Mechanismof 5 Azacytidine-inducedTransfer RNACytosine-5-methyltransferaseDeficiency 1

The administration of 5-azacytidine to mice leads to a spe cific, rapid, time-dependent, and dose-dependent decrease of transfer RNA (IRNA) cytosine-5-methyltransferase activity of mouse liver and the synthesis of lANA specifically lacking 5methylcytidlne. The mechanism of this enzyme deficiency was investigated. The pretreatment of mice with RNA synthesis inhibitors such as actinomycin D and D-galactosamine pre vented the enzyme deficiency induced by 5-azacytidine admin istration. These results suggested that RNA synthesis was a prerequisite for the induction by 5-azacytidine of the enzyme inhibition in vivo. Indeed, a slowly sedimenting RNA (4 to 75) from the livers of mice treated with 5-azacytidine, when present in an in vitro tRNA methyltransferase assay, decreased specif ically the activity of tRNA cytosine-5-methyltransferase. The pretreatment of mice with actinomycin D or D-galactosamine prior to the administration of 5-azacytidine effectively pre vented the formation of such inhibitory RNA in vivo as deter mined by an in vitro tRNA methyltransferase assay. These results indicate that the administration of 5-azacytidine to mice leads to the rapid synthesis of a low-molecular-weight RNA fraction which is capable of specifically inactivating tRNA cy tosine-5-methyltransferase activity in vivo and in vitro.