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

Abstract

The stimulatory effect of epidermal growth factor, alone or with insulin, on the rate of initiation of DNA synthesis in Swiss mouse 3T3 cells can be synergistically enhanced by the addition of either Colcemide or colchicine at 1 .mu.M. Colcemide and colchicine exert the synergistic effect only when added earlier than 8 h of the prereplicative period (lag phase). Removal of Colcemide (which allows for rapid reassembly of microtubules) earlier than 10 h of the lag phase results in a loss of the synergistic effect. Microtubules evidently 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 Colcemide or colchicine for 8 h prior to adding epidermal growth factor, alone or with insulin, shortens the lag phase by about 4 h, irrespective of the resulting rate of initiation of DNA synthesis. The state of microtubules apparently is affecting independently at least 2 different events involved in regulating the initiation of DNA synthesis.