Repetitive 5‐azacytidine treatments of Fao cells induce a stable and strong expression of gamma‐glutamyl transpeptidase

The role of DNA methylation in the expression of the rat gamma‐glutamyl transpeptidase (GGT) gene was assessed in the Fao cell line using a hypomethylating agent, 5‐azacytidine. Ten repetitive treatments of the cells, with 8 μM 5‐azacytidine for 24 h, led to 13‐ and 80‐fold increases, respectively, in GGT activity and in GGT mRNA level. The DNA methylation patterns generated by the isoschizomeric restriction enzymes Hpa II and Msp I indicated that the GGT gene, highly methylated in Fao cells, became strongly demethylated after 5‐azacytidine treatments. Thus, DNA demethylation increases the expression of the GGT gene. 5‐Azacytidine treatments also increased, but to a lesser extent, mRNAs level for actin, albumin, mitochondrial aspartate aminotransferase, aldolase BmRNAs (12‐ to 16‐fold) as wells as for tubulin, gluthathione transferese, and tyrosine aminotransferase mRNAs (2‐to 5‐fold). The GGT gene expression was further studied in B₄ cells, cloned from the demethylated Fao cell population. This clone B₄ exhibited a stable and strong GGT activity and a highly demethylated GGT gene. Among the three GGT mRNA I, II, or III, transcribed from three different promoters of the single rat GGT gene, only mRNA III was detected in Fao cells and was increased in clone B₄, indicating that the demethylation acts on the promoter for mRNA III. The analysis of the differentiation state of B₄ cells, as compared to Fao cells, showed a loss of the regulation of GGT and aspartate aminotransferase genes by dexamethasone, as well as a loss of the gluconeogenic pathway. Interestingly, B₄ cells have retained many other specific functions of hepatic differentiation and have acquired α‐fetoprotein expression; thus this clone exhibits the characteristics of a hepatic fetal phenotype. © 1992 Wiley‐Liss, Inc.