DNA replication in Syrian hamster cells transiently exposed to hydroxyurea

DNA rereplication has been observed in mammalian cells transiently treated during S phase with hydroxyurea, an inhibitor of DNA synthesis. Recently, evidence has been presented which does not support this proposal. We have performed a series of similar but more defined studies with the tumorigenic Syrian hamster cell line BP6T to further investigate the possibility of DNA rereplication after transient inhibition during S phase. Synchronized BP6T cells (greater than 75%) were given a 6-h exposure to 1 mM hydroxyurea after having progressed 2 h into S phase. The DNA species synthesized up to 24 h after removal of the inhibitor have been identified by bromodeoxyuridine pulse labeling and density gradient centrifugation studies. The results indicate that no DNA rereplication had occurred in the S phase of the same cell cycle as the transient hydroxyurea treatment. DNA replicated early in the S phase in which treatment was given was not replicated again until the next cell cycle. Our observations reveal that DNA rereplication does not occur in tumorigenic Syrian hamster cells transiently insulted with hydroxyurea during S phase. Thus, these findings imply that DNA synthesis is stringently controlled in these transformed cells.