A transcriptional repressor of c-myc

The effects of MYC on tumor immunity and immunotherapy

The oncogene MYC is dysregulated in a host of human cancers, and as an important point of convergence in multitudinous oncogenic signaling pathways, it plays a crucial role in tumor immune regulation in the tumor immune microenvironment (TIME). Specifically, MYC promotes the expression of immunosuppressive factors and inhibits the expression of immune activation regulators. Undoubtedly, a therapeutic strategy that targets MYC can initiate a new era of cancer treatment. In this review, we summarize the essential role of the MYC signaling pathway in tumor immunity and the development status of MYC-related therapies, including therapeutic strategies targeting MYC and combined MYC-based immunotherapy. These studies have reported extraordinary insights into the translational application of MYC in cancer treatment and are conducive to the emergence of more effective immunotherapies for cancer.

Plasma cell survival: The intrinsic drivers, migratory signals, and extrinsic regulators

Antibody-secreting cells (ASC) are the effectors of protective humoral immunity and the only cell type that produces antibodies or immunoglobulins in mammals. In addition to their formidable capacity to secrete massive quantities of proteins, ASC are terminally differentiated and have unique features to become long-lived plasma cells (LLPC). Upon antigen encounter, B cells are activated through a complex multistep process to undergo fundamental morphological, subcellular, and molecular transformation to become an efficient protein factory with lifelong potential. The ASC survival potential is determined by factors at the time of induction, capacity to migration from induction to survival sites, and ability to mature in the specialized bone marrow microenvironments. In the past decade, considerable progress has been made in identifying factors regulating ASC longevity. Here, we review the intrinsic drivers, trafficking signals, and extrinsic regulators with particular focus on how they impact the survival potential to become a LLPC.

Insights Into the Evolution of the prdm1/Blimp1 Gene Family in Teleost Fish

In mammals, Blimp1 (B lymphocyte-induced maturation protein 1) encoded by the prdm1 gene and its homolog Hobit (homolog of Blimp1 in T cells) encoded by znf683, represent key transcriptional factors that control the development and differentiation of both B and T cells. Despite their essential role in the regulation of acquired immunity, this gene family has been largely unexplored in teleosts to date. Until now, one prdm1 gene has been identified in most teleost species, whereas a znf683 homolog has not yet been reported in any of these species. Focusing our analysis on rainbow trout (Oncorhynchus mykiss), an in silico identification and characterization of prdm1-like genes has been undertaken, confirming that prdm1 and znf683 evolved from a common ancestor gene, acquiring three gene copies after the teleost-specific whole genome duplication event (WGD) and six genes after the salmonid-specific WGD. Additional transcriptional studies to study how each of these genes are regulated in homeostasis, in response to a viral infection or in B cells in different differentiation stages, provide novel insights as to how this gene family evolved and how their encoded products might be implicated in the lymphocyte differentiation process in teleosts.

Unraveling the mysteries of plasma cells

Antibody-secreting plasma cells are the central pillars of humoral immunity. They are generated in a fundamental cellular restructuring process from naive B cells upon contact with antigen. This outstanding process is guided and controlled by a complex transcriptional network accompanied by a fascinating morphological metamorphosis, governed by the combined action of Blimp-1, Xbp-1 and IRF-4. The survival of plasma cells requires the intimate interaction with a specific microenvironment, consisting of stromal cells and cells of hematopoietic origin. Cell-cell contacts, cytokines and availability of metabolites such as glucose and amino acids modulate the survival abilities of plasma cells in their niches. Moreover, plasma cells have been shown to regulate immune responses by releasing cytokines. Furthermore, plasma cells are central players in autoimmune diseases and malignant transformation of plasma cells can result in the generation of multiple myeloma. Hence, the development of sophisticated strategies to deplete autoreactive plasma cells and myeloma cells represents a challenge for current and future research.

Nuclear hormone receptor BmFTZ-F1 is involved in regulating the fibroin heavy chain gene in the silkworm, Bombyx mori

BACKGROUND

The synthesis of silk protein is controlled by hormones. The expression of the nuclear hormone Bmftz-f1 in the posterior silk gland (PSG) is induced by 20-hydroxyecdysone in vivo and in vitro. However, whether Bmftz-f1 regulates silk protein expression is unknown.

METHODS

In our study, western blotting and quantitative polymerase chain reactions were conducted to detect the expression of FTZ-F1 in the PSG. Electrophoretic mobility shift, chromatin immunoprecipitation, far-western blotting, bimolecular fluorescence complementation, and dual luciferase reporter assays were performed to investigate the effect of FTZ-F1 on the fibH promoter.

RESULTS

(1) The expression of the hormone receptor BmFTZ-F1 was opposite to that of fibH. It was highly expressed in the PSG during the fourth molting stage and the beginning of the fifth instar, and then its expression decreased gradually until it disappeared at the end of the fifth instar and the wandering stage. (2) We identified a FTZ-F1 response element 390bp upstream of the transcription initiation site of the fibH promoter. (3) BmFTZ-F1 interacted with the basic helix-loop-helix transcription factor Bmdimm. (4) BmFTZ-F1 down-regulated fibH promoter activity and counteracted the effect of Bmdimm on fibH expression.

CONCLUSIONS

Integrating these results, we conclude that BmFTZ-F1 regulates the transcription of fibH by binding to the FTZ-F1 response element in the fibH promoter and counteracts the effect of Bmdimm on fibH expression.

GENERAL SIGNIFICANCE

These findings provide new insights into the mechanism of regulation of the silk protein gene.

A signature for induced pluripotent stem cell-associated genes in colorectal cancer

Genes associated with induced pluripotent stem cells (iPS genes) are several pivotal transcriptional factors, which are used to induce pluripotent stem cells from some adult somatic cells. The roles of these iPS genes and especially the signature for these iPS genes in colorectal cancer (CRC) are still unclear. Overexpressed Oct4 and Lin28 but down-regulated Nanog were found in tumor tissues compared with that in their paired normal counterparts of CRC patients. Interestingly, we found that Oct4, Lin28 and Nanog were highly overexpressed in some patients. And the signature for iPS genes was correlated with tumor site (P = 0.012), lymph node status (P = 0.033), Dukes classification (P = 0.033) of CRC patients. Moreover, an independent public expression profiling data showed signature for the four iPS genes could successfully be used to predict the survival of CRC patients with Dukes stages B and C. Immunofluorescent staining of fresh CRC tissues from patients showed that strong co-expressions of Oct4 and Nanog proteins or Sox2 and Lin28 were present in some CRC cells. Then, CRC cell subclone with four iPS genes overexpression were establish by a mixed retroviral system. We found that iPS genes promote sphere-formation, proliferation, colony formation, migration of human CRC cells in vitro and tumor growth in vivo. Our study first shows the clinical significance of iPS signature in CRC patients.

Constitutive activation of SHP2 in mice cooperates with ICSBP deficiency to accelerate progression to acute myeloid leukemia

Myeloproliferative disorders (MPDs) are characterized by cytokine hypersensitivity and apoptosis resistance. Development of a block in myeloid differentiation is associated with progression of MPD to acute myeloid leukemia (AML) and portends poor prognosis. Identifying molecular markers of this transition may suggest targets for therapeutic intervention. Interferon consensus sequence binding protein (ICSBP, also known as IRF8) is an interferon-regulatory transcription factor that functions as a leukemia tumor suppressor. In mice, ICSBP deficiency induces an MPD that progresses to AML over time, suggesting that ICSBP deficiency is sufficient for myeloproliferation, but additional genetic lesions are necessary for AML. Since activity of ICSBP is influenced by tyrosine phosphorylation state, we hypothesized that mutations in molecular pathways that regulate this process might synergize with ICSBP deficiency for progression to AML. Consistent with this, we found that constitutive activation of SHP2 protein tyrosine phosphatase synergized with ICSBP haploinsufficiency to facilitate cytokine-induced myeloproliferation, apoptosis resistance, and rapid progression to AML in a murine bone marrow transplantation model. Constitutive SHP2 activation cooperated with ICSBP deficiency to increase the number of progenitors in the bone marrow and myeloid blasts in circulation, indicating a block in differentiation. Since SHP2 activation and ICSBP deficiency may coexist in human myeloid malignancies, our studies have identified a molecular mechanism potentially involved in disease progression in such diseases.

The Blimp-1 gene regulatory region directs EGFP expression in multiple hematopoietic lineages and testis in mice

The B lymphocyte-induced maturation protein-1 (Blimp-1) is a transcriptional repressor involved in B cell terminal differentiation. Recent studies suggest that it also plays roles in the differentiation of myeloid and T cells. Using reporter gene analysis, we found that the mouse Blimp-1 promoter constructs of 4.4 kb, 2.5 kb and 1.6 kb exerted similar transcription activity in vitro. To study the Blimp-1 promoter function in vivo, we generated a mouse transgenic line in which the expression of EGFP is driven by the 4.4 kb Blimp-1 promoter fragment. EGFP expression was detected in multiple hematopoietic lineages including hematopoietic progenitor cells (c-kit+), B cells at various developmental stages (B220+, IgM+, IgD+ and CD138+), myeloid cells (Gr1+ and CD11b+) and T cells (CD4+ and CD8+). Furthermore, we demonstrated, for the first time, that this Blimp-1 promoter fragment also drives EGFP expression in erythroid lineage (Ter119+) and embryonic livers. Finally, EGFP was expressed in the testes of transgenic mice in a cell-specific manner with expression weakly in primary spermatocytes and strongly in spermatids, but not in spermatogonia or spermatozoa. Our studies collectively suggest that Blimp-1 may play a general role in the development of hematopoietic lineages and a stage-specific role in spermatogenesis.

Positive regulatory domain I-binding factor 1 mediates repression of the MHC class II transactivator (CIITA) type IV promoter

MHC class II transactivator (CIITA), a co-activator that controls MHC class II (MHC II) transcription, functions as the master regulator of MHC II expression. Persistent activity of the CIITA type III promoter (pIII), one of the four potential promoters of this gene, is responsible for constitutive expression of MHC II by B lymphocytes. In addition, IFN-gamma induces expression of CIITA in these cells through the type IV promoter (pIV). Positive regulatory domain 1-binding factor 1 (PRDI-BF1), called B lymphocyte-induced maturation protein 1 (Blimp-1) in mice, represses the expression of CIITA pIII in plasma and multiple myeloma cells. To investigate regulation of CIITA pIV expression by PRDI-BF1 in the B lymphocyte lineage, protein/DNA-binding studies, and functional promoter analyses were performed. PRDI-BF1 bound to the IFN regulatory factor-element (IRF-E) site in CIITA pIV. Ectopic expression of either PRDI-BF1 or Blimp-1 repressed this promoter in B lymphocytes. In vitro binding and functional analyses of CIITA pIV demonstrated that the IRF-E is the target of this repression. In vivo genomic footprint analysis demonstrated protein binding at the IRF-E site of CIITA pIV in U266 myeloma cells, which express PRDI-BF1. PRDI-BF1beta, a truncated form of PRDI-BF1 that is co-expressed in myeloma cells, also bound to the IRF-E site and repressed CIITA pIV. These findings demonstrate for the first time that, in addition to silencing expression of CIITA pIII in B lymphocytes, PRDI-BF1 is capable of binding and suppressing CIITA pIV.

Plasma cell ontogeny defined by quantitative changes in blimp-1 expression

Plasma cells comprise a population of terminally differentiated B cells that are dependent on the transcriptional regulator B lymphocyte–induced maturation protein 1 (Blimp-1) for their development. We have introduced a gfp reporter into the Blimp-1 locus and shown that heterozygous mice express the green fluorescent protein in all antibody-secreting cells (ASCs) in vivo and in vitro. In vitro, these cells display considerable heterogeneity in surface phenotype, immunoglobulin secretion rate, and Blimp-1 expression levels. Importantly, analysis of in vivo ASCs induced by immunization reveals a developmental pathway in which increasing levels of Blimp-1 expression define developmental stages of plasma cell differentiation that have many phenotypic and molecular correlates. Thus, maturation from transient plasmablast to long-lived ASCs in bone marrow is predicated on quantitative increases in Blimp-1 expression.

Transcriptional control of B cell development and function

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

Regulatory mechanisms that determine the development and function of plasma cells

Plasma cells are terminally differentiated final effectors of the humoral immune response. Plasma cells that result from antigen activation of B-1 and marginal zone B cells provide the first, rapid response to antigen. Plasma cells that develop after a germinal center reaction provide higher-affinity antibody and often survive many months in the bone marrow. Transcription factors Bcl-6 and Pax5, which are required for germinal center B cells, block plasmacytic differentiation and repress Blimp-1 and XBP-1, respectively. When Bcl-6-dependent repression of Blimp-1 is relieved, Blimp-1 ensures that plasmacytic development is irreversible by repressing BCL-6 and PAX5. In plasma cells, Blimp-1, XBP-1, IRF4, and other regulators cause cessation of cell cycle, decrease signaling from the B cell receptor and communication with T cells, inhibit isotype switching and somatic hypermutation, downregulate CXCR5, and induce copious immunoglobulin synthesis and secretion. Thus, commitment to plasmacytic differentiation involves inhibition of activities associated with earlier B cell developmental stages as well as expression of the plasma cell phenotype.

Localization of Sequences Determining Cell Type Specificity and NGF Responsiveness in the Promoter Region of the Rat Choline Acetyltransferase Gene

A genomic clone containing 7 kb of 5' flanking sequences from the rat choline acetyltransferase (ChAT) gene was isolated and shown to contain a TATA box-like sequence and several consensus binding sites for the transcription factor AP1. Two constructs containing 450 and 1450 base pairs (bp), respectively, of 5' flanking sequences promoted expression of a fused chloramphenicol acetyltransfersase (CAT) gene when transfected into fibroblast FR3T3, Sertoli TM4, phaeochromocytoma PC12 and cholinergic neuronal SN6 cell lines. In contrast, a longer construct containing 3850 bp of 5' flanking sequence allowed CAT activity only in the cholinergic cell line SN6. CAT activity with this construct was suppressed in the three other cell lines, indicating that the distal region of the ChAT promoter contains a cell type-specific silencer-like element that restricts ChAT gene expression to cholinergic cells. Treatment of PC12 cells with nerve growth factor (NGF) increased the promoter activity of the -450 and -1450 constructs approximately four-fold and allowed promoter activity from the -3850 construct, indicating that elements involved in NGF responsiveness of the ChAT promoter are contained in the first 450 bp of upstream sequence. These results support a model in which gene transcription controlled by cell-type specific regulatory elements contribute to the establishment, maintenance and plasticity of the cholinergic transmitter phenotype in the nervous system.

Multiple myeloma: evolving genetic events and host interactions

Multiple myeloma is a neoplasm of terminally differentiated B cells (plasma cells) in which chromosome translocations frequently place oncogenes under the control of immunoglobulin enhancers. Unlike most haematopoietic cancers, multiple myeloma often has complex chromosomal abnormalities that are reminiscent of epithelial tumours. What causes full-blown myeloma? And can our molecular understanding of this common haematological malignancy be used to develop effective preventive and treatment strategies?

Chromosome translocations in multiple myeloma

Multiple myeloma (MM), a malignant tumor of somatically mutated, isotype-switched plasma cells (PC), usually arises from a common benign PC tumor called Monoclonal Gammopathy of Undetermined Significance (MGUS). MM progresses within the bone marrow, and then to an extramedullary stage from which MM cell lines are generated. The incidence of IgH translocations increases with the stage of disease: 50% in MGUS, 60-65% in intramedullarly MM, 70-80% in extramedullary MM, and >90% in MM cell lines. Primary, simple reciprocal IgH translocations, which are present in both MGUS and MM, involve many partners and provide an early immortalizing event. Four chromosomal partners appear to account for the majority of primary IgH translocations: 11q13 (cyclin D1), 6p21 (cyclin D3), 4p16 (FGFR3 and MMSET), and 16q23 (c-maf). They are mediated primarily by errors in IgH switch recombination and less often by errors in somatic hypermutation, with the former dissociating the intronic and 3' enhancer(s), so that potential oncogenes can be dysregulated on each derivative chromosome (e.g., FGFR3 on der14 and MMSET on der4). Secondary translocations, which sometimes do not involve Ig loci, are more complex, and are not mediated by errors in B cell specific DNA modification mechanisms. They involve other chromosomal partners, notably 8q24 (c-myc), and are associated with tumor progression. Consistent with MM being the malignant counterpart of a long-lived PC, oncogenes dysregulated by primary IgH translocations in MM do not appear to confer an anti-apoptotic effect, but instead increase proliferation and/or inhibit differentiation. The fact that so many different primary transforming events give rise to tumors with the same phenotype suggests that there is only a single fate available for the transformed cell.

Transcription factor AP-2 functions as a repressor that contributes to the liver-specific expression of serum amyloid A1 gene

We previously identified transcription factor AP-2 as the nuclear factor that interacts with the tissue-specific repressor element in the rat serum amyloid A1 (SAA1) promoter. In this report, we provide evidence for a second AP-2-binding site and show that both AP-2 sites participate in mediating the transcription repression of SAA1 promoter. This proximal AP-2 site overlaps with the NFkappaB-binding site known to be essential for SAA1 promoter activity. Protein binding competition experiments demonstrated that AP-2 and NFkappaB binding to these overlapping sites were mutually exclusive. Furthermore, the addition of AP-2 easily displaced prebound NFkappaB, whereas NFkappaB could not displace AP-2. These results thus suggest that one mechanism by which AP-2 negatively regulates SAA1 promoter activity may be by antagonizing the function of NFkappaB. Consistent with a repression function, transient expression of AP-2 in HepG2 cells inhibited conditioned medium-induced SAA1 promoter activation. This inhibition was dependent on functional AP-2-binding sites, since mutation of AP-2-binding sites abolished inhibitory effects of AP-2 in HepG2 cells as well as resulted in derepression of the SAA1 promoter in HeLa cells. In addition to SAA1, we found that several other liver gene promoters also contain putative AP-2-binding sites. Some of these sequences could specifically inhibit AP-2.DNA complex formation, and for the human complement C3 promoter, overexpression of AP-2 also could repress its cytokine-mediated activation. Finally, stable expression of AP-2 in hepatoma cells significantly reduced the expression of endogenous SAA, albumin, and alpha-fetoprotein genes. Taken together, our results suggest that AP-2 may function as a transcription repressor to inhibit the expression of not only SAA1 gene but also other liver genes in nonhepatic cells.

BLIMP-I mediates extinction of major histocompatibility class II transactivator expression in plasma cells

Class II transactivator (CIITA), a coactivator required for class II major histocompatibility complex (MHC) transcription, is expressed in B cells but extinguished in plasma cells. This report identifies B lymphocyte-induced maturation protein I (BLIMP-I), a transcriptional repressor that is capable of triggering plasma cell differentiation, as a developmentally regulated repressor of CIITA transcription. BLIMP-I represses the B cell-specific promoter of the human gene that encodes CIITA (MHC2TA) in a binding site-dependent manner. Decreased CIITA correlates with increased BLIMP-I during plasma cell differentiation in cultured cells. Ectopic expression of BLIMP-I represses endogenous mRNA for CIITA and the CIITA targets, class II MHC, invariant chain and H2-DM (the murine equivalent of HLA-DM) in primary splenic B cells as well as 18-81 pre-B cells. Thus, the BLIMP-I program of B cell differentiation includes loss of antigen presentation via extinction of CIITA expression.

Repression of c-myc is necessary but not sufficient for terminal differentiation of B lymphocytes in vitro

The importance of c-myc as a target of the Blimp-1 repressor has been studied in BCL-1 cells, in which Blimp-1 is sufficient to trigger terminal B-cell differentiation. Our data show that Blimp-1-dependent repression of c-myc is required for BCL-1 differentiation, since constitutive expression of c-Myc blocked differentiation. Furthermore, ectopic expression of cyclin E mimicked the effects of c-Myc on both proliferation and differentiation, indicating that the ability of c-Myc to drive proliferation is responsible for blocking BCL-1 differentiation. However, inhibition of c-Myc by a dominant negative form was not sufficient to drive BCL-1 differentiation. Thus, during Blimp-1-dependent plasma cell differentiation, repression of c-myc is necessary but not sufficient, demonstrating the existence of additional Blimp-1 target genes.

BLIMP-1: trigger for differentiation of myeloid lineage

B lymphocyte-induced maturation protein-1 (BLIMP-1 or PRDI-BF1) is induced when bone marrow-derived progenitors differentiate in response to macrophage-colony stimulating factor (M-CSF) and is present in peripheral blood monocytes and granulocytes. BLIMP-1 is also induced during differentiation of U937 and HL-60 cells into macrophages or granulocytes. Induction of BLIMP-1 mRNA during macrophage differentiation of U937 and HL-60 shows a biphasic pattern. Overexpression of BLIMP-1 is sufficient to initiate macrophage differentiation of U937 cells whereas blocking endogenous BLIMP-1 inhibits differentiation. One target of BLIMP-1-dependent transcriptional repression in U937 cells is c-myc, providing an explanation for cessation of cell division. Thus BLIMP-1 is a key regulator of terminal differentiation in two separate hematopoietic lineages: myeloid cells and B lymphocytes.

Transcriptional repression by blimp-1 (PRDI-BF1) involves recruitment of histone deacetylase

B-lymphocyte-induced maturation protein (Blimp-1) is a transcriptional repressor that is considered to be a master regulator of terminal B-cell development because it is sufficient to trigger differentiation in the BCL(1)-cell model. Transcription of the c-myc gene is repressed by Blimp-1 during B-cell differentiation. In this study, we have explored the mechanism by which Blimp-1 represses transcription by using Gal4-fusion protein assays and assays in which Blimp-1 represses the natural c-myc promoter. The results show that Blimp-1 represses the c-myc promoter by an active mechanism that is independent of the adjacently bound activator YY1. Blimp-1 contains two regions that independently associate with histone deacetylase (HDAC) and endogenous Blimp-1 in nuclear extracts binds in vitro to the c-myc Blimp-1 site in a complex containing HDAC. The functional importance of recruiting HDAC for Blimp-1-dependent repression of c-myc transcription is supported by two experiments. First, the HDAC inhibitor tricostatin A inhibits Blimp-1-dependent repression in cotransfection assays. Second, a chromatin immunoprecipitation assay shows that expression of Blimp-1 causes deacetylation of histone H3 associated with the c-myc promoter, and this deacetylation depends on the Blimp-1 binding site in the c-myc promoter.

Purification and identification of a tissue-specific repressor involved in serum amyloid A1 gene expression

We have previously demonstrated that the 5'-flanking regions from the rat serum amyloid A1 (SAA1) promoter are necessary and sufficient to confer specific cytokine-induced expression in cultured hepatoma cells. Deletion analysis identified a tissue-specific repressor (TSR) regulatory element, located between bp -289 and -256, that functioned as a silencer, contributing to the transcription repression on SAA1 promoter in nonliver cells. When this 34-base pair TSR-binding element was used as a probe in electrophoretic mobility shift assays, an intense DNA-protein complex was detected in nuclear extracts from HeLa and several other nonliver tissues. This TSR binding activity, however, was undetectable in extracts from liver or liver-derived cells. The distribution of TSR binding activity is therefore consistent with its regulatory role in repressing SAA1 expression in a tissue-specific manner. In this study, we purified TSR protein from HeLa nuclear extracts and showed that it has a molecular mass of approximately 50 kDa. Surprisingly, protein sequencing and antibody supershift experiments identified TSR as transcription factor AP-2. Subsequent functional analysis showed that forced expression of AP-2 in HepG2 cells could indeed inhibit conditioned medium-induced SAA1 promoter activation. Moreover, expression of a dominant-negative mutant of AP-2 in HeLa cells or mutation of the AP-2-binding site led to derepression of the SAA1 promoter, presumably by neutralizing the inhibitory effects of the endogenous wild-type AP-2. Our results therefore demonstrate a novel function for AP-2 in the transcriptional repression of SAA1 promoter. Together with its tissue distribution, AP-2 may contribute to SAA1's highly liver-specific expression pattern by restricting its expression in nonliver cells.

Transcriptional regulation of the MN/CA 9 gene coding for the tumor-associated carbonic anhydrase IX. Identification and characterization of a proximal silencer element

The MN/CA 9 (MN) gene encodes a tumor-associated isoenzyme of the carbonic anhydrase family. Functional characterization of the 3. 5-kilobase pair MN 5' upstream region by deletion analysis led to the identification of the -173 to +31 fragment as the MN promoter. In vitro DNase I footprinting revealed the presence of five protected regions (PRs) within the MN promoter. Detailed deletion analysis of the promoter identified PR1 and PR2 (numbered from the transcription start) as the most critical for transcriptional activity. PR4 negatively affected transcription, since its deletion led to increased promoter activity and was confirmed to function as a promoter-, position-, and orientation-independent silencer element. Mutational analysis indicated that the direct repeat AGGGCacAGGGC is required for efficient repressor binding. Two components of the repressor complex (35 and 42 kDa) were found to be in direct contact with PR4 by UV cross-linking. Increased cell density, known to induce MN expression, did not affect levels of PR4 binding in HeLa cells. Significantly reduced repressor level seems to be responsible for MN up-regulation in the case of tumorigenic CGL3 as compared with nontumorigenic CGL1 HeLa x normal fibroblast hybrid cells.

Three unrelated viral transforming proteins (vIRF, EBNA2, and E1A) induce the MYC oncogene through the interferon-responsive PRF element by using different transcription coadaptors

Kaposi sarcoma-associated herpesvirus vIRF is a viral transcription factor that inhibits interferon signaling and transforms NIH 3T3 cells, but does not bind interferon-stimulated response element (ISRE) DNA sequences. Here we show that induction of the MYC protooncogene is required for cell transformation by vIRF, and that vIRF increases MYC transcription up to 15-fold through specific promoter interactions at an ISRE sequence called the plasmacytoma repressor factor (PRF) element. These effects are resistant to cycloheximide but are inhibited by a dominant-negative ISRE-binding protein, indicating that vIRF acts together with a cellular cofactor at the PRF element to directly transactivate MYC. The coadaptor CREB-binding protein (CBP) binds vIRF and synergizes transactivation of MYC, but, unexpectedly, closely related histone acetyltransferases p300 and P/CAF potently suppress vIRF transactivation. On the basis of the prediction that other interferon-inhibiting viral transforming proteins behave similarly, we found that Epstein-Barr virus-induced nuclear antigen 2 (EBNA2) also binds p300/CBP, and that both EBNA2 and adenovirus E1A transactivate MYC through the PRF element. For E1A, P/CAF coactivates MYC, whereas both p300 and CBP suppress E1A transactivation. For EBNA2, both P/CAF and CBP coactivate the MYC promoter, whereas p300 suppresses EBNA2 transactivation. These findings demonstrate that viral transforming proteins can activate as well as inhibit transcription through coadaptor interactions. At some promoters CBP and p300 have previously unrecognized, competitive antagonism to each other. While all three viral proteins target the same promoter element, each has a different coadaptor use profile. These findings are consistent with cellular MYC repression playing a role in innate immunity as well as in control of cell proliferation.

PRDI-BF1/Blimp-1 repression is mediated by corepressors of the Groucho family of proteins

The PRDI-BF1/Blimp-1 protein is a transcriptional repressor required for normal B-cell differentiation, and it has been implicated in the repression of beta-interferon (IFN-beta) and c-myc gene expression. Here, we show that PRDI-BF1 represses transcription of the IFN-beta promoter and of an artificial promoter through an active repression mechanism. We also identified a minimal repression domain in PRDI-BF1 that is sufficient for transcriptional repression when tethered to DNA as a Gal4 fusion protein. Remarkably, this repression domain interacts specifically with hGrg, TLE1, and TLE2 proteins, all of which are members of the Groucho family of transcriptional corepressors. In addition, the hGrg protein itself can function as a potent repressor when tethered to DNA through the Gal4 DNA-binding domain. We also find that the amino-terminal glutamine-rich domains of hGrg and TLE1 are sufficient to mediate dimerization of the two Groucho family proteins. Proteins containing only this domain can function as a dominant-negative inhibitor of PRDI-BF1 repression, and can significantly increase the IFN-beta promoter activity after virus induction. We conclude that PRDI-BF1/Blimp-1 represses transcription by recruiting a complex of Groucho family proteins to DNA, and suggest that such corepressor complexes are required for the postinduction repression of the IFN-beta promoter.

Transcriptional regulation during B cell development

Information is increasingly available concerning the molecular events that occur during primary and antigen-dependent stages of B cell development. In this review the roles of transcription factors and coactivators are discussed with respect to changes in expression patterns of various genes during B cell development. Transcriptional regulation is also discussed in the context of developmentally regulated immunoglobulin gene V(D)J recombination, somatic hypermutation, and isotype switch recombination.

Transactivation of prothymosin alpha and c-myc promoters by human papillomavirus type 16 E6 protein

The human papillomavirus type 16 E6 protein exerts a transforming activity through inactivation of tumor suppressor p53. Recently E6 has been shown to have additional transforming activities independent of p53. E6 is able to transactivate or repress several specific viral promoters. However, underlying molecular mechanisms and cellular target genes for the activity are not well understood. Using a differential hybridization technique, we identified the prothymosin alpha gene as a cellular target of E6 transactivation. E6 was able to transactivate the prothymosin alpha promoter in H358 cells lacking p53 and in C33A cells harboring a mutant p53 allele. Disruption of the E-box in intron 1 of the prothymosin alpha promoter abolished the responsiveness to E6. Then we determined if E6 up-regulates the expression of Myc, by which the prothymosin alpha promoter is transactivated through the E-box. We found that E6 is also able to transactivate the c-myc promoter in H358 cells and in C33A cells. These results suggest that E6 is able to transactivate the c-myc promoter independently of p53, and that the prothymosin alpha promoter is subsequently transactivated by Myc.

Repression of c-myc transcription by Blimp-1, an inducer of terminal B cell differentiation

Transcription of c-myc in plasma cells, which are terminally differentiated B cells, is repressed by plasmacytoma repressor factor. This factor was identified as Blimp-1, known for its ability to induce B cell differentiation. Blimp-1 repressed c-myc promoter activity in a binding site-dependent manner. Treatment of BCL1 lymphoma cells with interleukin-2 (IL-2) plus IL-5 induced Blimp-1 and caused a subsequent decline in c-Myc protein. Ectopic expression of Blimp-1 in Abelson-transformed precursor B cells repressed endogenous c-Myc and caused apoptosis; Blimp-1-induced death was partially overcome by ectopic expression of c-Myc. Thus, repression of c-myc is a component of the Blimp-1 program of terminal B cell differentiation.

The ZiN/POZ domain of ZF5 is required for both transcriptional activation and repression

ZF5 is a ubiquitously expressed protein originally identified by its ability to bind and repress the murine c-myc promoter. It contains five C-terminal zinc fingers and a conserved N-terminal ZiN/POZ domain. This motif, found in a growing number of zinc finger proteins, can inhibit DNA binding and mediate dimerization [Bardwell, V.J. and Treisman,R. (1994) Genes Dev., 8,1664-1677]. In the current study, a cyclic amplification and selection of targets (CAST) protocol detected preferred ZF5 binding sites which are highly GC-rich. Binding to these sites by ZF5 depended upon the zinc fingers and was enhanced when the ZiN/POZ domain was removed. Using transient cotransfection assays, ZF5 was shown to activate the HIV-1 LTR and repress the beta-actin promoter. The ZiN/POZ domain was shown to mediate ZF5-dependent transcriptional activation and repression. From these data, we conclude that ZF5 can both activate and repress in the context of different natural promoters and that its ZiN/POZ domain can affect two functions; DNA binding and transcriptional modulation.

Sequence-specific DNA binding and transcription factor phosphorylation by Ku Autoantigen/DNA-dependent protein kinase. Phosphorylation of Ser-527 of the rat glucocorticoid receptor

NRE1 is a DNA sequence element through which Ku antigen/DNA-dependent protein kinase (DNA-PK) catalytic subunit represses the induction of mouse mammary tumor virus transcription by glucocorticoids. Although Ku is an avid binder of DNA ends and has the ability to translocate along DNA, we report that direct sequence-specific Ku binding occurs with higher affinity (Kd = 0.84 +/- 0.24 nM) than DNA end binding. Comparison of Ku binding to several sequences over which Ku can accumulate revealed two classes of sequence. Sequences with similarity to NRE1 competed efficiently for NRE1 binding. Conversely, sequences lacking similarity to NRE1 competed poorly for Ku and were not recognized in the absence of DNA ends. Phosphorylation of glucocorticoid receptor (GR) fusion proteins by DNA-PK reflected Ku DNA-binding preferences and demonstrated that co-localization of GR with DNA-PK on DNA in cis was critical for efficient phosphorylation. Phosphorylation of the GR fusion protein by DNA-PK mapped to a single site, Ser-527. This site occurs adjacent the GR nuclear localization sequence between the DNA and ligand binding domains of GR, and thus its phosphorylation, if confirmed, has the potential to affect receptor function in vivo.

Nucleosomal structure of active and inactive c-myc genes

The nucleosomal structure of active and inactive c-myc genes has been analyzed in detail in undifferentiated and differentiated cells of the promyelocytic leukemia cell line HL60. The c-myc P2 promoter was never found in nucleosomal configuration, no matter whether c-myc was expressed or not. Differences in the nucleosomal structure, however, were found in the promoter upstream region proximal to a previously described DNase I-hypersensitive site I, at the P0 promoter, and at the P1 promoter and upstream thereof. In these regions nucleosomes were detected in differentiated but not undifferentiated HL60 cells. Similar patterns of nucleosomes as found for active and inactive c-myc genes in HL60 cells were found for active and inactive episomal c-myc genes in stably transfected B cell lines. In these cell lines three activation stages could be described for episomal c-myc constructs: (i) uninducible, (ii) inducible, and (iii) induced. Significant differences in the nucleosomal structure of c-myc were observed for the uninducible and inducible stages, but not for the inducible and induced stages.

Inhibitory transcription factors

Although the majority of the transcription factors that were initially characterized had a stimulatory effect on gene expression, it is now clear that transcription factors that inhibit gene transcription are at least as important in regulating a wide variety of processes, including development. These factors can act either by interfering with the action of a positively acting factor (indirect repression) or by directly interfering with transcription by interacting with the basal transcriptional complex of RNA polymerase and associated factors (direct repression). Indirect repression often operates by the negative factor preventing the positively acting factor binding to DNA. This can involve reorganization of chromatin structure, blockage of the binding site in the DNA by binding of the inhibitory factor or formation of a non-DNA binding protein-protein complex. Indirect repression can also occur via quenching of the activity of a positive factor that remains bound to DNA. Direct repression can be produced by factors that interact with the basal transcriptional complex to reduce its activity or stability. This can be achieved either by factors that interact with the complex following binding to DNA or by those which bind directly to it. Factors that act by each of these means, and their mechanisms of action, are discussed.

The ability of the inhibitory domain of the POU family transcription factor Oct-2 to interfere with promoter activation by different classes of activation domains is dependent upon the nature of the basal promoter elements

The Oct-2 transcription factor contains an inhibitory domain which is able to repress transcription following DNA binding. Here we show that within the neuronally expressed Oct-2.5 form, the inhibitory domain can strongly inhibit activation by transcription factor activation domains which are either composed predominantly of acidic residues or contain the HOB motif, whereas it has a weaker effect or no effect on proline-rich activation domains and on a glutamine-rich domain. In contrast, the isolated inhibitory domain of Oct-2 can efficiently repress all types of activation domains. This effect is observed however, only on TATA box-containing promoters and not on promoters containing an initiator motif. This widespread inhibition of different activation domains and its dependence on the nature of the basal promoter elements indicate that the inhibitory domain is likely to act by contacting a common downstream target of activation domains within the basal transcriptional complex bound at the TATA box rather than quenching specific activation domains by direct interaction. These effects are discussed in terms of the functional role of the inhibitory domain within Oct-2.5 and the mechanism by which it acts.

Characterization of a silencer element in the first exon of the human osteocalcin gene

Osteocalcin, the major non-collagenous protein in bone, is transcribed in osteoblasts at the onset of extracellular matrix mineralization. In this study it was demonstrated that sequences located in the first exon of the human osteocalcin gene possess a differentiation-related osteocalcin silencer element (OSE). Osteocalcin was rendered transcribable in UMR-106 cells and proliferating normal osteoblasts after deletion of the -3 to +51 region. Site-specific mutagenesis of this region revealed that a 7 bp sequence (TGGCCCT) (+29 to +35) is critical for silencing function. Mobility shift assays demonstrated that a nuclear factor bound to the OSE. The OSE binding protein was present in proliferating normal pre-osteoblasts and in UMR-106 and ROS 17/2.8 osteosarcoma cells, but was absent from post-proliferative normal osteoblasts. The binding protein was inhibited by fragments containing the +29/+35 sequence, but not by other promoter fragments or by the consensus oligomers of unrelated nuclear factors AP-1 and Sp1. DNase 1 footprinting demonstrated that the OSE binding-protein protected the +17 to +36 portion of the first exon, consistent with the results of mapping studies and competitive mobility shift assays. It is hypothesized that this silencer is activated by complexing of the OSE binding protein to the OSE during the osteoblast proliferation stage and that the OSE binding protein is down-regulated at the onset of extracellular matrix mineralization.

Nuclear factor binding to a DNA sequence element that represses MMTV transcription induces a structural transition and leads to the contact of single-stranded binding proteins with DNA

NRE1 is a DNA sequence element in the long terminal repeat of mouse mammary tumor virus through which viral transcription is repressed. In addition to double-stranded DNA binding, both upper- and lower-stranded NRE1 binding activities occur in nuclear extracts. All three binding activities appear to be important for transcriptional effects. We report that occupancy of NRE1 within linear double-stranded NRE1 induces a structural transition in upstream flanking DNA that is facilitated by Mg2+. This transition was reflected by the striking DNase I sensitivity of the DNA. As Mg2+ concentration was increased, discrete DNase I hypersensitivity on one face of the DNA progressed to complete degradation of template. On the DNA face opposite the DNase I hypersensitivity, Mg2+ promoted regularly spaced cleavage by the single-strand-specific cleavage agents KMnO4 and S1 nuclease. Induction of degradation by DNase I occurred independently of MMTV sequences flanking NRE1, because nuclear extract-dependent DNase I sensitivity was conferred to an unrelated DNA fragment by introduction of a 23-bp NRE1-containing oligonucleotide. UV protein-DNA cross-linking revealed that addition of Mg2+ to a double-stranded NRE1 DNA binding assay induced conversion from a double- to a single-stranded protein-DNA cross-linking pattern. Thus, nuclear factor binding to NRE1 induces changes in DNA topology that promote the direct contact of single-stranded NRE1 binding factors with DNA.

Transcriptional down-regulation of c-myc expression in the MCF-7 breast tumor cell line by the topoisomerase II inhibitor, VM-26

In the MCF-7 human breast tumor cell line, the topoisomerase II inhibitor, VM-26, produces a concentration dependent reduction in expression of the oncogene c-myc which parallels growth inhibition. Down-regulation of c-myc expression was examined at transcriptional and post-transcriptional levels. VM-26, at 10 microM, produced a reduction in the transcription rate of both sense and antisense strands of c-myc as determined by nuclear run-off analysis. In contrast, in the presence of the RNA synthesis inhibitor, actinomycin D, VM-26 failed to alter the half-life of the c-myc message. The capacity of VM-26 to reduce c-myc expression was not abrogated in cells pretreated with the protein synthesis inhibitor, cycloheximide (despite superinduction of c-myc expression in both control and VM-26 treated cells); this observation suggests that de novo protein synthesis may not be required to mediate the effects of VM-26 on steady state c-myc transcript levels. An extended analysis of the time course of c-myc expression demonstrated that the decline of steady state c-myc mRNA levels induced by VM-26 was biphasic, 6 h after the initial reduction in c-myc expression to approx. 30% of control levels, c-myc levels rebounded to 70% of control; after 24 h, c-myc expression declined gradually and remained at depressed levels (40% of control) at 48 and 72 h. These observations suggest that the initial transient reduction in c-myc expression associated with inhibition of transcription may represent a component of an early signalling pathway leading to growth arrest in MCF-7 breast tumor cells exposed to VM-26.

Cellular biology of tubulointerstitial growth

The study of tubular growth has certainly become more complex since Pierre-Rayers's time and is progressing toward a molecular dissection of regulatory events. Understanding the mechanisms of tubular growth is important, because these cells represent the bulk of the nephron, and there is convincing evidence of a link between tubular hypertrophy and the progression of renal disease with irreversible tubulointerstitial fibrosis as an end point. Two tubular growth responses can be distinguished: hypertrophy and hyperplasia. These fundamentally different patterns of growth indicate that diverse molecular mechanisms may be involved in inducing distinct growth responses. It is likely that cytokines and polypeptide growth factors play a role in tubular hypertrophy and hyperplasia. Probably, a combination of growth factors including inhibitory polypeptides like TGF beta, rather than a single factor, is necessary for differentiated tubular growth responses. Such factors bind to their receptors, and signals are transduced to the nucleus by various second messengers involving protein kinases, cyclic nucleotides, Ca++, and inositolphosphates. The phosphorylation of nuclear trans-acting factors resulting in an expression of immediate early genes may be the common pathway of many of these mediators. Finally, whether the cell is to proliferate or to remain in the G1-phase of the cell cycle is determined by the very complex cascade phosphorylation of kinases and their associations with different cyclins. How the induction of immediate early genes is linked to events of the cell cycle is currently incompletely understood. Negative regulation of growth through protein growth suppressors like the retinoblastoma gene product or the expression of special genes only during cell rest may be mandatory for the fine tuning of tubular growth.

Computer assisted promoter analysis of a human sperm specific nucleoprotein gene cluster

The promoter regions of the clustered human spermatid-specific nucleoprotein PRM1, PRM2 arid TNP2 genes were compared to define regulatory elements that may govern their expression. Sequence alignment revealed two wll conserved motifs, despite a lack of extensive homology. They are located at similar positions within the first 400 nt of their 5' UTRs. Conservation of these motifs may reflect selective evolutionary pressure to maintain their structure. This supports the view that these elements assume a central role in the coordinate regulation of this gene cluster during spermiogenesis. The distribution of binding sites of known transcription factors was also assessed within the regions flanking the 5' ends of these genes. This analysis should prove useful in directing studies that define the signals necessary for the coordinate regulation of this spermatid specific gene cluster.

A silencer element for the lipoprotein lipase gene promoter and cognate double- and single-stranded DNA-binding proteins

Transfection experiments with constructs containing various 5'-deleted fragments of the human lipoprotein lipase (LPL) promoter and the chloramphenicol acetyltransferase reporter gene revealed an LPL silencer element (LSE) in the region of nucleotides -225 to -81 of the LPL gene that functioned in Chinese hamster ovary (CHO) and HeLa cells. Gel retardation competition analysis showed the presence of a nuclear factor(s) capable of binding to the sequence of nucleotides -169 to -152 of LSE (LSE-6) in a single-stranded (opposite-strand) and double-stranded specific fashion, the binding affinity being almost the same in the two binding forms. Site-directed mutagenesis indicated that almost the entire sequence of LSE-6 was necessary to form the complexes and also critical for silencing activity in CHO cells. The amounts of this binding factor(s) in CHO and HeLa cells were closely associated with transcriptional silencing activity. Photochemical cross-linking experiments indicated that the single- and double-stranded elements recognized the same binding factor(s) with molecular masses of 54 to 63 kDa and 109 to 124 kDa. The 109- to 124-kDa DNA binding factor(s) was found to be a doublet of that of the 54- to 63-kDa factor by isoelectric focusing or by increasing the time of exposure to UV irradiation. When inserted upstream of another gene such as that of the simian virus 40 enhancer/promoter of pSV2CAT, the sequence of nucleotides -190 to -143 (LSE-1) also suppressed transcription of the reporter gene in CHO cells. These results strongly suggest that the LSE plays a role in regulation of LPL gene expression by suppressing its transcription.

A novel hepatocytic transcription factor that binds the alpha-fetoprotein promoter-linked coupling element

We recently characterized a promoter-linked coupling element (PCE) in the rat alpha-fetoprotein (AFP) gene required for strong transcriptional stimulation by distant enhancers (P. Wen, N. Crawford, and J. Locker, Nucleic Acids Res. 21:1911-1918, 1993). In this study, oligonucleotide gel retardation and competition experiments defined the PCE as a 12-bp binding site, TGTCCTTGAACA, an imperfect inverted repeat from -166 to -155 near the AFP promoter. A factor that bound this site (PCF) was abundant in HepG2 nuclear extracts and detectable in extracts from several other AFP-producing hepatocarcinoma cell lines and fetal liver. Hepatocytic cell lines that did not express AFP, nonhepatocytic cell lines, adult liver, and fetal brain did not show the factor. Experiments excluded the possibility that PCF activity was due to binding of glucocorticoid receptor or an AP1-like factor that bound overlapping sites. Competition experiments with several mutant oligonucleotides determined that the optimum PCF binding site was TGTCCTTGAAC(A/T). Mutations decreased binding or totally abolished binding activity. In expression plasmids, PCE mutations strongly reduced gene expression. UV cross-linking to a PCE probe identified peptide bands near 34 kDa. PCF was purified by heparin-Sepharose chromatography followed by affinity binding to oligomerized PCE DNA. The product resolved as a complex of three peptides (PCF alpha 1, PCF alpha 2, and PCF beta, 32 to 34 kDa) on sodium dodecyl sulfate-acrylamide gels. The peptide sizes and gel patterns are unlike those of any of the well-described hepatic transcription factors, and the binding site has not been previously reported. PCF thus appears to be a novel transcription factor.

A novel hepatocytic transcription factor that binds the alpha-fetoprotein promoter-linked coupling element

We recently characterized a promoter-linked coupling element (PCE) in the rat alpha-fetoprotein (AFP) gene required for strong transcriptional stimulation by distant enhancers (P. Wen, N. Crawford, and J. Locker, Nucleic Acids Res. 21:1911-1918, 1993). In this study, oligonucleotide gel retardation and competition experiments defined the PCE as a 12-bp binding site, TGTCCTTGAACA, an imperfect inverted repeat from -166 to -155 near the AFP promoter. A factor that bound this site (PCF) was abundant in HepG2 nuclear extracts and detectable in extracts from several other AFP-producing hepatocarcinoma cell lines and fetal liver. Hepatocytic cell lines that did not express AFP, nonhepatocytic cell lines, adult liver, and fetal brain did not show the factor. Experiments excluded the possibility that PCF activity was due to binding of glucocorticoid receptor or an AP1-like factor that bound overlapping sites. Competition experiments with several mutant oligonucleotides determined that the optimum PCF binding site was TGTCCTTGAAC(A/T). Mutations decreased binding or totally abolished binding activity. In expression plasmids, PCE mutations strongly reduced gene expression. UV cross-linking to a PCE probe identified peptide bands near 34 kDa. PCF was purified by heparin-Sepharose chromatography followed by affinity binding to oligomerized PCE DNA. The product resolved as a complex of three peptides (PCF alpha 1, PCF alpha 2, and PCF beta, 32 to 34 kDa) on sodium dodecyl sulfate-acrylamide gels. The peptide sizes and gel patterns are unlike those of any of the well-described hepatic transcription factors, and the binding site has not been previously reported. PCF thus appears to be a novel transcription factor.

LR1 regulates c-myc transcription in B-cell lymphomas

LR1 is a 106-kDa sequence-specific DNA-binding protein first identified as a potential regulator of immunoglobulin class switch recombination in B lymphocytes. Here we report that LR1 binds to a site 310 nt upstream of the human c-myc P1 promoter. Mutation of this site decreases reporter gene expression 5.5-fold in the Burkitt lymphoma line Raji and 3.8-fold in the lymphoma line BJAB. These experiments show that LR1 can function as a transcription factor and identify it as a cell type-specific activator of c-myc expression. There are multiple matches to the LR1 recognition consensus at the immunoglobulin heavy-chain locus and at c-myc, which further suggests that LR1 may play a dual role, facilitating c-myc translocation as well as regulating c-myc transcription.