Microtubule‐disrupting agents can independently affect the prereplicative period and the entry into S phase stimulated by prostaglandin F2α and fibroblastic growth factor

Abstract Fibroblastic growth factor (FGF) and prostaglandin F2α (PGF2α) are two different growth factors in Swiss 3T3 cells: each stimulates the initiation of DNA synthesis after a lag phase of 14–15 hr. Colchicine, colcemid, and nocodazole have no mitogenic effect on their own, but each has a synergistic effect on the rate of initiation of DNA synthesis stimulated by either FGF or PGF2α. Independent of the growth factor, the microtubule‐disrupting drug had to be added before 8 hr of the lag phase to enhance the stimulation. Colchicine could be removed at 5 hr of the lag phase with no impairment of the synergistic effect. However, both colcemid and nocodazole had to remain present up to 10 hr of the lag phase to achieve the full synergy. This result can be accounted for by the fact that colchicine, in contrast to colcemid and nocodazole, binds almost irreversibly to microtubules, and it strengthens the interpretation that the state of microtubular organization plays a role in regulating the rate of entry into S phase. Preincubating quiescent Swiss 3T3 cells with colchicine, colcemid, or nocodazole before stimulation by FGF or PGF2α shortened the lag phase by about 3 hr. When the microtubule‐disrupting drug was removed prior to stimulation, the lag phase was also shortened, but there was no enhancement of the rate of initiation of DNA synthesis, except upon preincubation with colchicine. This suggests that microtubule disruption can independently affect the length of the lag phase and the rate of initiation of DNA synthesis, and that these two phenomena can be uncoupled.