Extinction of Ig genes expression in myeloma x fibroblast somatic cell hybrids is accompanied by repression of the oct-2 gene encoding a B-cell specific transcription factor.

Abstract

In most instances, fusion of differentiated cell types with fibroblasts has resulted in the extinction of differentiation‐specific traits of the nonfibroblast parental cell. To explore the genetic basis of this phenomenon, we have used a series of somatic cell hybrids between myeloma cells and fibroblasts. Previous findings show that in these hybrids expression of the immunoglobulin (Ig) genes was extinguished at the transcriptional level. Our present results show that NF‐kappa B transcription factor, known to be critical for kappa‐chain enhancer activity, is present although in a lower amount, in the nucleus and in the cytosolic fraction of most of these hybrids (probably attached to the previously postulated I‐kappa B inhibitor). In contrast, the expression of the NF‐A2/OTF‐2 transcription factor encoded by the oct‐2 gene, which binds to the octameric motif located in the Ig promoters and heavy chain gene enhancer, is extinguished at the transcriptional level. Our data thus suggest that extinction of Ig genes expression occurs via an indirect mechanism in which a fibroblast factor suppresses transcription factor(s) which are critical for Ig transcription.

Extinction of Oct-3/4 gene expression in embryonal carcinoma x fibroblast somatic cell hybrids is accompanied by changes in the methylation status, chromatin structure, and transcriptional activity of the Oct-3/4 upstream region.

“…Among the different phenotypes that appear after fusion, extinction of specific cellular functions is most frequently observed (12). Previous studies utilizing several different hybrid cell systems, notably the pituitary cell x fibroblast, B-cell x fibroblast, and hepatoma x fibroblast hybrids, have analyzed the extinction of tissue-specific gene expression of growth hormone, immunoglobulin, otl-antitrypsin, and albumin, respectively (2,3,8,10,26,36,56,59). These studies concentrated on assessing the expression of transcription factors known to play a pivotal role in expression of the above-mentioned tissue-specific genes.…”

The tissue-specific extinguisher locus TSE1 encodes a regulatory subunit of cAMP-Dependent protein kinase

“…The mechanism of extinction hasalso been investigated in hybrids between fibroblasts and pituitary cells, lymphoid cells, or hepatoma cells expressing growth hormone, immunoglobulin genes, or albumin, respectively. In those hybrids the transcription rate or mRNA level of a tissuespecific transcription factor required for expression of these genes (GHF-1, OCT2, or HNFl, respectively) was down-regulated (McCormick et al, 1988;Junker et al, 1988;Bergman et al, 1990;Junker et al, 1990;Cereghini et al, 1990). Extinction can also be mediated by activation of silencer-like target sequences (Triputti et al, 1988;Zaller et al, 1988;Yu et al, 1989).…”

A cyclic AMP response element mediates repression of tyrosine aminotransferase gene transcription by the tissue-specific extinguisher locus Tse-1

“…lmmunoglobulin heavy and K light chain gene promoters and the heavy chain enhancer are targets for extinction in myeloma x fibroblast somatic cell hybrids (Junker et al, 1988) and this is accompanied by down-regulation of the mRNA coding for the lymphoid-specific transcription factor Ott-2 (OTF-2) (Bergman et al, 1990;P. Matthias and S. Junker, personal communication).…”