The ubiquitous octamer-binding protein(s) is sufficient for transcription of immunoglobulin genes

Bright/ARID3A contributes to chromatin accessibility of the immunoglobulin heavy chain enhancer

Bright/ARID3A is a nuclear matrix-associated transcription factor that stimulates immunoglobulin heavy chain (IgH) expression and Cyclin E1/E2F-dependent cell cycle progression. Bright positively activates IgH transcriptional initiation by binding to ATC-rich P sites within nuclear matrix attachment regions (MARs) flanking the IgH intronic enhancer (Eμ). Over-expression of Bright in cultured B cells was shown to correlate with DNase hypersensitivity of Eμ. We report here further efforts to analyze Bright-mediated Eμ enhancer activation within the physiological constraints of chromatin. A system was established in which VH promoter-driven in vitro transcription on chromatin- reconstituted templates was responsive to Eμ. Bright assisted in blocking the general repression caused by nucleosome assembly but was incapable of stimulating transcription from prebound nucleosome arrays. In vitro transcriptional derepression by Bright was enhanced on templates in which Eμ is flanked by MARs and was inhibited by competition with high affinity Bright binding (P2) sites. DNase hypersensitivity of chromatin-reconstituted Eμ was increased when prepackaged with B cell nuclear extract supplemented with Bright. These results identify Bright as a contributor to accessibility of the IgH enhancer.

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.

Nitric oxide inhibits Oct-1 DNA binding activity in cultured vascular smooth muscle cells

Since Oct-1 is a ubiquitous DNA binding protein shown to play an important role in regulating cell proliferation and possess structural characteristics consistent with a nitric oxide (NO) target, we studied NO regulation of the DNA binding activity of Oct-1 in the A7R5 vascular smooth muscle cell (VSMC) line. Two NO donors, sodium nitroprusside (SNP) and S-nitroso-N-acetyl-penicillamine (SNAP) were directly added to the nuclear extract-oligonucleotide reaction mixture, respectively and the effect on Oct-1 DNA binding activity was evaluated by gel shift assay. Both NO donors (0.01-1 mM) inhibited the DNA binding activity of Oct-1. This inhibitory effect was not attenuated by dithiothreitol (DTT) (1 mM) while in contrast, DTT did antagonize the effect of diamide on Oct-1 DNA binding activity. The NO effect on Oct-1 has some specificity; as the NO donors had no effect on myc DNA binding activity. The inhibitory effect of NO donors was reproduced in A7R5 cells, without affecting their viability. These findings provide the first evidence that NO inhibits the DNA binding activity of Oct-1, probably through a cGMP independent mechanism and suggests that NO may inhibit mitogenesis in part through an effect on Oct-1 DNA binding activity in VSMCs.

NonO enhances the association of many DNA-binding proteins to their targets

NonO is an unusual nucleic acid binding protein not only in that it binds both DNA and RNA but that it does so via functionally separable domains. Here we document that NonO enhances the binding of some (E47, OTF-1 and OTF-2) but not all (PEA3) conventional sequence-specific transcription factors to their recognition sites in artificial substrates as well as in an immunoglobulin VHpromoter. We also show that NonO induces the binding of the Ku complex to DNA ends. Ku has no known DNA sequence specificity. These enhancement of binding effects are NonO concentration dependent. Using the E box activity of E47 as a model, kinetic studies demonstrate that the association rate of the protein-DNA complex increases in the presence of NonO while the dissociation rate remains the same, thereby increasing the sum total of the interaction. Oligo competition experiments indicate that NonO does not contact the target DNA in order to enhance the binding activity of DNA binding proteins. Rather, methylation interference analysis reveals that the induced E47 binding-activity has the same DNA-binding sequence specificity as the normal binding. This result suggests that one of the effects of NonO is to induce a true protein-DNA interaction. In this way, it might be possible for NonO to play a crucial role in gene regulation.

Ku is a general inhibitor of DNA-protein complex formation and transcription

Ku is a ubiquitous and abundant DNA binding protein. Recently, it has been shown that Ku plays a crucial role in double stranded-DNA (dsDNA) break repair such as occurs during the V(D)J recombination of Ig genes. Ku has also been found to provide DNA binding activity to the catalytic domain of DNA-PK which is known to phosphorylate several transcription factors, suggesting that Ku is a multifunctional protein that participates as a component of several functional DNA-protein complexes. Here, we examined the interaction of Ku with several DNA binding proteins. Firstly, the DNA binding interaction between Ku and well-characterized transcription factors (OTF-1, Sp-1, AP-1) was analysed by EMSA. Although sequence non-specific, Ku was strongly competitive with these sequence specific transcription factors on compatible DNA elements, displacing them because of its high affinity association with DNA ends. Secondly, to determine whether this competitive effect was functionally relevant, we tested Ku in an in vitro transcription system with the adenovirus major late promoter. We found that Ku inhibited transcription from linear, but not from circular template DNA. These results suggest that Ku inhibits transcription when it is able to bind to template DNA and that the inhibition is the result of Ku displacing specific transcription factors from DNA.

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.

Genetic analysis of transcription factors implicated in B lymphocyte development

The transcription factors Oct-2, NF-kappa B and PU.1 have been implicated in regulating the development of B lymphocytes. Genetic approaches have been used to analyze the developmental functions of these regulatory proteins. Using gene targeting in murine embryonic stem cells, PU.1 is shown to be required for the development of progenitor B cells. Strikingly, PU.1 is also essential for the development of T lymphoid, granulocytic and monocytic progenitors. Transcription factors of the NF-kappa B/Rel family, which appear to regulate immunoglobulin kappa gene expression, are shown to be a target of the viral transforming protein (v-abl) which arrests B lineage development at the precursor B stage. This suggests a mechanism by which v-abl blocks precursor B cell differentiation. The Oct-2 transcription factor was considered to represent a development regulator of immunoglobulin gene expression. Using gene targeting in a murine B cell, Oct-2 is shown to be dispensable for immunoglobulin gene expression. This suggests the existence of an alternate pathway, involving the ubiquitous related protein, Oct-1, in immunoglobulin gene regulation.

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.

Two different IFN-gamma nonresponsive variants derived from the B-cell lymphoma 70Z/3

The kappa immunoglobulin (Igk) light chain locus is transcriptionally silent in the mouse B-cell lymphoma 70Z/3. However, exposure to lipopolysaccharide (LPS) or interferon-gamma (IFN) causes a marked increase in Igk transcription. By immunoselection, we isolated two variants that are nonresponsive to IFN. One variant, AT7.2, has retained its response to LPS (IFN-LPS+), whereas the other, AT3.3, is also nonresponsive to LPS (IFN-LPS-). Stable transfection of an intact Igk gene does not rescue the phenotype of either variant. Both variants have intact Igk genes and neither is deficient in the binding or uptake of IFN. Nuclear extracts from LPS-treated wild-type 70Z/3 cells show strong increases in three transcription factors: OTF-2, NF-kappa B, and kBF-A. Remarkably, when the IFN-LPS- variant is treated with LPS, all three transcription factors are still observed in the nuclear extracts. Treatment of wild-type cells with either LPS or IFN also causes a decrease in nuclear complexes that bind to two other regions of the Igk intron enhancer, the octenh and the E kappa MHCIC regions. Both of these changes are also observed after LPS or IFN treatment of the IFN-LPS- variant. Thus, this variant transduces the IFN and LPS signals at least into the nuclear compartment, but still fails to activate Igk transcription. In contrast, the IFN-LPS+ variant decreases neither the octenh nor the E kappa MHCIC binding complexes in response to IFN. This variant may be defective in transducing the IFN signal to the nucleus. These variants will be useful in studying the activation of Igk transcription and the IFN signaling pathway in B cells.

Cloning, sequencing and restriction fragment length polymorphism analysis of a porcine cDNA for OCT2

A full-length pig cDNA for the POU-domain protein OCT2 has been isolated and sequenced. The 478 amino acid-long reading frame in pig OCT2 is 97% identical to human OCT2A, indicating strong conservation of function for this immunoglobulin regulatory protein. To investigate the potential use of this cDNA for mapping and identifying linkage of OCT2 to economic traits, restriction fragment length polymorphism (RFLP) analysis was used to identify a TaqI polymorphism. A population of 60 unrelated animals, as well as multigenerational families, were typed for this RFLP and showed variability in several American and Chinese breeds. The TaqI polymorphism was also detected by a non-POU-domain OCT2 probe, demonstrating that this RFLP is located in the OCT2 gene. This result suggests that OCT2 is likely to be a single-copy gene in swine as seen in other mammals.

Broad binding-site specificity and affinity properties of octamer 1 and brain octamer-binding proteins

The ubiquitous Pit-1-Oct-1-Unc-1 (POU)-domain protein octamer 1 (Oct-1) has been observed to bind specifically to a number of degenerate and dissimilar sequences. We have used antibodies directed against a C-terminal Oct-1 peptide to immunoselect binding sequences for HeLa cell Oct-1 from random-sequence oligonucleotides and we describe the isolation of binding sequences of considerable heterogeneity. Although our consensus alignment indicated a 9-bp TATGCAAAT motif with AT-rich flanking sequences, this binding motif is not immediately obvious in the population of sequences and no clone actually contained this sequence. Screening these Oct-1-binding sequences with a mouse whole-brain extract demonstrated that the neuronal octamer-binding proteins exhibit similar but distinct DNA sequence specificities. Unlike the reported selection of binding sequences for other transcription factors, the dependence of Oct-1-binding affinity upon sequence did not correspond tightly to the degree of conservation at particular positions of the consensus sequence. Our results suggest that either base-specific hydrogen bonding is not the only major determinant of binding affinity and specificity, or that Oct-1 binding to some sequences is mechanistically different from its binding to an octamer. These results exemplify the potential to overlook binding sites for some factors by searching gene sequences with a consensus nucleotide sequence.

In vivo and in vitro expression of octamer binding proteins in human melanoma metastases, brain tissue, and fibroblasts

The pattern of octamer sequence-specific DNA binding proteins expressed in human melanoma was examined in nuclear extracts of seven surgically-isolated tumors, short-term cultures of these tumors, and 25 human melanoma cell lines to determine the in vivo and in vitro distribution of the melanocytic-associated Oct-M1 and Oct-M2 octamer binding activities. In the biopsy tissue and cultured melanoma cells of a metastasis from the cerebellum, two other binding activities (N-Oct-2 and N-Oct-6) in addition to the Oct-M1, Oct-M2 and the generally expressed Oct-1 protein were detected; this profile was consistent with that seen in normal human and mouse brain tissue. Melanoma tissue removed from lymph nodes and cell lines established from them also showed Oct-1, Oct-M1, Oct-M2, and N-Oct-2. N-Oct-2 was distinguished from the comigrating Oct-2A activity by failure to react with Oct-2A-specific antibody. All but one of the 25 melanoma cell lines exhibited Oct-1, Oct-M1, and Oct-M2 and/or N-Oct-2 activity, whereas cultured normal melanocytes expressed only Oct-1 and Oct-M1. In contrast to murine fibroblasts, which express only Oct-1, human fibroblast strains also expressed Oct-2A binding activity, which was confirmed by reactivity with Oct-2A antibody and the presence of Oct-2A mRNA and indicated that Oct-2A has a more general role than that of a lymphoid-specific transcription factor. Overall, the results indicate that expression of neural-specific Oct factors in human melanoma is (1) aberrant compared with normal melanocytes, (2) can be modulated by the surrounding tissue in a brain metastasis, and (3) may be part of the altered program of differentiation accompanying transformation.

A novel B cell-derived coactivator potentiates the activation of immunoglobulin promoters by octamer-binding transcription factors

A novel B cell-restricted activity, required for high levels of octamer/Oct-dependent transcription from an immunoglobulin heavy chain (IgH) promoter, was detected in an in vitro system consisting of HeLa cell-derived extracts complemented with fractionated B cell nuclear proteins. The factor responsible for this activity was designated Oct coactivator from B cells (OCA-B). OCA-B stimulates the transcription from an IgH promoter in conjunction with either Oct-1 or Oct-2 but shows no significant effect on the octamer/Oct-dependent transcription of the ubiquitously expressed histone H2B promoter and the transcription of USF- and Sp1-regulated promoters. Taken together, our results suggest that OCA-B is a tissue-, promoter-, and factor-specific coactivator and that OCA-B may be a major determinant for B cell-specific activation of immunoglobulin promoters. In light of the evidence showing physical and functional interactions between Oct factors and OCA-B, we propose a mechanism of action for OCA-B and discuss the implications of OCA-B for the transcriptional regulation of other tissue-specific promoters.

Promoter-selective activation domains in Oct-1 and Oct-2 direct differential activation of an snRNA and mRNA promoter

The promoter specificity of transcriptional activators is generally thought to be conferred by the specificity of the DNA-binding domain, which brings the activation domain to the appropriate promoter sequence. We show here, however, that Oct-1 and Oct-2 can differentially activate transcription not through DNA binding specificity but instead through the use of promoter-selective activation domains. These distinct activation domains lead to stimulation of the U2 small nuclear RNA promoter by Oct-1 and an mRNA promoter by Oct-2. An Oct-2 variant, called Oct-2B, differs from Oct-2 by an Oct-1-related C-terminal extension that results from alternative splicing. This variant gains the ability to activate the U2 small nuclear RNA promoter. Thus, the promoter selectivity of a transcriptional activator can be changed, in this case by alternative splicing, without affecting its DNA binding specificity.

Regulation of immunoglobulin gene transcription

Analysis of the immunoglobulin gene suggests that their expression is controlled through the combinatorial action of tissue- and stage-specific factors (OTF-2, TF-microB, NF-kappa B), as well as more widely expressed E motif-binding factors such as E47/E12. Two basic issues cloud understanding of how these factors are involved in immunoglobulin gene regulation. First, cloning of these factors shows them to be members of families of proteins, all with similar DNA-binding specificities. OTF-2 is a member of the POU domain family, NF-kappa B is a related protein, and the microE5/kappa E2-binding factors are members of the bHLH family. Second, these binding sites and associated factors are involved in the regulation of many genes, not only the immunoglobulin genes, and in fact not only lymphoid-specific genes. These facts complicate understanding which member of a family is in fact responsible for interaction with, and activation of, a particular binding element in an enhancer/promoter. Recently, more detailed analysis of the interactions between such proteins and their related binding sites suggest that a certain level of specificity may in fact be encoded by the DNA element such that one family member of a protein is preferentially bound, or alternatively that the protein-DNA interactions that occur give subtle alterations in protein conformation that unmask an activation or protein-protein interactive domain. An additional level of regulation is imparted by combinatorial mechanisms such as adjacent DNA-binding elements and factors that may alter activity, as well as "cofactors" that, by forming a complex with the bound factor, affect its activation of a gene in a particular cell type. A third level of specificity may be obtained by factors such as NF-kappa B and the bHLH family due to their ability to create heterogeneous complexes, creating unique complexes in a tissue- or stage-specific manner. The multiple functions transcription factors such as NF-kappa B and OTF-2 play in the transcriptional regulation of multiple genes seems complex in contrast to a one factor, one gene regulation model. However, this type of organization may limit the number of factors lymphocytes would require if each lymphoid-specific gene were activated by a unique factor. Thus what appears to be complexity at the molecular level may reflect an economical organization at the cellular level. Investigation of the key factors controlling these genes suggests an ordered cascade of transcription factors becomes available in the cell during B cell differentiation.(ABSTRACT TRUNCATED AT 400 WORDS)

Differentiation of human B-cell malignant lymphomas is independent of the octamer lymphoid specific binding factor (Oct-2)

We have shown previously that the presence and action of immunoglobulin gene promoter specific trans-acting factors correlates with the stages of 'differentiation' of human lymphoid neoplasms. The regulatory sequence described by us was located upstream of the octamer motif which is known to bind lymphoid specific trans-acting factor Oct-2. In the present study we attempted to establish if the Oct-2 factor was present in fresh human tissue of B-cell origin and if the levels of Oct-2 also correlated with the stages of human lymphoid differentiation. We applied DNA mobility shift assay using the same cases which we utilized in our previous work. We compared the levels of Oct-2 with the levels of ubiquitous octamer binding factor Oct-1. Oct-2 was present in all lymphoid cells of B-cell origin (from fresh surgical specimens and in long-term tissue cultured cells) with the exception of a pre-B-cell line NALM-6. The relative abundance of Oct-2 varied, however, and the ratio of Oct-2 to Oct-1 was variable in different types of B cells. This phenomenon did not correlate with the stages of differentiation of human lymphoid neoplasms. There was also no correlation between the expression of Oct-2 and levels of immunoglobulin-specific messenger RNAs. These findings indicate that the control of, immunoglobulin expression in relation to the differentiation of human B-cell neoplasms requires factors other than Oct-2.

Differential phosphorylation of the transcription factor Oct1 during the cell cycle

Orderly progression through the somatic cell division cycle is accompanied by phase-specific transcription of a variety of different genes. During S phase, transcription of mammalian histone H2B genes requires a specific promoter element and its cognate transcription factor Oct1 (OTF1). A possible mechanism for regulating histone H2B transcription during the cell cycle is direct modulation of Oct1 activity by phase-specific posttranslational modifications. Analysis of Oct1 during progression through the cell cycle revealed a complex temporal program of phosphorylation. A p34cdc2-related protein kinase that is active during mitosis may be responsible for one mitotic phosphorylation of Oct1. However, the temporally controlled appearance of Oct1 phosphopeptides suggests the involvement of multiple kinases and phosphatases. These results support the idea that cell cycle-regulated transcription factors may be direct substrates for phase-specific regulatory enzymes.