Neocarzinostatin-induced DNA strand scission and subsequent cell cycle traverse in HeLa S3 cells

HeLa cells were synchronized at the G1-S boundary by double thymidine block and treated for 1 hr with varying concentrations of the antibiotic anticancer protein neocarzinostatin (NCS). Cells were then released from the block and allowed to resume their cycle. Aliquots were removed at various times in order to monitor cell cycle progression and to assess repair of DNA strand breaks. Dose-dependent DNA strand breakage occurred at all concentrations of NCS tested down to 0.05 microgram/ml. Flow cytometry revealed that NCS-treated cells were delayed in entering S phase and that once in S phase their rate of progression through it was retarded significantly. At all concentrations of NCS tested, the majority of cells did not enter G2 by 12 hr. Untreated cells, on the other hand, completed mitosis by this time. NCS-treated cells had little ability to repair DNA breaks. There appeared to be a correlation between the initial number of NCS-induced DNA breaks and the delayed entry into the S phase but little correlation between the lack of strand scission repair and the retarded progress through S phase.