Oct2 transactivation from a remote enhancer position requires a B-cell-restricted activity

OBF1 and Oct factors control the germinal center transcriptional program

OBF1 is a specific coactivator of the POU family transcription factors OCT1 and OCT2. OBF1 and OCT2 are B cell-specific and indispensable for germinal center (GC) formation, but their mechanism of action is unclear. Here, we show by chromatin immunoprecipitation-sequencing that OBF1 extensively colocalizes with OCT1 and OCT2. We found that these factors also often colocalize with transcription factors of the ETS family. Furthermore, we showed that OBF1, OCT2, and OCT1 bind widely to the promoters or enhancers of genes involved in GC formation in mouse and human GC B cells. Short hairpin RNA knockdown experiments demonstrated that OCT1, OCT2, and OBF1 regulate each other and are essential for proliferation of GC-derived lymphoma cell lines. OBF1 downregulation disrupts the GC transcriptional program: genes involved in GC maintenance, such as BCL6, are downregulated, whereas genes related to exit from the GC program, such as IRF4, are upregulated. Ectopic expression of BCL6 does not restore the proliferation of GC-derived lymphoma cells depleted of OBF1 unless IRF4 is also depleted, indicating that OBF1 controls an essential regulatory node in GC differentiation.

Downregulation of internal enhancer activity contributes to abnormally low immunoglobulin expression in the MedB-1 mediastinal B-cell lymphoma cell line

Primary mediastinal B-cell lymphoma (PMBL) is a highly aggressive tumour with a unique pattern of clinical, morphological, immunological and genetic features distinct from other diffuse large B-cell lymphomas. PMBLs are characterized by a mature B-cell phenotype, but they typically lack immunoglobulin (Ig) gene expression. The PMBL cell line MedB-1 shares many characteristic properties of the primary tumour, including low-level Ig production despite a functionally rearranged IgVH gene and absence of 'crippling' mutations. In this study, a search was undertaken for reasons for downregulated Ig expression. Similar levels of the B-cell-specific transcription factors BOB.1/OBF.1 and PU.1 were found in MedB-1 cells to those in the Ig-producing UM-1 lymphoblastoid cell line. However, MedB-1 lacked the Oct2 transcription factor. Reporter assays showed that Ig-type promoters were active in MedB-1 cells. In contrast, activity of the intronic heavy chain enhancer was dramatically reduced. Ectopic expression of Oct2 was able partially to restore enhancer activity but transcription from the endogenous IgVH gene could not be rescued. Therefore, the role of epigenetic factors in the downregulation of Ig was investigated. Methylated histone 3 lysine 9, a reliable marker of chromatin silencing, was not detected in MedB-1 promoter and enhancer regions. Inhibition of DNA methyltransferase and of histone deacetylases also did not reactivate Ig production. These data suggest the existence of alternative mechanisms of Ig inhibition in MedB-1 cells, different from chromatin silencing and the lack of Oct2.

Epigenetic silencing of the immunoglobulin heavy-chain gene in classical Hodgkin lymphoma-derived cell lines contributes to the loss of immunoglobulin expression

Immunoglobulin production is impaired in Hodgkin and Reed-Sternberg (HRS) cells of classical Hodgkin lymphoma (cHL) in spite of functional clonal rearrangements. The presence of "crippling" mutations in coding and regulatory regions, as well as down-regulation of B-cell-specific transcription factors, has been suggested as a potential reason for the lack of immunoglobulin (Ig) chain gene transcription. We have investigated the impact of epigenetic silencing in suppressing Ig heavy (H)-chain expression. Chromatin immunoprecipitation (ChIP) was used to analyze transcription factor binding to octamer motifs present in the IgH regulatory regions. Transcription factors were bound to these motifs in control cell lines, however, they were absent in the cHL-derived cell lines KMH2, L1236, and L428. Ectopic expression of octamer-binding transcription factor (Oct2) and/or B-cell Oct binding protein/Oct-binding factor (BOB.1/OBF.1) did not result in any measurable binding to these sites. Increased histone 3 Lysine 9 (H3-K9) methylation was observed in the promoter region of the IgH locus in L428 and L1236 cells. This is a typical feature of heterochromatic, transcriptionally silent regions. Treatment of cHL-derived cell lines with the DNA demethylating agent 5-aza-2'-deoxycytidine (5-aza-dC) partially reactivated IgH transcription and affected chromatin modifications. Our results suggest an important role of epigenetic silencing in the inhibition of IgH transcription in HRS cells.

Expression of the aldehyde dehydrogenase 2-like gene is controlled by BOB.1/OBF.1 in B lymphocytes

BOB.1/OBF.1 is a lymphocyte-restricted transcriptional coactivator. It binds to the Oct1 and Oct2 transcription factors and increases their transactivation potential. Targeted gene disruption experiments revealed that BOB.1/OBF.1 is critical at different stages of B cell development. A large number of genes expressed in B cells contain octamer motifs in their regulatory regions. However, only few genes have been described so far whose expression is dependent on BOB.1/OBF.1. To understand the molecular basis of BOB.1/OBF.1 function in B cell development, we searched for BOB.1/OBF.1 target genes by expression profiling. We have identified genes both induced and repressed by BOB.1/OBF.1. Using different genetic systems, we demonstrate regulation of a selection of these genes. Identified targets included genes encoding Ahd2-like, AKR1C13, Rbp1, Sdh, Idh2, protocadherin gamma, alpha-catenin, Ptprs, Id3, and Creg. Classification of BOB.1/OBF.1 target genes by function suggests that they affect various aspects of B cell physiology such as cellular metabolism, cell adhesion, and differentiation. To better understand the mechanism of BOB.1/OBF.1 action, we cloned the promoter of the gene encoding Ahd2-like, the gene showing the strongest regulation by BOB.1/OBF.1. This promoter indeed contains a perfect octamer motif. Furthermore, the motif was recognized by the Oct transcription factors as well as BOB.1/OBF.1 in vitro and in vivo, as shown by electromobility shift and chromatin immunoprecipitation assays. Transient transfections confirm that this promoter is activated by BOB.1/OBF.1. Our observations suggest that by regulating genes in different functional pathways, BOB.1/OBF.1 has a widespread effect on B cell development and function.

Oct2 transcription factor of a teleost fish: activation domains and function from an enhancer

Oct2 transcription factors of the catfish (Ictalurus punctatus) are expressed as alternatively spliced alpha and beta isoforms. Functional analysis revealed an N-terminal glutamine (Q)-rich transactivation domain common to both isoforms of catfish Oct2. A C-terminal proline, serine, threonine (PST)-rich activation domain was identified exclusively in the beta isoform. Activation domains of fish and mammalian Oct2 showed cell type- and species-specific activity correlated with their biochemical composition (Q-rich vs PST-rich). In contrast the activation domains of the aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator of fish and mammals showed no correlation of activity with biochemical composition or species of origin. Although isolated catfish Oct2 activation domains were unable to drive transcription from a site 1.9kb distal to the promoter, Oct2beta activated transcription from both an IgH enhancer and an array of octamer motifs at this distal position. The properties of catfish Oct2 activation domains differ depending on whether they are studied in isolation or as components of the intact transcription factor.

Unique function for carboxyl-terminal domain of Oct-2 in Ig-secreting cells

The activity of Ig gene promoters and enhancers is regulated by two related transcription factors, Oct-1 (ubiquitous) and Oct-2 (B lineage specific), which bind the octamer motif (ATTTGCAT) present in these elements. As Ig promoter-binding factors, Oct-1 and Oct-2 each work together with a B lymphocyte-specific cofactor OCA-B/OBF-1/Bob-1 that interacts with them through their POU (DNA-binding) domains. Because both can mediate Ig promoter activity in B cells, there has been some question as to whether these two octamer-binding factors serve distinct functions in lymphocytes. We have shown previously that the silencing of B lymphocyte-specific genes in plasmacytoma x T lymphoma hybrids can be prevented by preserving Oct-2 expression. The pronounced effect of this transcription factor on the phenotype of plasmacytoma x T lymphoma hybrids established a critical role for Oct-2 not only in maintaining Ig gene expression, but in maintaining the overall genetic program of Ig-secreting cells. In the present study, we have explored the functional differences between Oct-1 and Oct-2 using chimeric Oct-1/Oct-2 proteins in cell fusion assays. Our results provide further evidence for an essential role for Oct-2 in Ig-secreting cells and identify the C-terminal domain of Oct-2 as responsible for its unique function in these cells.

An IgH Enhancer That Drives Transcription through Basic Helix-Loop-Helix and Oct Transcription Factor Binding Motifs

The transcriptional enhancer (E(mu)3') of the IgH locus of the channel catfish, Ictalurus punctatus, shows strong B cell-specific activity and differs from the mammalian E(mu) enhancer in both location and structure. It occurs between the mu and delta genes and contains numerous transcription factor binding sites, predominantly octamer and muE5 motifs of consensus and variant sequences. It lacks the classical muA-muE3(CBF)-muB core array of binding motifs seen within mammalian IgH E(mu) enhancers. To determine the functionally important motifs, a series of mutant enhancers was created using sequence-targeted polymerase chain reaction. Whereas the mutation of consensus and variant octamer motifs (individually or in multiples) decreased enhancer function, mutation of a single consensus muE5 motif destroyed the function of this enhancer in mammalian plasmacytomas. Mutation of this consensus muE5 site, combined with mutations of certain octamer sites, destroyed function in catfish B cells. Experiments using artificial enhancers containing multimers of motifs or short regions of the native enhancer suggested that the minimal E(mu)3' enhancer (a) contains a consensus muE5 site and two octamer sites, (b) is B cell-specific, and (c) is active across species. The dependence of an Ig enhancer on sites that bind basic helix-loop-helix and Oct transcription factors has not previously been observed and confirms large differences in structure and function between fish and mammalian IgH enhancers.

The virtuoso of versatility: POU proteins that flex to fit

During the evolution of eukaryotes, a new structural motif arose by the fusion of genes encoding two different types of DNA-binding domain. The family of transcription factors which contain this domain, the POU proteins, have come to play essential roles not only in the development of highly specialised tissues, such as complex neuronal systems, but also in more general cellular housekeeping. Members of the POU family recognise defined DNA sequences, and a well-studied subset have specificity for a motif known as the octamer element which is found in the promoter region of a variety of genes. The structurally bipartite POU domain has intrinsic conformational flexibility and this feature appears to confer functional diversity to this class of transcription factors. The POU domain for which we have the most structural data is from Oct-1, which binds an eight base-pair target and variants of this octamer site. The two-part DNA-binding domain partially encircles the DNA, with the sub-domains able to assume a variety of conformations, dependent on the DNA element. Crystallographic and biochemical studies have shown that the binary complex provides distinct platforms for the recruitment of specific regulators to control transcription. The conformability of the POU domain in moulding to DNA elements and co-regulators provides a mechanism for combinatorial assembly as well as allosteric molecular recognition. We review here the structure and function of the diverse POU proteins and discuss the role of the proteins' plasticity in recognition and transcriptional regulation.

Hodgkin and Reed-Sternberg cells represent an expansion of a single clone originating from a germinal center B-cell with functional immunoglobulin gene rearrangements but defective immunoglobulin transcription

Single cell studies aimed at clarifying the nature and clonality of Hodgkin and Reed-Sternberg (HRS) cells of classical Hodgkin's disease (HD) have so far produced conflicting results. Using an improved single cell procedure, the HRS cells of 25 patients with nodular sclerosing HD lacking B- and T-cell antigens, with and without Epstein-Barr virus infection, were analyzed for the presence of immunoglobulin (Ig) gene rearrangements. One patient with HD developed follicular lymphoma 2 years later. Both lymphomas originated from a common precursor identified as a germinal center B cell. The data show that all but one of the investigated cases harbored rearranged Ig genes, which were clonal in all instances and carried a high load of somatic mutations. The Ig coding capacity was preserved in 18 of the 24 cases (75%) with rearrangements. However, expression of Ig messenger RNA was not detectable in the HRS cells with the exception of Ig kappa light chain expression in some tumor cells of 1 case. The lack of Ig gene transcription in HRS cells was confirmed by analyzing the HD cell lines L428 and KM-H2 in transient transfection experiments. An Ig promoter/enhancer reporter construct showed virtually no activity in these cells compared to 5 control B-cell lines. We conclude that (1) classical HD is a B-cell lymphoma in most instances, (2) HRS cells are clonal without any exception, (3) they are derived from germinal center B-cells that (4) mostly lack crippling mutations but (5) have consistently lost their Ig gene transcription ability, due to functional defects in the Ig gene regulatory elements.

Up-regulation of BOB.1/OBF.1 expression in normal germinal center B cells and germinal center-derived lymphomas

The BOB.1/OBF.1/OCAB.1 protein is a lymphocyte-specific transcriptional coactivator. It interacts with the Oct1 and Oct2 transcription factors and contributes to the transcriptional activity of octamer motifs. The analysis of established B cell lines had suggested that BOB.1/OBF.1 is constitutively expressed at all stages of B cell development. Here we show that expression of BOB. 1/OBF.1 is regulated within the B cell lineage. Specifically, germinal center B cells show highly increased BOB.1/OBF.1 levels. We can induce the up-regulation by stimulating primary splenic B cells, eg, by triggering CD40 signaling in the presence of interleukin-4. Expression of BOB.1/OBF.1 is detectable but reduced in spleens from mice unable to undergo the germinal center reaction due to mutations in the TNF receptor p55 or lymphotoxin beta (LTbeta) receptor genes. Furthermore, we demonstrate that BOB.1/OBF.1 expression is highly regulated in human B cell lymphomas. Whereas lymphomas representing pre- and postfollicular B cell developmental stages are negative for BOB.1/OBF.1, high-level expression of BOB.1/OBF.1 is characteristic of germinal center-derived tumors. In these tumors BOB.1/OBF.1 is typically coexpressed with high levels of Bcl6. These results imply that overexpression of BOB.1/OBF.1, like overexpression of Bcl6, might play a role in the pathogenesis of germinal center-derived B cell lymphomas. Furthermore, overexpression of BOB.1/OBF.1 represents a characteristic feature of these tumors that is useful in their identification.

Characterization of the regulatory domains of the human skn-1a/Epoc-1/Oct-11 POU transcription factor

The Skn-1a POU transcription factor is primarily expressed in keratinocytes of murine embryonic and adult epidermis. Although some POU factors expressed in a tissue-specific manner are important for normal differentiation, the biological function of Skn-1a remains unknown. Previous in vitro studies indicate that Skn-1a has the ability to transactivate markers of keratinocyte differentiation. In this study, we have characterized Skn-1a's transactivation domain(s) and engineered a dominant negative protein that lacked this transactivation domain. Deletional analysis of the human homologue of Skn-1a with three target promoters revealed the presence of two functional domains: a primary C-terminal transactivation domain and a combined N-terminal inhibitory domain and transactivation domain. Skn-1a lacking the C-terminal region completely lost transactivation ability, irrespective of the promoter tested, and was able to block transactivation by normal Skn-1a in competition assays. Compared with full-length, Skn-1a lacking the N-terminal region demonstrated either increased transactivation (bovine cytokeratin 6 promoter), comparable transactivation (human papillomavirus type 1a long control region), or loss of transactivation (human papillomavirus type 18 long control region). The identification of a primary C-terminal transactivation domain enabled us to generate a dominant negative Skn-1a factor, which will be useful in the quest for a better understanding of this keratinocyte-specific gene regulator.

Regulation of the human interleukin-5 promoter by Ets transcription factors. Ets1 and Ets2, but not Elf-1, cooperate with GATA3 and HTLV-I Tax1

Interleukin-5 (IL-5), expressed primarily by type-2 T helper (Th2) cells, plays an important role in the development of allergic diseases, such as allergic asthma. Studying the regulation of IL-5 gene expression by Ets transcription factors, we found that Ets1 and Ets2, but not Elf-1, were able to activate the human IL-5 promoter in Jurkat T-cells. This required the presence of either phorbol 12-myristate acetate (PMA) plus ionomycin or PMA plus the viral protein HTLV-I Tax1. By mutation studies, it could be shown that Ets1 and Ets2 exerted their effects on the IL-5 promoter through a GGAA motif within the Cle0 element. In myeloid Kasumi cells, Ets1 and Ets2 failed to stimulate IL-5 promoter activity, unless the T-cell specific transcription factor GATA3 was added. These results show, for the first time, that Ets1 and Ets2 are able to cooperate with GATA3. Both ionomycin and Tax1 increased the combined effect of GATA3 with Ets1 and Ets2 in the presence of PMA. The data further demonstrate that, in addition to Ets1, Ets2 is also able to functionally cooperate with Tax1. The synergism of GATA3 with either Ets1 or Ets2 may play an important role in calcium- or Tax1-dependent regulation of IL-5 expression in Th2 cells or in HTLV-I transformed adult T-cell leukemia cells, respectively.

Formation of pluripotent stem cells in the mammalian embryo depends on the POU transcription factor Oct4

Oct4 is a mammalian POU transcription factor expressed by early embryo cells and germ cells. We report that the activity of Oct4 is essential for the identity of the pluripotential founder cell population in the mammalian embryo. Oct4-deficient embryos develop to the blastocyst stage, but the inner cell mass cells are not pluripotent. Instead, they are restricted to differentiation along the extraembryonic trophoblast lineage. Furthermore, in the absence of a true inner cell mass, trophoblast proliferation is not maintained in Oct4-/- embryos. Expansion of trophoblast precursors is restored, however, by an Oct4 target gene product, fibroblast growth factor-4. Therefore, Oct4 also determines paracrine growth factor signaling from stem cells to the trophectoderm.

Induction of Nuclear Factor-κB During Primary B Cell Differentiation

We have investigated activation of nuclear factor-κB (NF-κB) in the process of primary B cell differentiation in vitro. In this system, NF-κB is strongly induced when B cells develop from the pre-B cell to the immature B cell stage. Unlike the typical NF-κB activation in response to exogenous stimuli, induction proceeds with a slow time course. NF-κB induction is only observed in B cells that undergo differentiation, not in Rag2-deficient cells. Nuclear DNA binding complexes predominantly comprise p50/RelA heterodimers and, to a lesser extent, c-Rel-containing dimers. The increase in NF-κB binding activity is accompanied by a slow and steady decrease in IκBβ protein levels. Interestingly, absolute RelA protein levels remain unaffected, whereas RelB and c-Rel synthesis is induced. The reason for preferential nuclear translocation of RelA complexes appears to be selective inhibition by the IκBβ protein. IκBβ can efficiently inhibit p50/RelA complexes, but has a much reduced ability to interfere with p50/c-Rel DNA binding both in vitro and in vivo. Interestingly, p50/RelB complexes are not at all targeted by IκBβ, and coimmunoprecipitation experiments show no evidence for an association of IκBβ and RelB in vivo. Consistent with these observations, IκBβ cotransfection can inhibit p50/RelA-mediated trans-activation, but barely affects p50/RelB mediated trans-activation.

Identification of target genes of the lymphoid-specific transcription factor Oct2

The Oct2 transcription factor is expressed predominantly in B lymphocytes and plays an essential role during the terminal phase of B cell differentiation. The regulatory regions of several genes specifically expressed in B cells contain functional binding sites for Oct2. Nevertheless, none of the genes originally thought to be regulated by Oct2 were affected in their expression in Oct2-deficient B cells. In an attempt to find such elusive Oct2 target genes and to understand the molecular function of Oct2 in B cell development, we isolated cDNAs for Oct2 target genes. So far, we have identified five potential targets for Oct2: the membrane glycoprotein CD36, the cysteine-rich secreted protein 3 (CRISP-3), a mouse homolog of the human monocyte/neutrophil elastase inhibitor (mEI) and two unknown cDNA sequences Nov1 and Nov2. These target genes show quite distinct expression patterns demonstrating that transcription factors in addition to Oct2 are involved in their regulation. Whereas CD36 and mEI were expressed in all hematopoetic cell lines containing Oct2,. CRISP-3 is pre-B cell-specific, Nov1 is plasma B cell-specific and Nov2 is B cell-specifically expressed.

A three-protein-DNA complex on a B cell-specific domain of the immunoglobulin mu heavy chain gene enhancer

The lymphoid-specific immunoglobulin mu heavy chain gene intron enhancer (muE) contains multiple binding sites for trans-acting nuclear factors. We have used a combination of in vitro and in vivo assays to reconstruct protein-DNA interactions on a minimal B cell-specific mu enhancer that contains three motifs, muA, muB, and muE3. Using ETS-domain proteins that transactivate the minimal enhancer in non-lymphoid cells, we show that (i) PU.1 binds coordinately to both muA and muB sites in vitro and (ii) in the presence of Ets-1, this factor binds to the muA site and PU.1 to the muB site. Two factors, TFE3 and USF, bind to the muE3 element. When the ETS proteins are present together with muE3 binding proteins, a three-protein-DNA complex is generated. Furthermore, we provide evidence for protein-protein interactions between Ets-1 and PU.1 proteins that bind to muA and muB sites, and between Ets-1 and TFE3 bound to the muA and mu3 sites. We propose that this domain of the mu enhancer is assembled into a nucleoprotein complex that contains two tissue-restricted ETS domain proteins that recognize DNA from the same side of the helix and one ubiquitously expressed bHLH-leucine zipper protein that binds between them, recognizing its site from a different side of the helix.

Binding of transcription factors to widely-separated cis-regulatory elements of the murine FGF-4 gene

Embryonal carcinoma (EC) cells and their embryo-derived counterparts, embryonic stem (ES) cells, have been used extensively to study the transcriptional regulation of the fibroblast growth factor-4 (FGF-4) gene. The FGF-4 gene is expressed in EC cells and ES cells, but it is repressed in their retinoic acid (RA)-induced differentiated counterparts. Previous studies have shown that the transcription of the FGF-4 gene is controlled by cis-regulatory elements located in the 5' flanking region of the gene, and by a powerful enhancer located approximately 3 kb downstream from the transcription start site. In the current study, gel mobility shift analysis was used to examine the binding of nuclear proteins to cis-regulatory elements involved in the transcription of the FGF-4 gene. We demonstrate that the transcription factors Sp1 and Sp3 in nuclear extracts prepared from EC cells bind to three Sp1 motifs, one located in the downstream enhancer, and two located in the 5' flanking region of the gene. We also show that Sp1 and Sp3 bind to each of the Sp1 motifs when nuclear extracts prepared from EC-derived differentiated cells are used. In contrast, differentiation of EC cells and ES cells drastically reduces the ability of nuclear factors to bind to an octamer motif and an adjacent High Mobility Group (HMG) motif, which have been shown previously to play essential roles in the functioning of the FGF-4 enhancer. Together, these findings provide a mechanistic explanation of how the distant FGF-4 enhancer promotes transcription of this gene in EC cells and ES cells, and how differentiation of these cells represses transcription of the FGF-4 gene.

The B cell coactivator Bob1 shows DNA sequence-dependent complex formation with Oct-1/Oct-2 factors, leading to differential promoter activation

We have shown previously that both octamer binding transcription factors, namely the ubiquitous Oct-1 and the B cell-specific Oct-2A protein, can be enhanced in transcriptional activity by their association with the B cell-specific coactivator protein Bob1, also called OBF-1 or OCA-B. Here we study the structural requirements for ternary complex formation of DNA-Oct-Bob1 and coactivation function of Bob1. In analogy to DNA-bound transcription factors, Bob1 has a modular structure that includes an interaction domain (amino acids 1-65) and a C-terminal domain (amino acids 65-256), both important for transcriptional activation. A mutational analysis has resolved a region of seven amino acids (amino acids 26-32) in the N-terminus of Bob1 that are important for contacting the DNA binding POU domain of Oct-1 or Oct-2. In contrast to the viral coactivator VP16 (vmw65), which interacts with Oct-1 via the POU homeosubdomain, Bob1 association with Oct factors requires residues located in the POU-specific subdomain. Because the same residues are also involved in DNA recognition, we surmised that this association would affect the DNA binding specificity of the Oct-Bob1 complex compared with free Oct factors. While Oct-1 or Oct-2 bind to a large variety of octamer sequences, Bob1 ternary complex formation is indeed highly selective and occurs only in a subset of these sequences, leading to the differential coactivation of octamer-containing promoters. The results uncover a new level in selectivity that furthers our understanding in the regulation of cell type-specific gene expression.

The hormone responsive region of mouse mammary tumor virus positions a nucleosome and precludes access of nuclear factor I to the promoter

The mouse mammary tumor virus (MMTV) promoter is transcriptionally silent prior to hormonal induction, partly because its organization into phased nucleosomes precludes access of transcription factors to their cognate sites. A T47D-derived cell line carrying a single integrated copy of the MMTV promoter exhibited a positioned nucleosome, which prevented binding of nuclear factor I (NFI). To study the molecular mechanisms controlling promoter accessibility we have made use of a strong chimeric transactivator, NFI-VP16, composed of NFI linked to the transactivation function of VP16. T47D cells transiently transfected with an MMTV-CAT reporter show little transcription even after cotransfection of an expression vector for NFI-VP16. However, a truncated MMTV promoter, lacking the hormone regulatory region (HRR) was transactivated by cotransfected NFI-VP16. The repressive effect of the HRR was not due to binding of a sequence-specific transcriptional repressor, and was evident with the DEAE-Dextran transfection procedure but not with the calcium phosphate technique. A similar behavior was observed in Saccharomyces cerevisiae carrying wild type or truncated MMTV-lacZ reporters and expressing NFI-VP16. Reconstitution experiments suggest that the promoter lacking the HHR generates less stable nucleosomes, a fraction of which contain a more accessible NFI site. Recombinant NFI binds to nucleosomes assembled on this truncated promoter but not to nucleosomes encompassing the HRR. These results are compatible with the notion that transiently transfected MMTV promoters behave like their stably integrated counterparts, in that the HRR drives positioning of a nucleosome and mediates transcriptional repression by preventing access of NFI to its cognate site.

Functional characterization of the murine homolog of the B cell-specific coactivator BOB.1/OBF.1

B cell-specific transcriptional promoter activity mediated by the octamer motif requires the Oct1 or Oct2 protein and additional B cell-restricted cofactors. One such cofactor, BOB.1/OBF.1, was recently isolated from human B cells. Here, we describe the isolation and detailed characterization of the murine homolog. Full-length cDNAs and genomic clones were isolated, and the gene structure was determined. Comparison of the deduced amino acids shows 88% sequence identity between mouse and human BOB.1/OBF.1. The NH2-terminal 126 amino acids of BOB.1/OBF.1 are both essential and sufficient for interaction with the POU domains of either Oct1 or Oct2. This protein-protein interaction does not require the simultaneous binding of Oct proteins to DNA, and high resolution footprinting of the Oct-DNA interaction reveals that binding of BOB.1/OBF.1 to Oct1 or Oct2 does not alter the interaction with DNA. BOB.1/OBF.1 can efficiently activate octamer-dependent promoters in fibroblasts; however, it fails to stimulate octamer-dependent enhancer activity. Fusion of subdomains of BOB.1/OBF.1 with the GAL4 DNA binding domain reveals that both NH2- and COOH-terminal domains of BOB.1/OBF.1 contribute to full transactivation function, the COOH-terminal domain is more efficient in this transactivation assay. Consistent with the failure of full-length BOB.1/OBF.1 to stimulate octamer-dependent enhancer elements in non B cells, the GAL4 fusions likewise only stimulate from a promoter-proximal position.

Transcriptional activation and repression, two properties of the lymphoid-specific transcription factor Oct-2a

The lymphoid-specific transcription factor Oct-2a contains two transcriptional activation domains which are located within the N-terminal and C-terminal regions. To study their differential activation properties, we linked the isolated effector domains to the GAL4 DNA-binding domain. We have shown that both activating regions of Oct-2a, isolated from their natural context, can activate transcription as promoter factors. In contrast to the C-terminus, activation by the N-terminal domain is dependent on a yet unidentified factor(s) binding to the simian virus 40 enhancer. The results obtained by duplication of activation domains or their mixed combination suggest that the domains are functionally independent. However, activation from a remote position could only be achieved with the C-terminus of Oct-2a in B cells. In lymphoid cells, higher activation levels were observed, suggesting that distinct B-cell-specific cofactors in concert with the effector domains of Oct-2a might be involved in mediating transcription from proximal and remote positions. Furthermore, we identified a repression domain at the N-terminus of Oct-2a. When transferred to a potent activator, transcriptional stimulation was inhibited efficiently. These results underscore the modular structure of Oct-2a with separable domains for activation and repression and suggest that Oct-2a might have complex regulatory functions in vivo.

Molecular principles of Oct2-mediated gene activation in B cells

The octamer motif is a crucial regulatory element for immunoglobulin promoter and enhancer function. We have investigated the molecular mechanisms that underlie octamer-mediated gene activation in B cells. This B cell-specific transcriptional regulation is subject to a novel type of regulatory mechanism. We could demonstrate that octamer-dependent transcription is not only regulated by specific DNA-binding transcription factors, but in addition requires the activity of B cell-restricted cofactors. Both octamer-dependent promoter and enhancer activation depend on such a combination of transcription factor and cofactors. However, the exact requirements differ for these two situations. Promoter activity can be achieved with either one of two distinct transcription factors, Oct1 and/or Oct2, together with the cofactor OCA-B1. In contrast, only Oct2 in conjunction with an additional cofactor, OCA-B2, can confer enhancer activity.

Cloning, sequencing and expression of two isoforms of the murine oct-1 transcription factor

Oct-1 is a ubiquitously expressed regulatory gene of the POU domain family. The Oct-1 protein binds to the octamer motif present in the control regions of a variety of genes such as the immunoglobulins, histone H2B and snRNAs. To learn about Oct-1 and its possible role in B-cell maturation, we have used oct-2 cDNA to screen a murine pre-B cell, cDNA library. Two cDNA clones were identical in their POU-homeo box DNA binding domain, but differed in their 3'-region. Whereas one clone (oct-1a) was very similar to its human oct-1 homologue, the other (oct-1b), contained an additional 72 bp sequence (designated E1) at the serine threonine rich coding region (position 1485 of the human oct-1), and a deletion of another 72 bp sequence (designated E2) downstream (position 1920). These changes preserve the protein reading frame. DNA blot analysis indicates that murine oct-1 is a single copy gene and that the two oct-1 isoforms oct-1 is expressed as a large approximately 10 kb transcript in all the cell are generated by alternative RNA splicing. RNA blots showed that oct-1 is expressed as a large approximately 10 kb transcript in all the cell lines tested. PCR analysis of the E1 and E2 72 bp regions, indicated the presence of a third isoform containing both E1 and E2 (Oct-1c). Oct-1a and Oct-1b were present in all cell types examined, but the level of expression was lower in liver and spleen as compared to testis, thymus and kidney. The ratio of Oct-1b to Oct-1a ranged between 0.2 to 0.5, for all tissues examined except for testis which expressed higher amounts of oct-1b and/or oct-1c. Our findings thus show that the pattern of expression of the oct-1 gene is more complex than hitherto thought.

OBF-1, a novel B cell-specific coactivator that stimulates immunoglobulin promoter activity through association with octamer-binding proteins

Recent biochemical and genetic studies indicate that in addition to the octamer-binding proteins Oct-1 and Oct-2, other B cell components are required for lymphoid-restricted, octamer site-mediated immunoglobulin gene promoter activity. Using a genetic screen in yeast, we have isolated B cell-derived cDNAs encoding Oct-binding factor 1 (OBF-1), a novel protein that specifically associates with Oct-1 and Oct-2. Biochemical studies demonstrate that OBF-1 has no intrinsic DNA-binding activity and recognizes the POU domains of Oct-1 and Oct-2, but not those of Oct-4 and Oct-6. The OBF-1 mRNA is expressed in a highly cell-specific manner, being most abundant in B cells and essentially absent in most of the other cells or tissues tested. Furthermore, expression of OBF-1 in HeLa cells selectively stimulates the activity of a natural immunoglobulin promoter in an octamer site-dependent manner. Thus, OBF-1 has all the properties expected for a B cell-specific transcriptional coactivator protein.

Stimulation of kappa transcription by a decamer-dependent, synergistic mechanism

The intact SP6 kappa promoter stimulated transcription 30 times more efficiently than did a control promoter consisting of a TATA motif as the only promoter element. Mutation of the SP6 kappa promoter decamer in two positions reduced the transcriptional stimulation activity by over 90%. Promoters containing the SP6 kappa promoter octamer or a consensus octamer in front of a TATA box were ineffective immunoglobulin promoters and stimulated at the most 15% of maximal transcription. Identical results were obtained after transfection of untransformed mouse splenic B cells stimulated by lipopolysaccharide, that express high levels of Oct2A, or of S194 cells that express negligible levels of Oct2A. Selective mutations in the penta-decamer (pd), kappa Y or early B cell factor (EBF) elements of the promoter reduced transcriptional stimulation by 20-30% in untransformed B cells. In S194 plasmacytoma cells the EBF mutation was functionally silent while the kappa Y and pd mutations reduced transcriptional activation by 60-70% in this cell line. A mutation in a TATA-proximal E-box motif did not alter the functional activity of the promoter in either cell population. It can be concluded that kappa promoter function is highly dependent on complex interactions between individual promoter elements and that the decamer motif is pivotal for these interactions. The relative functional activity of a given promoter varied according to the target cell population used for the functional assay.

The inhibitory domain in the Oct-2 transcription factor represses gene activity in a cell type-specific and promoter-independent manner

The Oct-2 transcription factor contains an N-terminal inhibitory domain which can act to inhibit promoter activity when linked to either its corresponding DNA-binding POU domain or the heterologous DNA binding domain of the yeast transcription factor GAL4. This inhibitory effect is independent of the number of DNA binding sites or their context in the target promoter. In contrast the effect is cell type-specific and can be relieved by over-expression of the isolated inhibitory domain in the absence of a DNA binding domain. These results suggest that the inhibitory domain acts by decreasing the activity of the basal transcriptional complex but that it operates indirectly by recruiting a second cell type-specific factor to the promoter which then interacts with the basal complex decreasing its activity.

Constitutively expressed Oct-2 prevents immunoglobulin gene silencing in myeloma x T cell hybrids

Recent experiments involving disruption of the Oct-2 gene have shown that this largely B cell-restricted transcription factor is not required in the early stages of B cell development. However, B cells that lack Oct-2 may be blocked from differentiation past the surface immunoglobulin-positive stage. To identify a possible function for Oct-2 in the late stage immunoglobulin-secreting cell, we have used the method of somatic cell fusion. When the immunoglobulin-producing myeloma MPC11 is fused to a T lymphoma, Oct-2 production ceases, as does the expression of immunoglobulin, J chain, and several other B cell-specific gene products. In the present study, we show that by preventing the loss of Oct-2 in the hybrid cells, we can preserve expression of all other tested B cell-specific genes. These results establish a central role for Oct-2 in maintaining the genetic program of the immunoglobulin-secreting plasmacyte.

Oct-2 is required early in T cell-independent B cell activation for G1 progression and for proliferation

Oct-2, a POU homeodomain protein expressed primarily in B cells, is a powerful transcriptional activator that binds to DNA at sites appropriately placed for major effects on immunoglobulin gene expression. Our examination of B cell development and function in Oct-2 null mice did not support an essential role for Oct-2 early in B cell development. Rather, Oct-2 was required later, when B cells were induced to differentiate to antibody-secreting cells. We show here that Oct-2 is not required for normal immunoglobulin production by mature B lymphocytes. Instead, it is essential for a normal proliferative response to polyclonal mitogens. Responses to signals from activated T cells are unaffected. The requirement for Oct-2 maps to an early activation step in G1, during which B cells make the commitment to progress through the cell cycle and to divide.

Pit-1 and Pit-2 role in growth hormone gene regulation

The role of the POU domain transcription factor Pit-1 (GHF-1) in differentiation and proliferation of the somatotrophic lineage is well known. In our study of differential splicing of the PIT1 gene we found a new protein, Pit-2, in which 26 amino acids are inserted into the transactivation domain. Pit-2 can activate the growth hormone promoter but has lost the ability to activate the prolactin promoter or PIT1 promoter itself.