[Rat liver methylation of nuclear DNA following hydrocortisone induction].

After intravenous hydrocortisone injection (2 mg per 100 g of animal weight) DNA methylase activity in rat liver increases 1.5-2 times. Rat liver DNA is capable of being methylated in vitro by homologous and heterologous (from rat spleen and ascite carcinoma cells) enzymes. Rat liver DNA isolated 40 min after hydrocortisone injection contains 1.5 times more 5-methylcytosine and is able to accept 1.5 times less methyl groups from (3H-methyl)-S-adenosylmethionine in the in vitro methylation reaction by enzymes from rat spleen as compared to liver DNA isolated from nontreated rats. Thus, there is DNA supermethylation in rat liver cells occurring under the action of the hormone. This induced change in the methylation level of DNA in rat liver is reversible: 6 hours after a single hydrocortisone injection the amount of 5-methylcytosine in DNA decreases to normal, and the DNA ability to accept methyl groups in the in vitro methylation is the same as compared to that of liver DNA from control animals. The induced reversible DNA methylation is to be considered as a mechanism for the regulation of DNA transcription and cell genetic activity.