 expression in both donor T cells and B cells is necessary for cGVHD development, pointing to BCL6 as a therapeutic cGVHD target. A small-molecule BCL6 inhibitor reversed active cGVHD in a mouse model of multiorgan system injury with bronchiolitis obliterans associated with a robust GC reaction, but not in cGVHD mice with scleroderma as the prominent manifestation. For cGVHD patients with antibody-driven cGVHD, targeting of BCL6 represents a new approach with specificity for a master GC regulator that would extend the currently available second-line agents.

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.

Chromatin remodeling by the NuRD complex regulates development of follicular helper and regulatory T cells.

Lineage commitment and differentiation into CD4+ T cell subsets reflect an interplay between chromatin regulators and transcription factors (TF). Follicular T cell development is regulated by the Bcl6 TF, which helps determine the phenotype and follicular localization of both CD4+ follicular helper T cells (TFH) and follicular regulatory T cells (TFR). Here we show that Bcl6-dependent control of follicular T cells is mediated by a complex formed between Bcl6 and the Mi-2ß-nucleosome-remodeling deacetylase complex (Mi-2ß-NuRD). Formation of this complex reflects the contribution of the intracellular isoform of osteopontin (OPN-i), which acts as a scaffold to stabilize binding between Bcl6 and the NuRD complex that together regulate the genetic program of both TFH and TFR cells. Defective assembly of the Bcl6-NuRD complex distorts follicular T cell differentiation, resulting in impaired TFR development and skewing of the TFH lineage toward a TH1-like program that includes expression of Blimp1, Tbet, granzyme B, and IFNγ. These findings define a core Bcl6-directed transcriptional complex that enables CD4+ follicular T cells to regulate the germinal center response.

External signals regulate germinal center fate-determining transcription factors in the A20 lymphoma cell line.

Due to the apoptosis-prone nature of primary germinal center B (GCB) cells, it remains a huge challenge to dissect signals that guide their differentiation towards memory B cells and plasma cells in vitro. Here we show that the murine lymphoma cell line A20 resembles primary GCB cells in expression of GC-specific surface markers and the master transcription factor BCL6 and may serve as a useful system to model certain GCB cell behaviors in vitro. Using these cells, we found that both CD40 and B cell receptor (BCR) signaling are able to drive BCL6 downregulation, which is a prerequisite of post-GC B-cell differentiation. Under the steady state, BCL6 is constantly and rapidly degraded in A20 cells by the proteasome in a strictly FBXO11-dependent manner. This process can be further enhanced by signals downstream of the BCR. Both CD40 and BCR stimulation can upregulate IRF4, a transcription factor that suppresses BCL6 expression. However, only BCR signaling downregulate PAX5 and BACH2, two transcription factors that help maintain the GCB identity. Together, these results validate the A20 cell line as an experimental system suitable for studying regulation of BCL6 and potentially other transcription factors relevant to post-GC fate determination, and they support that combined signaling from BCR and CD40 receptors would drive termination of the GC program.

The E3 ubiquitin ligase Itch is required for the differentiation of follicular helper T cells.

Follicular helper T cells (T(FH) cells) are responsible for effective B cell-mediated immunity, and Bcl-6 is a central factor for the differentiation of T(FH) cells. However, the molecular mechanisms that regulate the induction of T(FH) cells remain unclear. Here we found that the E3 ubiquitin ligase Itch was essential for the differentiation of T(FH) cells, germinal center responses and immunoglobulin G (IgG) responses to acute viral infection. Itch acted intrinsically in CD4(+) T cells at early stages of T(FH) cell development. Itch seemed to act upstream of Bcl-6 expression, as Bcl-6 expression was substantially impaired in Itch(-/-) cells, and the differentiation of Itch(-/-) T cells into T(FH) cells was restored by enforced expression of Bcl-6. Itch associated with the transcription factor Foxo1 and promoted its ubiquitination and degradation. The defective T(FH) differentiation of Itch(-/-) T cells was rectified by deletion of Foxo1. Thus, our results indicate that Itch acts as an essential positive regulator in the differentiation of T(FH) cells.

AMPKα2 exerts its anti-inflammatory effects through PARP-1 and Bcl-6.

B-cell lymphoma-6 protein (Bcl-6) is a corepressor for inflammatory mediators such as vascular cell adhesion molecule-1 and monocyte chemotactic protein-1 and -3, which function to recruit monocytes to vascular endothelial cells upon inflammation. Poly [ADP ribose] polymerase 1 (PARP-1) is proinflammatory, in part through its binding at the Bcl-6 intron 1 to suppress Bcl-6 expression. We investigated the mechanisms by which PARP-1 dissociates from the Bcl-6 intron 1, ultimately leading to attenuation of endothelial inflammation. Analysis of the PARP-1 primary sequence suggested that phosphorylation of PARP-1 Serine 177 (Ser-177) by AMP-activated protein kinase (AMPK) is responsible for the induction of Bcl-6. Our results show that AMPK activation with treatment of 5-aminoimidazole-4-carboxamide ribonucleotide, metformin, or pulsatile shear stress induces PARP-1 dissociation from the Bcl-6 intron 1, increases Bcl-6 expression, and inhibits expression of inflammatory mediators. Conversely, AMPKα suppression or knockdown produces the opposite effects. The results demonstrate an anti-infamatory pathway linking AMPK, PARP-1, and Bcl-6 in endothelial cells.

Noxa mediates p18INK4c cell-cycle control of homeostasis in B cells and plasma cell precursors.

Inhibition of Cdk4/Cdk6 by p18(INK4c) (p18) is pivotal for generation of noncycling immunoglobulin (Ig)-secreting plasma cells (PCs). In the absence of p18, CD138(+) plasmacytoid cells continue to cycle and turnover rapidly, suggesting that p18 controls PC homeostasis. We now show that p18 selectively acts in a rare population of rapidly cycling CD138(hi)/B220(hi) intermediate PCs (iPCs). While retaining certain B-cell signatures, iPCs are poised to differentiate to end-stage PCs although the majority undergo apoptosis. p18 is dispensable for the development of the PC transcriptional circuitry, and Blimp-1 and Bcl-6 are expressed fully and mutually exclusively in individual iPCs. However, a minor proportion of iPCs express both, and they are preferentially protected by p18 or Bcl-xL overexpression, consistent with expansion of the iPC pool by Bcl-xL overexpression, or loss of proapoptotic Bim or Noxa. Expression of Noxa is induced during B-cell activation, peaks in iPCs, and selectively repressed by p18. It is required to promote apoptosis of cycling B cells, especially in the absence of p18. These findings define the first physiologic function for Noxa and suggest that by repressing Noxa, induction of G1 arrest by p18 bypasses a homeostatic cell-cycle checkpoint in iPCs for PC differentiation.

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.

Male-specific hepatic Bcl6: growth hormone-induced block of transcription elongation in females and binding to target genes inversely coordinated with STAT5.

The transcriptional repressor Bcl6 is a male-specific rat liver gene product and one of 24 early GH-response genes encoding DNA-binding proteins. Presently, the sex specificity of Bcl6 was shown to emerge at puberty, when hepatic Bcl6 mRNA was induced in males and repressed in females by the female plasma GH profile. Hepatic Bcl6 mRNA was increased to near-normal male levels in hypophysectomized females and was extinguished in intact males given a continuous GH infusion (female-like GH pattern). Bcl6 was also repressed in adult male somatostatin-deficient mice, where plasma GH profiles are female like. Hepatic Bcl6 RNA was rapidly down-regulated by GH pulse treatment, both in hypophysectomized male rats and in primary rat hepatocytes. Bcl6 was substantially induced in female mice deficient in hepatic signal transducer and activator of transcription (STAT)5a/STAT5b, suggesting that these STAT transcriptional mediators of GH signaling repress Bcl6. Indeed, STAT5 was bound to Bcl6 STAT5-binding region-B, previously associated with Bcl6 repression, in both male and female liver chromatin. STAT5 also bound to Bcl6 region-A in male chromatin but only during a plasma GH pulse. Analysis of primary transcripts (heterogeneous nuclear RNA) across the Bcl6 gene revealed a novel mechanism of GH-dependent sex specificity, with two apparent blocks in Bcl6 transcription elongation seen in female liver and in continuous GH-treated male liver, one early in intron 4 and one in exon 5, which together reduced transcription beyond exon 5 more than 300-fold. Finally, Bcl6 was bound to a subset of STAT5-binding sites in male liver chromatin, including a Socs2 STAT5-binding site where Bcl6 binding increased substantially between plasma GH pulses, i.e. when STAT5 binding was low. Bcl6 and STAT5 binding are thus inversely coordinated by the endogenous pulses of pituitary GH release, suggesting this male-specific transcriptional repressor modulates hepatic GH signaling to select STAT5 target genes.

The BCL6 transcriptional program features repression of multiple oncogenes in primary B cells and is deregulated in DLBCL.

The BCL6 transcriptional repressor is required for development of germinal center (GC) B cells and when expressed constitutively causes diffuse large B-cell lymphomas (DLBCLs). We examined genome-wide BCL6 promoter binding in GC B cells versus DLBCLs to better understand its function in these settings. BCL6 bound to both distinct and common sets of functionally related gene in normal GC cells versus DLBCL cells. Certain BCL6 target genes were preferentially repressed in GC B cells, but not DLBCL cells. Several such genes have prominent oncogenic functions, such as BCL2, MYC, BMI1, EIF4E, JUNB, and CCND1. BCL6 and BCL2 expression was negatively correlated in primary DLBCLs except in the presence of BCL2 translocations. The specific BCL6 inhibitor retro-inverso BCL6 peptidomimetic inhibitor-induced expression of BCL2 and other oncogenes, consistent with direct repression effects by BCL6. These data are consistent with a model whereby BCL6 can directly silence oncogenes in GC B cells and counterbalance its own tumorigenic potential. Finally, a BCL6 consensus sequence and binding sites for other physiologically relevant transcription factors were highly enriched among target genes and distributed in a pathway-dependent manner, suggesting that BCL6 forms specific regulatory circuits with other B-cell transcriptional factors.

B-cell lymphoma 6 and the molecular pathogenesis of diffuse large B-cell lymphoma.

PURPOSE OF REVIEW: The B-cell lymphoma 6 transcriptional repressor is the most commonly involved oncogene in B-cell lymphomas. Sustained expression of B-cell lymphoma 6 causes malignant transformation of germinal center B cells. Understanding the mechanism of action of B-cell lymphoma 6 is crucial for the study of how aberrant transcriptional programming leads to lymphomagenesis and development of targeted antilymphoma therapy. RECENT FINDINGS: Identification of B-cell lymphoma 6 target genes indicates a critical role for B-cell lymphoma 6 in facilitating a state of physiological genomic instability required for germinal center B cells to undergo affinity maturation, and suggests its contribution to several additional cellular functions. The discovery of several layers of counterregulatory mechanisms reveals how B cells can control and fine-tune the potentially lymphomagenic actions of B-cell lymphoma 6. From the biochemical standpoint, B-cell lymphoma 6 can regulate distinct biological pathways through different cofactors. This observation explains how the biological actions of B-cell lymphoma 6 can be physiologically controlled through separate mechanisms and affords the means for improved therapeutic targeting. The fact that patients with B-cell lymphoma 6-dependent lymphoma can be identified on the basis of gene signatures suggests that therapeutic trials of B-cell lymphoma 6 inhibitors could be personalized to these individuals. SUMMARY: B-cell lymphoma 6 plays a fundamental role in lymphomagenesis and is an excellent therapeutic target for development of improved antilymphoma therapeutic regimens.

BCL6 represses NFkappaB activity in diffuse large B-cell lymphomas.

BCL6 is a transcriptional repressor whose deregulated expression plays a key role in diffuse large B-cell lymphomas (DLBCLs). BCL6 expression characterizes one of the two main subtypes (GC type) of DLBCL, while the other (ABC type) is recognized by increased NFkappaB activation. The mechanistic basis of this distinction remains unclear and the BCL6 targets have been only partially explored. Here we describe how NFkappaB activity is increased after BCL6 silencing by shRNA in DLBCL cells, leading us to propose that BCL6 represses NFkappaB activity. We also demonstrate that this repression is brought about by a mechanism involving protein-protein interaction between BCL6 and NFkappaB members, both in vitro and in vivo. Analysis of a series of DLBCLs shows a negative correlation between the expression of NFkappaB target genes and BCL6. This combined approach using silenced cells and a series of human DLBCL samples leads us to a better understanding of the role of BCL6 as an NFkappaB regulator in B-cells.

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

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

BCL-6: a possible missing link for anti-inflammatory PPAR-delta signalling in pancreatic beta cells.

AIMS/HYPOTHESIS: Inflammatory mediators contribute to pancreatic beta cell death in type 1 diabetes. Beta cells respond to cytokine exposure by activating gene networks that alter cellular metabolism, induce chemokine release (thereby increasing insulitis), and cause apoptosis. We have previously shown by microarray analysis that exposure of INS-1E cells to IL-1beta + IFN-gamma induces the transcription factor peroxisome proliferator-activated receptor (Ppar)-delta and several of its target genes. PPAR-delta controls cellular lipid metabolism and is a major regulator of inflammatory responses. We therefore examined the role of PPAR-delta in cytokine-treated beta cells. MATERIALS AND METHODS: Primary beta cells that had been purified by fluorescence-activated cell sorting and INS-1E cells were cultured in the presence of the cytokines TNF-alpha, IL-1beta, or IL-1beta + IFN-gamma, or the synthetic PPAR-delta agonist GW501516. Gene expression was analysed by real-time PCR. PPAR-delta, monocyte chemoattractant protein (MCP-1, now known as CCL2) promoter and NF-kappaB activity were determined by luciferase reporter assays. RESULTS: Exposure of primary beta cells or INS-1E cells to cytokines induced Ppar-delta mRNA expression and PPAR-delta-dependent CD36, lipoprotein lipase, acyl CoA synthetase and adipophilin mRNAs. Cytokines and the PPAR-delta agonist GW501516 also activated a PPAR-delta response element reporter in beta cells. Unlike immune cells, neither INS-1E nor beta cells expressed the transcriptional repressor B-cell lymphoma-6 (BCL-6). As a consequence, PPAR-delta activation by GW501516 did not decrease cytokine-induced Mcp-1 promoter activation or mRNA expression, as reported for macrophages. Transient transfection with a BCL-6 expression vector markedly reduced Mcp-1 promoter and NF-kappaB activities in beta cells. CONCLUSIONS/INTERPRETATION: Cytokines activate the PPAR-delta gene network in beta cells. This network does not, however, regulate the pro-inflammatory response to cytokines because beta cells lack constitutive BCL-6 expression. This may render beta cells particularly susceptible to propagating inflammation in type 1 diabetes.