Normal G1/S transition and prolonged S phase within one cell cycle after seeding cells in the presence of an ornithine decarboxylase inhibitor.

We have previously found that DNA replication was affected within one cell cycle after seeding Chinese hamster ovary (CHO) cells in the presence of the polyamine biosynthesis inhibitor 2-difluoromethylornithine (DFMO). We could, however, not rule out if this was due to an effect on the G1/S transition and/or on DNA synthesis elongation. In the present paper, we use a bromodeoxyuridine- flow cytometric method to more specifically study the G1/S transition, the S phase length, and the progression of cells from S phase through G2+M and into G1, after seeding plateau phase CHO cells at low density in the absence or presence of 5 mM DFMO. We report here that DFMO-induced polyamine depletion increased the length of the S phase within one cell cycle after seeding of CHO cells in the presence of the inhibitor. No effect on the G1/S transition was observed until 2 days after seeding, suggesting that a DFMO-induced lengthening of the G1 phase occurred later than the effect on S phase progression. These results imply that the G2+M phase was not prolonged until 2 days after seeding CHO cells in the presence of DFMO.