Induction of immunoglobulin heavy-chain transcription through the transcription factor Bright requires TFII-I.

Bright/ARID3a/Dril1, a member of the ARID family of transcription factors, is expressed in a highly regulated fashion in B lymphocytes, where it enhances immunoglobulin transcription three- to sixfold. Recent publications from our lab indicated that functional, but not kinase-inactive, Bruton's tyrosine kinase (Btk) is critical for Bright activity in an in vitro model system, yet Bright itself is not appreciably tyrosine phosphorylated. These data suggested that a third protein, and Btk substrate, must contribute to Bright-enhanced immunoglobulin transcription. The ubiquitously expressed transcription factor TFII-I was identified as a substrate for Btk several years ago. In this work, we show that TFII-I directly interacts with human Bright through amino acids in Bright's protein interaction domain and that specific tyrosine residues of TFII-I are essential for Bright-induced activity of an immunoglobulin reporter gene. Moreover, inhibition of TFII-I function in a B-cell line resulted in decreased heavy-chain transcript levels. These data suggest that Bright functions as a three-component protein complex in the immunoglobulin locus and tie together previous data indicating important roles for Btk and TFII-I in B lymphocytes.

The transcription factor, Bright, is not expressed in all human B lymphocyte subpopulations.

Bright is an ARID family transcription factor that increases immunoglobulin heavy chain transcription. In the mouse, Bright expression is tightly regulated and B cell-restricted and the Bright protein associates with Bruton's tyrosine kinase (Btk), the defective enzyme in X-linked immunodeficiency. Human X-linked agammaglobulinemia results from defects in Btk and leads to early blocks in B lymphocyte development. Because so little is known about human Bright, we sought to determine where human Bright is expressed in normal B cell differentiation and whether it also forms complexes with Btk. Although human and mouse Bright exhibited similar expression patterns in normal B cells, many human transformed B cell lines did not express Bright protein. However, the human protein bound prototypic Bright DNA-binding motifs and, like mouse Bright, was capable of associating with Btk. These data suggest potentially important similarities exist in Bright expression and activity in human and mouse B lymphocytes.

High frequency of matrix attachment regions and cut-like protein x/CCAAT-displacement protein and B cell regulator of IgH transcription binding sites flanking Ig V region genes.

A major component in controlling V(D)J recombination is differential accessibility through localized changes in chromatin structure. Attachment of DNA to the nuclear matrix via matrix attachment region (MAR) sequences, and interaction with MAR-binding proteins have been shown to alter chromatin conformation, promote histone acetylation, and influence gene transcription. In this study, the flanking regions of several human and mouse Ig V(H) and Ig Vkappa genes were analyzed extensively for the presence of MARs by in vitro matrix-binding assay, and for interaction with the MAR-binding proteins cut-like protein x/CCAAT-displacement protein (Cux/CDP), B cell regulator of IgH transcription (Bright), and special AT-rich sequence-binding protein (SATB1) by EMSA. Cux/CDP and SATB1 are associated with repression, while Bright is an activator of Ig transcription. Binding sites were identified in the vicinity of all analyzed Ig V genes, and were also found flanking TCR Vbeta genes. We also show that the binding sites of the different factors do not always occur at MAR sequences. MAR sequences were also found within the Ig V loci at a much higher frequency than throughout the rest of the genome. Overall, the frequency and location of binding sites relative to the coding regions, and the strength of DNA-protein interaction showed much heterogeneity. Thus, variations in factor binding and MAR activity could potentially influence the extent of localized accessibility to V(D)J recombination and thus could play a role in unequal rearrangement of individual V genes. These sites could also contribute to effective transcription of Ig genes in mature and/or activated B cells, bringing both the promoter as well as the enhancer regions into close proximity at the nuclear matrix.