Effects of 5-azacytidine on transfer RNA methyltransferases.

Abstract

We previously reported that the administration of 5-azacytidine to mice causes a marked reduction of the 5-methylcytidine content of mouse liver transfer RNA (tRNA) without having an effect on any other methylated nucleoside of tRNA. In the present work, it was found that tRNA methyltransferases isolated from livers of mice given 5-azacytidine exhibited specific dose-dependent decreases in the activity and capacity of tRNA cytosine-5-methyltransferase. This effect could be detected as early as 20 min and as late as 72 hr after a single i.p. administration of the drug (18 mg/kg), with a minimum at 4 to 7 hr (3% of control). 5-Methylcytidine-deficient mouse liver tRNA was also shown to serve as a substrate for homologous tRNA methyltransferases in vitro , with 5-methylcytidine accounting for 93% of the total nucleosides formed under extent assay conditions. While 5-azacytidine did not affect the activities and capacities of tRNA adenine and guanine methyltransferases during the first 7 hr after drug administration, a subsequent delayed rise in the activities and capacities of tRNA 1-methyladenosine, N 2, N 2-dimethylguanosine, and N 2-methylguanosine methyltransferases was observed in addition to continued inhibition of tRNA cytosine-5-methyltransferase. These data indicate that the in vivo effect of 5-azacytidine on the 5-methylcytidine content of mouse liver tRNA is caused by decreased activity and capacity of tRNA cytosine-5-methyltransferase. Since related effects have been noted recently for 5-fluoropyrimidines (5-fluorouracil, 5-fluorouridine, and 5-fluorocytidine), the specific inhibition of post-transcriptional modification reactions of RNA represent a hitherto unrecognized common property of 5-azapyrimidine and 5-fluoropyrimidine antimetabolites.