Regulation of expression of tyrosine aminotransferase in somatic cell hybrids between rat hepatoma cells and mouse fibroblasts

Molecular mechanisms of extinction: old findings and new ideas

Fusion experiments between somatic cells have been used for a long time as a means to understand the regulation of gene expression. In hybrids between differentiated cells such as hepatocytes or lymphocytes and undifferentiated cells such as fibroblasts a phenomenon called extinction has been described. In such hybrids expression of cell-specific genes derived from the more differentiated parental cell is selectively turned off (extinguished), whereas genes expressed from both cells like housekeeping genes remain active after fusion. Study of the molecular basis of extinction of the liver-specifically expressed tyrosine aminotransferase gene and of the B-cell-specifically expressed immunoglobulin genes has revealed that in hybrids the transcriptional program of the differentiated cells is reset. This is accompanied by a loss of expression or activity of many of the regulatory molecules that were operating in the differentiated cells. In the light of new insights in eukaryotic gene regulation we speculate that molecular mechanisms such as chromatin remodelling, recruitment to heterochromatin or subnuclear localization could underly the extinction process.

Extinction of an immunoglobulin kappa promoter in cell hybrids is mediated by the octamer motif and correlates with suppression of Oct-2 expression

When immunoglobulin-expressing B cells are fused with fibroblasts, immunoglobulin expression is rapidly and selectively suppressed. here we demonstrate that the conserved octamer motif of a kappa light chain gene promoter plays a crucial role in mediating this "extinction" phenomenon. Replacement of this octamer site by an Sp1 or NF1 binding site is sufficient to bypass extinction. Furthermore, in early cell hybrids, immunoglobulin suppression is correlated with absence of the cell-specific transcription factor Oct-2 and its transcripts. Such hybrids cannot support transcription of a transiently introduced reporter plasmid, driven by an octamer-containing promoter, unless an expression vector encoding Oct-2 is cotransfected. Transfection of the same Oct-2 expression vector into hybrid cells is also sufficient to "reactivate" an integrated kappa promoter construct. Thus, our data further establish the role of Oct-2 for immunoglobulin transcription and show that in B cell x fibroblast hybrids, the lack of a necessary cell-specific transcription factor is involved in the extinction of immunoglobulin expression.

Time course of arrest of immunoglobulin expression in heterokaryons and early hybrids of human lymphoma cells and mouse fibroblasts. A study of transcriptional and translational events

Early events in arrest of immunoglobulin expression were investigated at the levels of both translation and transcription in heterokaryons and early hybrids between human Daudi lymphoma cells and mouse cl. 1D cells. Large populations of 1s: 1s hybrids, isolated by fluorescence-activated cell sorting (FACS) a few hours after fusion, were grown for up to 5 days. A survey at the light-microscopical level of peroxidase-antiperoxidase-immunostained cell populations showed that arrest of expression of IgM heavy chain (mu) occurred in up to 98% of the cells. Furthermore, quantitation of mu chain contents, by using an ELISA technique, suggested that synthesis of IgM was blocked shortly after fusion. The levels of cytoplasmic mRNA specific for mu and kappa chains, respectively, decreased at rates similar to those induced in unfused Daudi cells by treatment with actinomycin D. It is concluded that arrest of immunoglobulin expression in these hybrids occurs immediately or very shortly after fusion by mechanisms that affect the levels of their cytoplasmic mRNAs.

Inhibitory or stimulatory effect of human genes on the expression of adrenal function in human Leydig X Y1 cell hybrids

In order to investigate the expression and the regulation of steroidogenesis, human Leydig cells were fused with a functional mouse adrenal cell line (Y1). Six independent hybrid clones were analysed for hormone receptors and for cAMP and steroid response to ACTH, hCG, 8Br-cAMP or forskolin. All hybrids had lost hCG receptors and their ability to produce testosterone. With respect to the response of adenylate cyclase to ACTH and/or forskolin, hybrids could be classed into two groups. In the first group, the pattern of response was qualitatively similar to Y1 parental cells; The second group was far less responsive to ACTH than are Y1 cells, and when added together, forskolin and ACTH only had an additive effect. All hybrids responded to ACTH and 8Br-cAMP with an increased production of pregnenolone (P5). The amounts of P5 produced both under basal conditions and following 8Br-cAMP stimulation were significantly higher in three hybrids when compared to Y1 cells. However, the ability of two of these three hybrids to produce 20 alpha-dihydroprogesterone (20 alpha OHP4) was very low. The metabolism of [14C]P5 revealed that in one of these hybrids, there was a loss of 3 beta-hydroxysteroid dehydrogenase/isomerase whereas in the other case, there was a low 20 alpha-hydroxylase activity. The inhibition of cell growth by ACTH was related to the ability of the hormone to stimulate cAMP. Conversely, the inhibitory growth effects of 8Br-cAMP were not always inversely correlated with the ability of this nucleotide to stimulate P5 production. Since hybrids contained two mouse genomes and retained variable human chromosomes, these results suggest that extinction or enhancement of murine genes coding for some of the enzymes involved in steroidogenic response to ACTH was due to the regulation by human genes.