Hydroxyurea treatment affects the G1 phase in next generation CHO cells

DNA replication kinetics were studied in populations of synchronized CHO cells treated in the previous generation with hydroxyurea. These CHO cells were re-synchronized by selective detachment of mitotic cells after previously synchronized G1 traversing cultures were treated with 0.1 mM and 2 mM hydroxyurea for 9 and 13 h. Our results show that these cells exhibit a shortening of G1 of at least 1 h relative to cells selected in mitosis from untreated exponentially growing cultures. Survival studies indicated that the hydroxyurea treatments did not affect plating efficiencies. Cell viability was reduced when the initially synchronized populations were blocked with 2 mM, but not 0.1 mM hydroxyurea for greater than 13 h. DNA replication measurements after these blocks showed that all cultures treated with 2 mM hydroxyurea for either 9, 13 or 15 h were blocked at the same point near the G1/S boundary, and then progressed through S phase with similar kinetics. The observed shortening of G1 in the next generation of these cells was independent of both the concentration (0.1 or 2.0 mM) and the time (9 or 13 h) of the hydroxyurea block. These results suggest that specific events relating to the next cell generation can be uncoupled from DNA synthesis and can occur when hydroxyurea inhibits normal cell cycle traverse of G1 cells into and through S phase.