Inhibition of RNA methylation by intercalating agents.

Abstract

The effects of antitumor intercalating agents on the methylation of RNA were studied by the use of soluble transfer RNA (tRNA) methylases from the Novikoff ascites tumor and nucleolar ribosomal RNA (rRNA) methylases from the tumor and liver of the rat. In the soluble tRNA methylase system, ellipticine at low drug levels preferentially inhibited methylation of tRNA adenine, whereas ethidium bromide and hycanthone preferentially inhibited methylation of tRNA guanine. At higher drug levels, methylation of other tRNA bases was also significantly inhibited by ethidium bromide and hycanthone; however, methylation of tRNA pyrimidine bases was not at all affected by ellipticine. Perturbation of methyl acceptor sites by interaction of ellipticine with tRNA was indicated by decreased inhibition with increased tRNA concentrations. With a partially purified adenine methylase preparation, ellipticine was competitive with tRNA. Ethidium bromide and hycanthone appeared to interact with the enzymes rather than with the tRNA substrate, since the inhibition of a partially purified guanine methylase preparation was noncompetitive with respect to tRNA. All three drugs also inhibited rRNA synthesis and methylation in isolated nucleoli. Methylation of RNA in isolated tumor nucleoli was more susceptible to these drugs than was methylation of liver nucleolar RNA. The differential was the most marked in the case of ethidium bromide. Ethidium bromide administered to tumor-bearing rats caused a marked reduction in the content of RNA and of rRNA methylase activity in nucleoli prepared 16 hr later. Under these conditions ellipticine also caused a decrease in nucleolar RNA polymerase and rRNA methylase activities. The potential of these drugs in perturbation of RNA processing may make a significant contribution to their antitumor activity.