The Role of Methylation in Regulating the Expression of the Alpha‐Fetoprotein Gene in Developing Rat Liver and Hepatoma Cell Lines

We have examined four possible sites of methylation in the 5′ flanking region of the alpha‐fetoprotein (AFP) gene during liver development in the rat, paying particular attention to the neonatal period, in which AFP gene transcription changes rapidly. These sites are found in Mspl/Hpall sites located at ‐ 4197, ‐ 3038, ‐ 2431, and + 3 bp relative to the transcription start site. Three of these sites are associated with sequence regions important for the regulation of AFP gene transcription. We found that, in general, the 5′ flanking region of the gene was methylated more in the adult liver than in the livers of fetal and neonatal rats. In addition, the degree of methylation of all four sites examined was increased in the adult liver. One of these sites showed increased methylation as AFP gene activity decreased, whereas the others became more methylated only after transcriptional activity of the gene had ceased. In particular, the site ( + 3 bp) just adjacent to the transcriptional initiation site of the gene was fully methylated in the adult liver In various rat hepatoma and liver cell lines methylation of this same site showed a particularly close correlation with the amount of transcriptional activity of the AFP promoter in these cell lines. Treatment of the hepatoma and liver cell lines with dexamethasone, which influences AFP gene expression, did not result in any changes in methylation of these sites in the 5′ flanking region.