Newly synthesised DNA in ageing human cells in culture treated with 4-nitroquinoline-1-oxide.

Two lines of normal human embryonic lung fibroblasts, MRC-5 and F2002, were serially subcultured until senescence was attained. When cells were exposed to varying concentrations of the chemical carcinogen 4-nitroquinoline-1-oxide (4-NQO), the rate of DNA synthesis (as measured by thymidine incorporation) was reduced in a dose-dependent fashion in cells from both early and late passages. While the overall amount of incorporation was considerably lower in old cells, the extent of inhibition caused by 4-NQO treatment (relative to appropriate controls) was not related to culture age. Alkaline sucrose density gradient analysis of newly synthesised DNA from cells pre-treated with 4-NQO failed to detect any significant variation in the size of labelled DNA from cells examined immediately after incubation with radioactive thymidine. The shift of this labelled material to high molecular weight in 4-NQO-treated cells also showed no age-related difference.