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

Vasanwala, Farha H.; Kusam, Saritha; Toney, Lisa M.; Dent, Alexander L.

doi:10.4049/jimmunol.169.4.1922

Cited by 177 publications

(168 citation statements)

References 29 publications

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“…2). Possible candidate cis-elements located in this larger construct could be the binding sites for AP1 TF, which has been reported to increase the promoter activity in the absence of Bcl-6 [15]. In addition, a binding site (Maf response element site) for the repressor factor Bach2 that binds to the same site as the AP1 factor has recently been described [21].…”

Section: Identification Of Dna Sequences Required For Prdm1 Transcripmentioning

confidence: 91%

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Transcription of PRDM1, the master regulator for plasma cell differentiation, depends on an SP1/SP3/EGR‐1 GC‐box

et al. 2008

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The positive regulatory domain containing 1, encoded by the PRDM1 gene, is a transcriptional repressor considered as a master regulator that is required and sufficient for plasma cell differentiation. In the present study we have performed sequence analysis of the upstream region of the human PRDM1 gene to detect the minimal promoter region necessary for PRDM1 gene transcription. This region comprises the region upstream of the initiation site, as well as the first exon. Collectively, deletion and mutation analysis in conjunction with luciferase reporter assays, EMSA and supershift assays identified a phylogenetically conserved GC-box as an essential element for PRDM1 expression. This GC-box element matches to a binding site for multiple transcription factors such as SP1 and SP3 isoforms as well as early growth response 1. Chromatin immunoprecipitation assays confirmed the in vivo binding capability of these factors to the human PRDM1 promoter. These studies together characterize for the first time the basal activity of the human PRDM1 promoter, through which several factors, including SP1, SP3 and early growth response 1, modulate its expression through a conserved GC-box.Key words: B cell differentiation Á Human Á PRDM1 Á Transcription factors Supporting Information available online IntroductionPlasma cells (PC), i.e. terminally differentiated B lymphocytes in a post-mitotic state, are immunoglobulin (Ig)-secreting cells playing a central role in the antigen-specific immune response. The plasmacytic differentiation process is regulated by the "turn off" and "turn on" of many specific transcription factors (TF), which are now starting to be elucidated [1][2][3][4]. One of these, the human positive regulatory domain containing 1 with zinc-finger domain (PRDM1), is considered as a key factor that promotes the nonproliferative and differentiated stage of PC [4, 5]. The PRDM1 factor was initially identified as a 98-kDa DNA-binding protein that contains five zinc-finger domains and acts as a repressor of interferon-b gene expression [6]. Its murine homolog, initially called B lymphocyte-induced maturation protein-1 (Blimp-1), was isolated as a mouse gene that is induced during B lymphocyte maturation [7]. It is expressed at high levels only in late B cells and PC, and the ectopic overexpression of PRDM1 is sufficient to drive B lymphocytes to differentiate into PC [7]. Moreover, its expression is required for the formation of Ig-secreting cells [8] and for the maintenance of long-lived PC in bone marrow [9].MYC Here we have defined the minimal promoter for human PRDM1 transcription enclosing a GC-rich element necessary for basal transcription. Moreover, this GC-rich element contains an SP1/early growth response 1 (EGR-1) overlapping binding site to which the regulatory factors SP1, SP3 and EGR-1 can bind, which activate the PRDM1 transcription. Results and discussionStructure of the 5 0 -flanking region and initiation sites of the human PRDM1 gene To obtain insight into the regulation of the human PRDM1 gene and b...

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Section: Identification Of Dna Sequences Required For Prdm1 Transcripmentioning

confidence: 91%

“…On the basis of the PRDM1 transcript levels changes [13] it has been proposed that Bcl-6 represses PRDM1 expression at the germinal center B cell stage. Regulation studies on the human PRDM1 gene promoter have shown that Bcl-6 and Bach2 have recently been proposed as a repressor for human PRDM1 gene transcription [15][16][17].…”

Section: Introductionmentioning

confidence: 99%

Transcription of PRDM1, the master regulator for plasma cell differentiation, depends on an SP1/SP3/EGR‐1 GC‐box

et al. 2008

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“…A similar mechanism seems to function also in T cells [10]. Although direct binding of Bcl6 to PRDM1 has been characterized, it may not be a prominent mechanism, since inhibition of AP-1 function has been proposed as a mechanism for Bcl6-mediated PRDM1 repression [26]. Also, IRF4 has been suggested as a Bcl6 target that could contribute to PRDM1 expression [27].…”

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confidence: 99%

“…Bach2 has been shown to repress PRDM1 expression through two different MAREelements, one within the promoter [40] and the other next to the Bcl6 response element 1 (BRE1) within the fifth intron of the PRDM1 gene [41]. Thus, Bcl6 appears to use yet another mechanism to ensure the repressed state of PRDM1 in B cells in addition to the previously described competition for DNA binding with STAT3 [51], inhibition of AP-1 function [26] and direct binding to BRE1 in the PRDM1 gene [23]. Importantly, the mechanism that involves Bach2 would employ activation of transcription by Bcl6 as suggested for regulation of Cxcr5, Il6r and Il21r in the context of follicular T helper-cell development [11].…”

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confidence: 99%

Alternate pathways for Bcl6‐mediated regulation of B cell to plasma cell differentiation

et al. 2011

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The transcription factor Bcl6 regulates germinal center formation and differentiation of B cells into high-affinity antibody-producing plasma cells. The direct double-negative regulatory circuit between Bcl6 and Blimp-1 is well established. We now reveal alternative mechanisms for Bcl6-mediated regulation of B-cell differentiation to plasma cells and show with DT40 cells that Bcl6 directly promotes the expression of Bach2, a known suppressor of Blimp-1. Moreover, Bcl6 suppresses Blimp-1 expression through direct binding to the IRF4 gene, as well as by promoting the expression of MITF, a known suppressor of IRF4. We also provide evidence that Bcl6 is needed for the expression of AID and UNG, the indispensable proteins for somatic hypermutation and class-switch recombination, and UNG appears to be a direct Bcl6 target. Our findings reveal a complex regulatory network in which Bcl6 acts as a key element dictating the transition of DT40 B cells to plasma cells. Supporting Information available online See accompanying Commentary by Tarlinton IntroductionThe transition of activated B cells into high-affinity antibodyproducing plasmablasts, plasma cells and memory B cells in the germinal centers (GCs) is a biologically unique process involving rapid B-cell proliferation coupled with a high rate of mutation in the variable regions of immunoglobulin (Ig) genes. Considering that the majority of B-cell lymphomas originate from the GC B cells [1], a comprehensive understanding of the transcriptional regulation controlling the transition of B cells to plasma cells is essential. The transcription factors Blimp-1, Xbp-1 and IRF4 have been identified to be critical regulators promoting the Ig secretion and plasma cell phenotype, whereas Pax5, Bcl6, MITF and Bach2 are the known inhibitors of plasma cell development [2].Previous studies have shown that the downregulation of B-cell identity factor Pax5 occurs before the induction of the plasma cell transcription program [3,4], and that the loss of Pax5 expression [4] leads to the downregulation of BCL6. Furthermore, B-cell receptor (BCR) signaling of sufficient affinity leads to rapid phosphorylation and degradation of Bcl6 protein [5], while Bcl6 can also be functionally inactivated by acetylation [6]. These features of Bcl6 make it a prominent molecular trigger that controls the plasma cell differentiation.The transcription factor Bcl6 has been demonstrated to be essential for GC B-cell development, as Bcl6 knockout mice fail to develop GCs and do not produce high-affinity antibodies [7][8][9]. Recent work revealed that Bcl6 is also needed for follicular T helper cell development [10][11][12]. Analyses of Bcl6 target genes in B cells indicate that suppression of apoptosis and cell cycle arrest responses occur through p53, p21 and CHEK1 and Bcl6 also regulates DNA damage responses by controlling ATR [13][14][15][16]. Thus, Bcl6 appears to function as a factor permitting rapid B-cell 2404proliferation and tolerance for DNA damage in GCs. Bcl6 also represses genes that are involve...

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“…GC are dynamic and specialized structures in the secondary lymphoid organs within which B cells undergo immunoglobulin class switch recombination and somatic hypermutation to produce diverse, high-affinity antibodies (17,26). Widely considered to be the master regulator of the GC stage of B-cell development, BCL6 maintains the GC-specific gene expression program by silencing genes involved in B-cell activation (CD69, CD80, and NF-B1), response to DNA damage (TP53 and ATR), cell-cycle regulation (CCND2, CDKN1B, and CDKN1A), and plasma cell differentiation (PRDM1) (25,29,34,37,38,40,41). Thus, neither the memory nor the plasma cell differentiation program can be initiated until expression and activity of BCL6 are extinguished by GC exit signals.…”

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confidence: 99%

CtBP Is an Essential Corepressor for BCL6 Autoregulation

Mendez¹,

Polo

Yu³

et al. 2008

Molecular and Cellular Biology

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The transcription repressor BCL6 plays an essential role in the formation and function of germinal centers (GCs). While normal B cells promptly shut off BCL6 when they exit the GC, many GC-derived B-cell lymphomas sustain BCL6 expression through chromosomal translocations and activating mutations. We have previously shown that a common effect of lymphoma-associated BCL6 gene alterations is to bypass a negative autoregulatory loop that controls its transcription. In this study, we report that BCL6 autoregulation is independent of several known corepressor complexes including silencing mediator for retinoid and thyroid hormone receptors, nuclear receptor coreceptor, BCL6 corepressor, and MTA3/NuRD. Furthermore, we show that BCL6 can interact with the CtBP (C-terminal binding protein) corepressor both in vitro and in vivo and that CtBP is recruited by BCL6 to its 5 regulatory region. In lymphoma cell lines carrying BCL6 translocations, small interfering RNA-mediated CtBP knock-down selectively relieved the previously silenced wild-type BCL6 allele but not the translocated alleles, which are driven by heterologous promoters. These results demonstrate that CtBP is a novel BCL6 corepressor and suggest that a unique corepressor requirement for BCL6 autoregulation may allow GC B cells to differentially control the expression of BCL6 and other BCL6 target genes in response to environmental stimuli during the GC stage of B cell development.BCL6 is a sequence-specific transcription repressor that is required for the formation of germinal centers (GC), and its deregulated expression underlies development of many GCderived B-cell lymphomas (12,44,47). Expression of BCL6 is developmentally regulated such that in the B-cell lineage, high levels of BCL6 are restricted to the GC stage. GC are dynamic and specialized structures in the secondary lymphoid organs within which B cells undergo immunoglobulin class switch recombination and somatic hypermutation to produce diverse, high-affinity antibodies (17,26). Widely considered to be the master regulator of the GC stage of B-cell development, BCL6 maintains the GC-specific gene expression program by silencing genes involved in B-cell activation (CD69, CD80, and NF-B1), response to DNA damage (TP53 and ATR), cell-cycle regulation (CCND2, CDKN1B, and CDKN1A), and plasma cell differentiation (PRDM1) (25,29,34,37,38,40,41). Thus, neither the memory nor the plasma cell differentiation program can be initiated until expression and activity of BCL6 are extinguished by GC exit signals.BCL6 is a 95-kDa phosphoprotein with six Krüppel-type zinc fingers (ZF) at the C terminus and an N-terminal POZ/BTB domain. Our earlier work demonstrated that the maximum repression activity of BCL6 requires the entire POZ/BTB domain as well as a separate middle region, repression domain II (RDII) (7). To date, a variety of BCL6 corepressors have been described. Among the corepressors with well-documented in vivo functions, nuclear receptor coreceptor (NCoR), silencing mediator for retinoid and thyroid hormon...

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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

Cited by 177 publications

References 29 publications

Transcription of PRDM1, the master regulator for plasma cell differentiation, depends on an SP1/SP3/EGR‐1 GC‐box

Transcription of PRDM1, the master regulator for plasma cell differentiation, depends on an SP1/SP3/EGR‐1 GC‐box

Alternate pathways for Bcl6‐mediated regulation of B cell to plasma cell differentiation

CtBP Is an Essential Corepressor for BCL6 Autoregulation

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