The octamer/mu E4 region of the immunoglobulin heavy-chain enhancer mediates gene repression in myeloma x T-lymphoma hybrids

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.

Developmental regulation of VDJ recombination by the core fragment of the T cell receptor alpha enhancer

The role of T cell receptor alpha enhancer (E alpha) cis-acting elements in the developmental regulation of VDJ recombination at the TCR alpha/delta locus was examined in transgenic mice containing variants of a minilocus VDJ recombination substrate. We demonstrate that the 116-bp T alpha 1,2 core enhancer fragment of the 1.4-kb E alpha is sufficient to activate the enhancer-dependent step of minilocus rearrangement, and that within T alpha 1,2, intact binding sites for TCF/LEF and Ets family transcription factors are essential. Although minilocus rearrangement under the control of the 1.4-kb E alpha initiates at fetal day 16.5 and is strictly limited to alpha beta T cells, we find that rearrangement under the control of T alpha 1,2 initiates slightly earlier during ontogeny and occurs in both gamma delta and alpha beta T cells. We conclude that the core fragment of E alpha can establish accessibility to the recombinase in developing thymocytes in vivo in a fashion that is dependent on the binding of TCF/LEF and Ets family transcription factors, but that these and other factors that bind to the E alpha core cannot account for the precise developmental onset of accessibility that is provided by the intact E alpha. Rather, our data suggests a critical role for factors that bind E alpha outside of the core T alpha 1,2 region in establishing the precise developmental onset of TCR alpha rearrangement in vivo.

E47 activates the Ig-heavy chain and TdT loci in non-B cells

The E2A proteins, E12 and E47, are basic helix-loop-helix (bHLH) proteins essential for the B-cell lineage. Initially identified as immunoglobulin enhancer-binding proteins, they have also been shown to activate immunoglobulin enhancer-based reporters in transient transfection assays. Here, we examine the relationship between E2A DNA binding activity and activation of the endogenous, chromosomal immunoglobulin heavy chain (IgH) locus. Using sterile I(mu) transcription as an indicator of IgH enhancer activity, we see a direct correlation between E2A DNA binding activity and I(mu) transcription in stable BxT hybrids. We also observe a 1000-fold stimulation of endogenous I(mu) transcription in fibroblasts that express high levels of E47 and less stimulation in cells that express E12. By contrast, none of the other IgH enhancer-binding proteins tested (E2-2, Pu.1, Oct-2, OCA-B, TFE3 and USF) were able to activate I(mu) transcription. E47 overexpression also resulted in transcriptional activation of the endogenous gene encoding TdT, indicating that it, too, is a target of E2A proteins early in the B-cell lineage. Our results indicate that E2A proteins have the distinctive property of activating silent, chromatin-embedded B-cell-specific genes, underscoring their crucial role in B-cell development.

The human lambda immunoglobulin enhancer is controlled by both positive elements and developmentally regulated negative elements

We have recently reported the localization of the first transcriptional enhancer in the human lambda (lambda) immunoglobulin light chain locus. Enhancer activity was contained on a 1.2 kb SstI fragment, with partial activity retained on a core 111 bp PstI-SstI fragment. This enhancer is located 11.7 kb downstream of C lambda 7, the most 3' lambda constant region gene. Using a chloramphenicol acetyl transferase (CAT) assay system, we have now determined the boundaries of the complete enhancer and find it is two- to four-fold as active as the core fragment in both pre-B and B cell lines. Interestingly, a larger fragment, containing the complete enhancer as well as 5' and 3' flanking sequences has four- to eight-fold reduced activity when tested in pre-B cell lines, but full activity in B cell lines. This suggests the presence of developmentally regulated negative elements flanking the human lambda enhancer which prevent or reduce its activity at a developmentally incorrect time. By using in vivo footprinting we have begun to examine the protein interactions within this enhancer in a more physiologically relevant manner and have identified motifs which are shared with the murine lambda enhancers, as well as motifs unique to the human lambda enhancer.

Coordinate suppression of myeloma-specific genes and expression of fibroblast-specific genes in myeloma X fibroblast somatic cell hybrids

In most instances, fusion of differentiated cell types with fibroblasts has resulted in the extinction of the differentiation-specific traits of the non-fibroblast parental cell. To explore the genetic basis of this phenomenon, we have studied a series of somatic cell hybrids between mouse myeloma and fibroblasts. All the hybrids were adherent having a fibroblast-like phenotype. Molecular analysis revealed that plasma cell specific genes like the productively rearranged Ig genes, the J chain gene and genes for the cell surface markers CD20 and PC1, were extinguished in the hybrids. In contrast, fibroblast specific genes like fibronectin, alpha 2(I) and III collagens, as well as the receptor for fibroblast growth factor (flg), were expressed. Extinction was not due to chromosomal loss or lack of the relevant genes. To learn about the mechanism(s) of this phenomenon we have looked for the presence of positive and negative transcription factors in our hybrids. Expression of the PU.1 transcription factor, a member of the Ets transcription factor family normally expressed in B cells and macrophages, was lost in the cell hybrids. Interestingly, we found that the B-cell-specific Oct-2 transcription factor was still expressed at somewhat variable levels in several of the hybrid cell lines. In contrast, expression of the recently identified octamer coactivator BOB.1/OBF.1 was extinguished in all cell hybrids. This supports a critical role of this transcriptional coactivator for B-cell-specific gene expression. In addition, the Id and HLH462 genes coding for proteins known to repress bHLH transcription factors by formation of heterodimers, were found to be expressed at increased levels in fibroblasts and in the hybrids, indicating that their increased levels might also contribute to the suppression of myeloma-specific genes. Our results show that in myeloma x fibroblast hybrids, the phenotype of the fibroblast is dominant. It is suggested that fibroblasts contain regulatory "master" genes that are responsible for activation of the fibroblast differentiation pathway and suppress differentiation programs of other cell types.

Mammalian cell fusion in an electroporation device

We and others have been interested in the phenomenon of gene 'extinction' in somatic cell hybrids, reasoning that the study of this process is likely to reveal underlying mechanisms responsible for limiting the expression of specialized genes only to appropriate cell types. In the course of our studies in this area, we have developed a simple and economical method of fusing mammalian cells, using an electroporation device. In fusions between murine myeloma and T lymphoma lines, hybrid cell recoveries were typically one per 10(5) [corrected] input myeloma cells. Because of our interest in the regulation of immunoglobulin heavy chain (IgH) gene expression, we analyzed the hybrids for both IgH gene composition and expression. The hybrid lines were phenotypically indistinguishable from those generated by the more conventional, polyethylene glycol (PEG)-induced fusion protocol. There was a notable increase, however, in the number of hybrids that retained IgH-encoding chromosomes from both parental lines.

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.

Negative regulation of transcription in eukaryotes

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