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

Long Non-Coding RNA ARAP1-AS1 Facilitates the Progression of Cervical Cancer by Regulating miR-149-3p and POU2F2

BACKGROUND

Emerging research has demonstrated that long non-coding RNAs (lncRNAs) attach great importance to the progression of cervical cancer (CC). LncRNA ARAP1-AS1 was involved in the development of several cancers; however, its role in CC is far from being elucidated.

METHODS

Real-time PCR (RT-PCR) was employed to detect ARAP1-AS1 and miR-149-3p expression in CC samples. CC cell lines (HeLa and C33A cells) were regarded as the cell models. The biological effect of ARAP1-AS1 on cancer cells was measured using CCK-8 assay, colony formation assay, flow cytometry, Transwell assay and wound healing assay in vitro, and subcutaneous xenotransplanted tumor model and tail vein injection model in vivo. Furthermore, interactions between ARAP1-AS1 and miR-149-3p, miR-149-3p and POU class 2 homeobox 2 (POU2F2) were determined by bioinformatics analysis, qRT-PCR, Western blot, luciferase reporter and RNA immunoprecipitation assay, respectively.

RESULTS

The expression of ARAP1-AS1 was enhanced in CC samples, while miR-149-3p was markedly suppressed. Additionally, ARAP1-AS1 overexpression enhanced the viability, migration, and invasion of CC cells. ARAP1-AS1 downregulated miR-149-3p via sponging it. ARAP1-AS1 and miR-149-3p exhibited a negative correlation in CC samples. On the other hand, ARAP1-AS1 enhanced the expression of POU2F2, which was validated as a target gene of miR-149-3p.

CONCLUSION

ARAP1-AS1 was abnormally upregulated in CC tissues and indirectly modulated the POU2F2 expression via reducing miR-149-3p expression. Our study identified a novel axis, ARAP1-AS1/miR-149-3p/POU2F2, in CC tumorigenesis.

POU2F2-oriented network promotes human gastric cancer metastasis

BACKGROUND AND AIMS

Aberrant upregulation of POU2F2 expression has been discovered in metastatic gastric cancer (GC). However, the mechanisms underlying the aberrant upregulation and the potential functions of POU2F2 remain uncertain.

DESIGN

The role and mechanism of POU2F2 in GC metastasis were investigated in gastric epithelial cells, GC cell lines and an experimental metastasis animal model by gain of function and loss of function. Upstream and downstream targets of POU2F2 were selected by bioinformatics and identified by luciferase reporter assay, electrophoretic mobility shift assay and chromatin immunoprecipitation PCR. The influence of miR-218 on its putative target genes (POU2F2, ROBO1 and IKK-β) and GC metastasis was further explored via in vitro and in vivo approaches.

RESULTS

Increased POU2F2 expression was detected in metastatic GC cell lines and patient samples. POU2F2 was induced by the activation of nuclear factor (NF)-κB and, in turn, regulated ROBO1 transcription, thus functionally contributing to GC metastasis. Finally, miR-218 was found to suppress GC metastasis by simultaneously mediating multiple molecules in the POU2F2-oriented network.

CONCLUSIONS

This study demonstrated that NF-κB and the SLIT2/ROBO1 interaction network with POU2F2 as the central part may exert critical effects on tumour metastasis. Blocking the activation of the POU2F2-oriented metastasis network using miR-218 precursors exemplified a promising approach that sheds light on new strategies for GC treatment.

A tumorigenic factor interactome connected through tumor suppressor microRNA-198 in human pancreatic cancer

PURPOSE

The majority of pancreatic cancers overexpress mesothelin (MSLN), which contributes to enhanced proliferation, invasion, and migration. However, the MSLN regulatory network is still unclear. Here, we investigated the regulation of a panel of tumorigenic factors and explored the potential of MSLN-regulated miR-198 treatment in vivo.

EXPERIMENTAL DESIGN

The expression and functional regulation of the tumorigenic factors MSLN, NF-κB, and the homeobox transcription factors (TF) POU2F2 (OCT-2), Pre-B-cell leukemia homeobox factor 1 (PBX-1), valosin-containing protein (VCP), and miR-198 were studied in pancreatic cancer cell lines, patient tumor samples, and xenograft pancreatic cancer mouse models.

RESULTS

We found that miR-198 is downregulated in pancreatic cancer and is involved in an intricate reciprocal regulatory loop with MSLN, which represses miR-198 through NF-κB-mediated OCT-2 induction. Furthermore, miR-198 repression leads to overexpression of PBX-1 and VCP. The dysregulated PBX-1/VCP axis leads to increased tumorigenicity. Reconstitution of miR-198 in pancreatic cancer cells results in reduced tumor growth, metastasis, and increased survival through direct targeting MSLN, PBX-1, and VCP. Most interestingly, reduced levels of miR-198 in human tissue samples are associated with upregulation of these tumorigenic factors (MSLN, OCT-2, PBX-1, VCP) and predict poor survival. Reduced miR-198 expression links this tumor network signature and prognosticates poor patient outcome. High miR-198 disrupts the network and predicts better prognosis and increased survival.

CONCLUSIONS

miR-198 acts as a central tumor suppressor and modulates the molecular makeup of a critical interactome in pancreatic cancer, indicating a potential prognostic marker signature and the therapeutic potential of attacking this tumorigenic network through a central vantage point.

Expression of the Oct-2 transcription factor in mouse mammary gland and cloning and characterization of a novel Oct-2 isoform

Oct-2 is a member of the POU family of transcription factors, which specifically bind to the octamer DNA motif ATGCAAAT and its closely related sequences. Unlike its ubiquitous counterpart Oct-1, Oct-2 is thought to be expressed only in B lymphocytes and neuronal cells and is mainly involved in immunoglobulin gene expression. We show here that Oct-2 is also expressed in the epithelial cells of mouse mammary gland, and that this expression is developmentally regulated. Rapid amplification of cDNA ends and subsequent cDNA cloning indicate that the mammary gland expresses multiple Oct-2 isoforms, including a novel isoform, named Oct-2.7. Compared with Oct-2 (isoform 2.1), the deduced Oct-2.7 sequence has an additional 22 amino acids close to the N-terminus and a novel 76-amino-acid C-terminus resulting from alternative splicing, with retention of the last intron that is spliced out in all other isoforms. Although Oct-2.7 has intact POU-specific and POU-homeo domains, it is unable to bind to the octamer motif, unlike all other known isoforms. Like Oct-1, both Oct-2.1 and Oct-2.7 can activate basal beta-casein gene promoter activity. However, activation by Oct-2.7, which is independent of DNA binding, is significantly lower than that by Oct-2.1. Moreover, deletion of the first 114 amino acids at the N-terminus of Oct-2.1 has no effect on activation; this does not support previous reports of the presence of an inhibitory domain in this region.

DNA-dependent conversion of Oct-1 and Oct-2 into transcriptional repressors by Groucho/TLE

POU domain proteins contain a bipartite DNA-binding element that can confer allosteric control of coactivator recruitment. Dimerization of Oct-1 and Oct-2 on palindromic response elements results in the conformational dependent inclusion or exclusion of the transcriptional coactivator OBF-1. In this paper, we demonstrate that Oct-1 and Oct-2 can function as transcriptional repressors by recruiting and physically interacting with members of the Grg/TLE family of corepressors. In accordance with a model of DNA induced cofactor assembly, and analogous to the recruitment of the OBF-1 coactivator, the different Grg/TLE members can discriminate between both Oct-1 and Oct-2, and the monomeric or dimeric nature of the POU/DNA complex.

All known in vivo functions of the Oct-2 transcription factor require the C-terminal protein domain

Oct-2, a transcription factor expressed in the B lymphocyte lineage and in the developing CNS, functions through of a number of discrete protein domains. These include a DNA-binding POU homeodomain flanked by two transcriptional activation domains. In vitro studies have shown that the C-terminal activation domain, a serine-, threonine- and proline-rich sequence, possesses unique qualities, including the ability to activate transcription from a distance in a B cell-specific manner. In this study, we describe mice in which the endogenous oct-2 gene has been modified through gene targeting to create a mutated allele, oct-2DeltaC, which encodes Oct-2 protein isoforms that lack all sequence C-terminal to the DNA-binding domain. Surprisingly, despite the retention of the DNA-binding domain and the glutamine-rich N-terminal activation domain, the truncated protein(s) encoded by the oct-2DeltaC allele are unable to rescue any of the previously described defects exhibited by oct-2 null mice. Homozygous oct-2DeltaC/DeltaC mice die shortly after birth, and B cell maturation, B-1 cell self renewal, serum Ig levels, and B lymphocyte responses to in vitro stimulation are all reduced or absent, to a degree equivalent to that seen in oct-2 null mice. We conclude that the C-terminal activation domain of Oct-2 is required to mediate the unique and indispensable functions of the Oct-2 transcription factor in vivo.

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.

Analysis of octamer-binding transcription factors Oct2 and Oct1 and their coactivator BOB.1/OBF.1 in lymphomas

Oct1 and Oct2 are transcription factors of the POU homeo-domain family that bind to the Ig gene octamer sites, regulating B-cell-specific genes. The function of these transcription factors is dependent on the activity of B-cell-restricted coactivators such as BOB.1/OBF.1. Independent studies of the expression of these proteins in non-Hodgkin's lymphoma have been restricted to single markers, and most lack data concerning immunohistochemical expression. Thus, we have investigated the expression of Oct1, Oct2, and BOB.1/OBF.1 in human reactive lymphoid tissue and in a series of 140 Hodgkin and non-Hodgkin's lymphomas. None of these proteins was found to be restricted to B cells, although only B cells expressed high levels of all three markers. Additionally, germinal center B cells showed stronger Oct2 and BOB.1/OBF.1 staining. Consequently, most B-cell lymphomas showed reactivity for all three antibodies. Oct2 expression was significantly higher in germinal center-derived lymphomas, although other B-cell lymphomas also displayed a high level of Oct2 expression. Although T-cell lymphomas and Hodgkin's lymphomas expressed some of these proteins, they commonly exhibited less reactivity than B-cell lymphomas. Despite not being entirely cell-specific, the strong nuclear expression of Oct2 and BOB.1/OBF.1 by germinal center- derived lymphomas makes these antibodies a potentially useful tool in lymphoma diagnosis.

Biological activity of mammalian transcriptional repressors

Research on the regulation of transcription in mammals has focused in recent years mainly on the mechanism of transcriptional activation. However, transcriptional repression mediated by repressor proteins is a common regulatory mechanism in mammals and might play an important role in many biological processes. To understand the molecular mechanism of transcriptional repression, the activity of eight mammalian repressors or repressor domains was investigated using a set of model promoters in combination with two different transcriptional detection methods. The repressors studied were: REST, the thyroid hormone receptors alpha and beta, the zinc finger protein NK10 containing a 'krüppel-associated box' (KRAB), repressor domains derived from the proteins Egr-1, Oct2A and Dr1 and the repressor/activator protein YY1. Here we show that the repressor domains of REST, Egr-1, the thyroid hormone receptors alpha< and beta and NK10 were transferable to a heterologous DNA-binding domain and repressed transcription from proximal and distal positions. Moreover, these repressor domains also blocked the activity of a strong viral enhancer in a 'remote position'. Thus, these domains are 'general' transcriptional repressor domains. The 'krüppel-associated box' was the most powerful repressor domain tested. In contrast, the repressor domains derived from Oct2A and Dr1 were inactive when fused to a heterologous DNA-binding domain. The repressor domain of YY1 exhibited transcriptional repression activity only in one of the transcriptional assay systems. The recruitment of histone deacetylases to the proximity of the basal transcriptional apparatus was recently discussed as a mechanism for some mammalian transcriptional repressor proteins. Here we show here that histone deacetylase 2, targeted to the reporter gene via DNA-protein interaction, functions as a transcriptional repressor protein regardless of the location of its binding site within the transcription unit.

POU domain factors in the neuroendocrine system: lessons from developmental biology provide insights into human disease

POU domain factors are transcriptional regulators characterized by a highly conserved DNA-binding domain referred to as the POU domain. The structure of the POU domain has been solved, facilitating the understanding of how these proteins bind to DNA and regulate transcription via complex protein-protein interactions. Several members of the POU domain family have been implicated in the control of development and function of the neuroendocrine system. Such roles have been most clearly established for Pit-1, which is required for formation of somatotropes, lactotropes, and thyrotropes in the anterior pituitary gland, and for Brn-2, which is critical for formation of magnocellular and parvocellular neurons in the paraventricular and supraoptic nuclei of the hypothalamus. While genetic evidence is lacking, molecular biology experiments have implicated several other POU factors in the regulation of gene expression in the hypothalamus and pituitary gland. Pit-1 mutations in humans cause combined pituitary hormone deficiency similar to that found in mice deleted for the Pit-1 gene, providing a striking example of how basic developmental biology studies have provided important insights into human disease.

The opposing transcriptional activities of the two isoforms of the human progesterone receptor are due to differential cofactor binding

The human progesterone receptor (PR) exists as two functionally distinct isoforms, hPRA and hPRB. hPRB functions as a transcriptional activator in most cell and promoter contexts, while hPRA is transcriptionally inactive and functions as a strong ligand-dependent transdominant repressor of steroid hormone receptor transcriptional activity. Although the precise mechanism of hPRA-mediated transrepression is not fully understood, an inhibitory domain (ID) within human PR, which is necessary for transrepression by hPRA, has been identified. Interestingly, although ID is present within both hPR isoforms, it is functionally active only in the context of hPRA, suggesting that the two receptors adopt distinct conformations within the cell which allow hPRA to interact with a set of cofactors that are different from those recognized by hPRB. In support of this hypothesis, we identified, using phage display technology, hPRA-selective peptides which differentially modulate hPRA and hPRB transcriptional activity. Furthermore, using a combination of in vitro and in vivo methodologies, we demonstrate that the two receptors exhibit different cofactor interactions. Specifically, it was determined that hPRA has a higher affinity for the corepressor SMRT than hPRB and that this interaction is facilitated by ID. Interestingly, inhibition of SMRT activity, by either a dominant negative mutant (C'SMRT) or histone deacetylase inhibitors, reverses hPRA-mediated transrepression but does not convert hPRA to a transcriptional activator. Together, these data indicate that the ability of hPRA to transrepress steroid hormone receptor transcriptional activity and its inability to activate progesterone-responsive promoters occur by distinct mechanisms. To this effect, we observed that hPRA, unlike hPRB, was unable to efficiently recruit the transcriptional coactivators GRIP1 and SRC-1 upon agonist binding. Thus, although both receptors contain sequences within their ligand-binding domains known to be required for coactivator binding, the ability of PR to interact with cofactors in a productive manner is regulated by sequences contained within the amino terminus of the receptors. We propose, therefore, that hPRA is transcriptionally inactive due to its inability to efficiently recruit coactivators. Furthermore, our experiments indicate that hPRA interacts efficiently with the corepressor SMRT and that this activity permits it to function as a transdominant repressor.

Transcriptional regulation of the murine 3' IgH enhancer by OCT-2

Targeted disruption of either of the B cell-specific transcription factors Oct-2 or OCA-B/BOB-1/OBF-1 dramatically affects B cell terminal differentiation. The 3' enhancer of immunoglobulin heavy chain (IgH) locus is important for transcription of the locus in terminal plasma cells. Allele-specific suppression of mutant Oct-2 binding sites in this enhancer by a variant Oct-2 protein revealed that in a mature B cell line this enhancer was specifically dependent upon Oct-2, as contrasted to the closely related Oct-1 transcription factor. Phosphorylation of the Oct-2 protein was important for this activation and was synergistic for coactivation by the OCA-B factor. These results indicate that Oct-2 and OCA-B interact with the 3' enhancer in regulation of the IgH locus during B cell activation.

POU family transcription factors in the nervous system

The POU (Pit-Oct-Unc) family of transcription factors was originally defined on the basis of a common DNA binding domain in the mammalian factors Pit-1, Oct-1, and Oct-2 as well as the nematode protein Unc-86. Subsequently, a number of other POU family factors have been identified in both vertebrates and invertebrates. Many of these original and subsequently isolated members of the family have been shown to play critical roles in the development and functioning of the nervous system. To exemplify this, studies are described involving the functional characterisation of the Oct-2 factor, one of the original POU factors, and of the Brn-3 factors, which were isolated subsequently and are the mammalian factors most closely related to Unc-86.

Adjacent proline residues in the inhibitory domain of the Oct-2 transcription factor play distinct functional roles

A 40 amino acid region of Oct-2 from amino acids 142 to 181 functions as an active repressor domain capable of inhibiting both basal activity and activation of promoters containing a TATA box, but not of those that contain an initiator element. Based on our observation that the equivalent region of the closely related Oct-1 factor does not act as an inhibitory domain, we have mutated specific residues in the Oct-2 domain in an attempt to probe their importance in repressor domain function. Although mutations of several residues have no or minimal effect, mutation of proline 175 to arginine abolishes the ability to inhibit both basal and activated transcription. In contrast, mutation of proline 174 to arginine confers upon the domain the ability to repress activation of an initiator-containing promoter by acidic activation domains, and also suppresses the effect of the proline 175 mutation. Hence, adjacent proline residues play key roles in the functioning of the inhibitory domain and in limiting its specificity to TATA-box-containing promoters.

Characterization of Oct2 from the Channel Catfish: Functional Preference for a Variant Octamer Motif

The Ig heavy chain enhancer of the channel catfish (Ictalurus punctatus) has an unusual position and structure, being found in the 3′ region of the μ gene and containing eight functional octamer motifs of consensus (ATGCAAAT) and variant sequences. The presence of multiple octamer motifs suggests that an Oct2 homologue may play an important role in driving expression of the Ig heavy chain locus in a teleost fish. To test this hypothesis, two catfish Oct2 cDNAs (α and β) were cloned by screening a catfish B cell cDNA library. Catfish Oct2 α and β isoforms are derived by alternative RNA splicing; as determined by Southern analysis, Oct2 is a single copy gene. In comparisons with mammalian Oct2, the catfish Oct2 isoforms show high sequence conservation in their N-terminal regions and POU domains, but extensive divergence in their C-terminal regions. Catfish Oct2 α and β are tissue restricted, bind both consensus and variant octamer motifs, and activate transcription in both catfish and murine cells. In contrast, mouse Oct2 activated transcription in mouse but not catfish cells. Catfish Oct2 β is a more potent transcriptional activator than Oct2 α. In transient expression assays, catfish Oct2 β showed a marked preference for the octamer variant, ATGtAAAT, which occurs twice in the catfish enhancer. Mouse Oct2 also showed increased activity with the variant octamer when tested in mouse B cells. Gel-shift analysis competition assays indicated that catfish Oct2 binds the consensus octamer motif with an apparently higher affinity than it does the variant motif.

An N-terminal inhibitory function, IF, suppresses transcription by the A-isoform but not the B-isoform of human progesterone receptors

The B-isoform of human progesterone receptors (PR) contains three activation functions (AF3, AF1, and AF2), two of which (AF1 and AF2) are shared with the A-isoform. AF3 is in the B-upstream segment (BUS), the far N-terminal 164 amino acids of B-receptors; AF1 is in the 392-amino acid N-terminal region common to both receptors; and AF2 is in the C-terminal hormone binding domain. B-receptors are usually stronger transactivators than A-receptors due to transcriptional synergism between AF3 and one of the two downstream AFs. We now show that the N terminus of PR common to both isoforms contains an inhibitory function (IF) located in a 292-amino acid segment lying upstream of AF1. IF represses the activity of A-receptors but is not inhibitory in the context of B-receptors due to constraints imparted by BUS. As a result, IF inhibits AF1 or AF2 but not AF3, regardless of the position of IF relative to BUS. IF is functionally independent and strongly represses transcription when it is fused upstream of estrogen receptors. These data demonstrate the existence of a novel, transferable inhibitory function, mapping to the PR N terminus, which begins to assign specific roles to this large undefined region.

The different inhibitory domains of the Oct-2 transcription factor have distinct functional activities

The Oct-2 POU family transcription factor contains three distinct regions whose deletion reduces its ability to inhibit transcription via its octamer binding site. Here we show that only one of these inhibitory domains is capable of also inhibiting the activity of activating molecules bound at adjacent sites upstream of a TATA box-containing promoter whereas the other two regions are inactive in this assay. None of the three regions is able to achieve this effect when located upstream of the same promoter containing an initiator motif. The mechanisms of action of these domains and their role in the functioning of the Oct-2 factor are discussed.

Activation of enhancer elements by the homeobox gene Cdx2 is cell line specific

Cdx2 is a caudal-related homeodomain transcription factor that is expressed in complex patterns during mouse development and at high levels in the intestinal epithelium of adult mice. Cdx2 activates transcription of intestinal gene promoters containing specific binding sites. Moreover, Cdx2 has been shown to induce intestinal differentiation in cell lines. In this study, we show that Cdx2 is able to bind to two well defined enhancer elements in the HoxC8 gene. We then demonstrate that Cdx2 is able to activate transcription of heterologous promoters when its DNA binding element is placed in an enhancer context. Furthermore, the ability to activate enhancer elements is cell-line dependent. When the Cdx2 activation domain was linked to the Gal4 DNA binding domain, the chimeric protein was able to activate Gal4 enhancer constructs in an intestinal cell line, but was unable to activate transcription in NIH3T3 cells. These data suggest that there are cell-specific factors that allow the Cdx2 activation domain to function in the activation of enhancer elements. We hypothesize that either a co-activator protein or differential phosphorylation of the activation domain may be the mechanism for intestinal cell line-specific function of Cdx2 and possibly in other tissues in early development.

The carboxy-terminal transactivation domain of Oct-4 acquires cell specificity through the POU domain

The POU transcription factor Oct-4 is expressed in totipotent and pluripotent cells of the early mouse embryo and the germ cell lineage. Transactivation capacities of regions flanking the DNA binding domain of Oct-4 were analyzed in undifferentiated and differentiated cell lines. The amino- and carboxy-terminal regions (N domain and C domain) fused to the Gal4 DNA binding domain both functioned as transactivation domains in all cell lines tested. However, the C domain failed to activate transcription in some cell lines in the context of the native protein. The underlying regulatory mechanism appears to involve the POU domain of Oct-4 and can discriminate between different POU domains, since constructs in which the C domain was instead fused to the POU domain of Pit-1 were again equally active in all cell lines. These results indicate that the C domain is subject to cell-type-specific regulation mediated by the Oct-4 POU domain. Phosphopeptide analysis revealed that the cell-type-specific difference of C-domain activity correlates with a difference in Oct-4 phosphorylation status. Since Oct-4 is expressed in a variety of distinct cell types during murine embryogenesis, these results suggest an additional regulatory mechanism for determining Oct-4 function in rapidly changing cell types during development.

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.

Mapping of the transcriptional repression domain of the lymphoid-specific transcription factor oct-2A

The lymphoid-specific transcription factor Oct-2a is implicated in B cell-specific transcriptional activity via the octamer motif. Structure/function analysis of various Oct-2a effector regions in the context of the GAL4 DNA-binding domain revealed that Oct-2a contains two functionally different activation domains at the N and the C termini. The transcriptional activity of both domains is strongly potentiated by interactions with distinct B cell-specific coactivators. Recently, we have identified a repression domain located within the N terminus of Oct-2a (amino acids 2-99). When this domain was transferred to a potent activator, transcription was strongly inhibited. In this study we present a deletion analysis of the N-terminal region of Oct-2a to determine the minimal repression domain. We identified a stretch of 23 amino acids, rich in serine and threonine residues, which was responsible for most of the repression activity. We show that repression is strongly dependent on the type of enhancer present in the reporter plasmid as well as on the cell line tested. The possibility that Oct-2a can act as an activator and/or a repressor may have important consequences for the function of Oct-2a in B cell differentiation and other developmental processes.

Gene structure and characterization of the murine homologue of the B cell-specific transcriptional coactivator OBF-1

The B cell-specific activity of immunoglobulin (Ig) gene promoters is to a large extent mediated by the conserved octamer motif ATTTGCAT. This requires the DNA binding octamer factors Oct-1 and/or Oct-2, as well as an additional B cell-restricted non-DNA binding cofactor. We recently cloned such a coactivator specific for Oct-1 or Oct-2 from human B cells and called it OBF-1. Here we report the isolation and characterization of the murine homologue. Full-length cDNA clones as well as genomic clones were isolated and the gene structure was determined. The deduced protein sequence shows that the mouse protein has an identical length, is likewise proline rich and shows 89% overall identity to the human protein. The OBF-1 gene is expressed in a very highly B cell-specific manner and is transcribed in cells representative of all stages of B cell differentiation, including the earliest ones. We show that OBF-1 interacts in the absence of DNA with the POU domain of Oct-1 or Oct-2 and also with the general transcription factors TBP and TFIIB. Furthermore, we demonstrate that although OBF-1 efficiently activates promoter octamer sites, it does not activate enhancer octamer sites.