Microtubule-disrupting agents affect two different events regulating the initiation of DNA synthesis in Swiss 3T3 cells

The stimulatory effect of epidermal growth factor, alone or with insulin, on the rate of initiation of DNA synthesis in Swiss 3T3 cells can be synergistically enhanced by the addition of either Colcemid or colchicine at 1 microM. However, both Colcemid and colchicine can exert the synergistic effect only when added earlier than 8 hr of the prereplicative period (lag phase). Removal of Colcemid (which allows for rapid reassembly of microtubules) earlier than 10 hr of the lag phase results in a loss of the synergistic effect. This suggests that microtubules must remain disrupted for longer times to accomplish some putative event(s) necessary for increasing the rate of initiation of DNA synthesis. Preincubation of quiescent cells with either Colcemid or colchicine for 8 hr prior to adding epidermal growth factor, alone or with insulin, shortens the lag phase by about 4 hr, irrespective of the resulting rate of initiation of DNA synthesis. These results suggest that the state of microtubules is affecting independently at least two different events involved in regulating time initiation of DNA synthesis.