Interferon inhibition of thymidine incorporation into DNA through effects on thymidine transport and uptake

Replenishment of medium after 72 hr of growth of HeLa‐S3 cells in dense suspension cultures increased [ 3 H]‐thymidine uptake into cells and incorporation into DNA, with the levels reaching a peak ∼ 12 hr following medium change; β interferon inhibits the enhanced uptake of [ 3 H]‐thymidine and labeling of DNA in a dose‐dependent manner. Some reduction in these processes is observed at a concentration as low as 1 u/ml, and ∼ 75% inhibition at 640 u/ml. Kinetic analysis has revealed that the rate of labeling of the acid‐soluble pool with [ 3 H]‐thymidine, measured either at 22°C, or 37°C, is reduced in interferon‐treated (640 u/ml, 24 hr) HeLa‐S3 cells. At 22°C, the initial rate of thymidine transport at a high (500 μM) thymidine concentration, determined within the first 30 sec of [ 3 H]‐thymidine addition was depressed by 44% in interferon‐treated HeLa cells. At 37°C, labeled precursors accumulate in acid‐soluble material for ∼ 8 min after the addition of [ 3 H]‐thymidine, after which an apparent equilibrium level is attained. At this temperature, the rate of thymidine uptake and the apparent equilibrium level attained were depressed by 70% in interferon‐treated HeLa cells. The reduced incorporation of [ 3 H]‐thymidine into DNA in interferon‐treated HeLa‐S3 cells can be largely explained by interferon inhibition of thymidine transport and phosphorylation.