Expression of a liver-specific function by mouse fibroblast nuclei transplanted into rat hepatoma cytoplasts

Abstract

Data from many somatic cell fusion experiments support the notion that the cytoplasm of eukaryotic cells contains elements which can alter gene expression and replication¹. More definitive evidence is perhaps lacking because the cell hybrids studied were complex cells comprising mixed genomes, frequently undergoing chromosomal losses and other aberrations, within mixed cytoplasms. Such problems have been alleviated by the development of the techniques of cybridisation, red cell-mediated microinjection and nuclear transplantation². Using this last technique, we showed that up to 40% of a monolayer culture of cytoplasts could be renucleated to form whole viable cells^3,4. Because of the high efficiency of fusion and the purity of cytoplast and karyoplast cultures used, large quantities of true cytoplasmic–nuclear hybrid cells suitable for immediate morphological and biochemical analysis could be formed. Here we describe the application of the technique to an investigation of the effects of rat liver cytoplasm on gene expression by mouse fibroblast nuclei. We show that the synthesis of mouse tryosine amino transferase (TAT)—a liver-specific enzyme never detected in the fibroblast cell line was induced in hybrids constructed by transplantation of mouse (A9) fibroblast nuclei into rat liver (HTC) cytoplasts. Furthermore, as occurs in the rat cell line, its activity was modulated by addition of the synthetic corticosteroid, dexamethasone. Clearly, this hybrid cell system will be useful in the further characterisation and eventual isolation of the newly identified regulatory substance.