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 cells, tissue-specific extinguisher 1 (TSE1), localized to mouse chromosome 11, extinguishes the expression of tyrosine aminotransferase and phospho(enol)pyruvate carboxykinase. Recently, it was demonstrated that TSE1 corresponds to R1 alpha, a regulatory subunit of protein kinase A. Here, we have analyzed whether R1 alpha could play a role in differentiation of the hepatocyte. It is known that the TSE1/R1 alpha target genes belong to the group of neonatal functions that are repressed until birth. High expression of R1 alpha characterizes fetal-type BW1J hepatoma cells in which the neonatal target genes are silent. This R1 alpha is active in trans to extinguish these genes in hybrids between BW1J and Fao adult-type rat hepatoma cells. Reexpression of the target genes is correlated with loss of R1 alpha and/or overexpression of the mRNA for the hepatocyte-enriched transcription factors HNF4 and HNF3 alpha. Phenylalanine hydroxylase is shown to be another function negatively regulated by R1 alpha. In BW cells in which expression of phenylalanine hydroxylase has been activated (after either 5-aza-cytidine treatment or transfection with genomic DNA from adult-type hepatoma cells), no down-regulation of R1 alpha expression occurs: an independent mechanism overcomes R1 alpha repression. Finally, dedifferentiated derivatives of the adult-type rat hepatoma cells express neither the R1 alpha target genes nor the R1 alpha gene itself. Thus, in three different situations in which modulation of R1 alpha expression could be anticipated, it fails to occur.(ABSTRACT TRUNCATED AT 250 WORDS)