Depression of thymidylate synthetase activity in response to cytosine arabinoside.

Depression of thymidylate synthetase activity by cytosine arabinoside in CCRF-CEM cells was correlated with a blocking of precursor incorporation into RNA and with cell lysis. With increasing concentrations of cytosine arabinoside in the culture media, a proportional decrease of uridine incorporation into RNA and decrease in thymidylate synthetase activity occurred and was accompanied by lysis of the cells. These effects continued to develop with drug concentrations in excess of that required to block thymidine incorporation into DNA. The addition of deoxycytidine at concentrations that failed to reverse inhibition by cytosine arabinoside of thymidine incorporation into DNA reversed the drug effects on uridine incorporation, thymidylate synthetase activity, and cell lysis. Inhibition of precursor incorporation into RNA was also observed with isotopically labeled glycine and hypoxanthine. Cytidine could not be substituted for deoxycytidine to reverse drug effects on uridine incorporation and thymidylate synthetase. Deoxycytidine blocked intracellular accumulation of cytosine arabinoside, while cytidine was ineffective in blocking the drug uptake. We conclude that depression of thymidylate synthetase activity by cytosine arabinoside is a secondary drug action requiring drug concentrations in excess of that required to inhibit thymidine incorporation into DNA and is attributed to an inhibition of RNA synthesis that results from the action of the drug.