Induction of alpha-fetoprotein synthesis in differentiating F9 teratocarcinoma cells is accompanied by a genome-wide loss of DNA methylation.

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F9 teratocarcinoma cells can be grown as monolayers or aggregates, and upon treatment with retinoic acid they will differentiate into parietal or visceral endoderm, respectively. Visceral endoderm specifically synthesizes alpha-fetoprotein and albumin mRNAs, which are not found in parietal endoderm. In contrast, both endoderms produce enhanced levels of the major histocompatibility antigen (H2) mRNA compared with F9 cells. F9 cells contain highly methylated DNA as judged by restriction enzyme digestion. However, upon differentiation into visceral endoderm, there is a genome-wide loss of methylation in induced, silent, and constitutively expressed genes. Experiments in which methylation loss is induced via the methyltransferase inhibitor 5-azacytidine result in no induction of alpha-fetoprotein mRNA and no morphological differentiation, suggesting that methylation loss alone is not sufficient to induce the visceral endoderm phenotype. Likewise, 5-azacytidine treatment of differentiated cells does not result in enhanced expression of alpha-fetoprotein mRNA. However, the patterns of loss of DNA methylation at all sites examined after differentiation or 5-azacytidine treatment were remarkably similar, suggesting that the two occur by a similar mechanism, the inhibition of DNA methyltransferase activity. These results argue that the specificity for methylation loss at a given site is an inherent property of aggregated F9 cell chromatin. This system provides a model for studying a tissue-specific change in DNA methylation upon differentiation.

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