CONSTANCY OF EXPRESSION OF THE PROTEIN KINASE-A REGULATORY SUBUNIT R1-ALPHA IN HEPATOMA-CELL LINES OF DIFFERENT PHENOTYPES

In somatic hybrids between fibroblast microcells and rat hepatoma cell s, tissue-specific extinguisher 1 (TSE1), localized to mouse chromosom e 11, extinguishes the expression of tyrosine aminotransferase and pho spho(enol(pyruvate carboxykinase. Recently, it was demonstrated that T SE1 corresponds to R1 alpha, a regulatory subunit of protein kinase A. Here, we have analyzed whether R1 alpha could play a role in differen tiation of the hepatocyte. It is known that the TSE1/R1 alpha target g enes belong to the group of neonatal functions that are repressed unti l birth. High expression of R1 alpha characterizes fetal-type BW1J hep atoma cells in which the neonatal target genes are silent. This R1 alp ha is active in trans to extinguish these genes in hybrids between BW1 J and Fao adult-type rat hepatoma cells. Reexpression of the target ge nes is correlated with loss of R1 alpha and/or overexpression of the m RNA for the hepatocyte-enriched transcription factors HNF4 and HNF3alp ha. Phenylalanine hydroxylase is shown to be another function negative ly regulated by R1 alpha. In BW cells in which expression of phenylala nine hydroxylase has been activated (after either 5-aza-cytidine treat ment or transfection with genomic DNA from adult-type hepatoma cells), no down-regulation of R1 alpha expression occurs: an independent mech anism overcomes R1 alpha repression. Finally, dedifferentiated derivat ives of the adult-type rat hepatoma cells express neither the R1 alpha target genes nor the R1 alpha gene itself. Thus, in three different s ituations in which modulation of R1 alpha expression could be anticipa ted, it fails to occur. It is concluded that somatic cells in culture show cis-heritability of the R1 alpha gene, even under conditions wher e its effect is selected against.