High Induction of Poly(ADP‐ribose) Polymerase Activity in Bleomycin‐resistant HeLa Cells

Poly(ADP‐ribose) polymerase activity was measured in bleomycin (BLM) ‐ resistant HeLa (HeLa‐BLMr) and the parental HeLa cells after BLM treatment. HeLa‐BLMr cells, which had been subcultured in growth medium containing 1 μg/ml of BLM, showed a 3.75‐fold higher enzyme activity than did HeLa cells, but this activity was decreased to the same level as that of HeLa cells after 48 h of BLM‐free cultivation. When HeLa and HeLa‐BLMr cells after a 48‐h cultivation in BLM‐free growth medium were treated with BLM, the enzyme activity was induced at a higher level (2.5–3.8 times) in HeLa‐BLMr than in HeLa cells and was inhibited markedly in HeLa‐BLMr and slightly in HeLa cells by nicotinamide, an inhibitor of this enzyme. The BLM‐induced cell killing by nicotinamide was highly potentiated (about 18 times) in HeLa‐BLMr as compared to HeLa cells.