Variations of DNA-(cytosine-5-)-methyltransferase activities after administration of N-hydroxy- N-aminofluorene to Sprague-Dawley rats

DNA methylation in eukaryotic cells is a post-replicative process involving the transfer of methyl groups from S-adenosyl-L-methionine to the 5 position of cytosine residues through the action of DNA (cytosine-5-)-methyltransferase (DNA-methylase). There are two types of methylation within the cell: a maintenance methylation and a de novo methylation. Its major function is the maintenance methylation of hemimethylated sites after replication in order to preserve the pattern from one generation to the next. Nevertheless DNA-methylase is also able to transfer methyl groups to unmethylated sites in various substrates in a de novo reaction. Male Sprague-Dawley rats have a low specific activity of liver maintenance DNA-methylase and are sensitive to the toxic and carcinogenic effects of N-hydroxy-N-acetylaminofluorene (N-OH-AAF). Female Sprague-Dawley rats, on the contrary, have a 4–5 times higher maintenance DNA-methylase activity and are 6–7 times less sensitive to this carcinogenic effect. Their de novo DNA-methylase activity is the same. Whem female Sprague-Dawley rats are treated with N-OH-AAF their total DNA-methylase activity diminishes. On the contrary, the maintenance DNA-methylase activity of male Sprague-Dawley rats increases, whereas the de novo activity remains constant. In the spleen, which is not a target organ, the total DNA-methylase activity decreases after injection of N-OH-AAF. These variations of DNA-methylase activity are due to a variation of extractable nuclear DNA-methylase. When Swiss mice, which are not sensitive to the carcinogenic effect, are treated with N-OH-AAF, their total DNA-methylase activity decreases. A decrease of DNA-methylase activity in response to this carcinogen seems to be correlated to the resistance of the animals in developing a hepatocarcinoma.