Addition of growth factors, such as prostaglandin F2. or fibroblastic growth factor, to quiescent Swiss mouse 3T3 cells resulted in an abrupt increase in the rate of initiation of DNA synthesis after a lag phase of [13][14][15] hr. This increase could be quantified by a rate constant k. Addition of colchicine, Colcemid, or vinblastine had a synergistic effect on the initiation of DNA synthesis triggered by PGF2. or FGF by increasing the value of k. These drugs alone had no effect. Colchicine had a synergistic effect only if added within 8 hr of the PGF2. or FGF addition. Also, colchicine exerted its full effect when it was present only for the first 5 hr with either growth factor. These results suggest that an intact cytoskeleton is not required for the initiation of DNA synthesis. Furthermore, cytoskeleton-disrupting drugs enhance the stimulatory effect of the growth factors. (8,9) or growth factors (5, 10, 11) to quiescent fibroblastic cells is followed by a constant lag phase, before an abrupt increase in the rate of initiation of DNA synthesis is observed. The latter process follows first-order kinetics and can be quantified by a rate constant k (4,8,12). Growth factors initially interact with receptors of the plasma membrane to deliver the mitogenic signals required for cell proliferation (13)(14)(15). Recently, it has been suggested that in chicken embryo and Swiss mouse 3T3 cells the association of microtubules (MT) and microfilaments with proteins of the plasma membrane may play a regulatory role in conveying the signals delivered by the growth factors from the cell surface to their specific intracellular targets (16, 17).Here we show the effect of different cytoskeleton-disrupting drugs at various times after the addition of prostaglandin F2ca (PGF22,) (18) or fibroblastic growth factor (FGF) (19), alone or in combination with hormones, on the initiation of DNA synthesis and cell division in confluent quiescent Swiss 3T3 cells.t In our system, colchicine, Colcemid, and vinblastine, which disrupt MT (20, 21), do not prevent the progression through the lag phase. In fact, these drugs increase the rate of initiation of DNA synthesis stimulated by growth factors. (Fig. 1C). Vinblastine at concentrations above 10 MM caused detachment of cells from the substratum. Similarly, colchicine, Colcemid, or vinblastine increased the stimulatory effect of FGF (50 ng/ml) on the initiation of DNA synthesis from 30 to 80% over 28 hr. The maximal effect of colchicine or Colcemid was at 0.4,uM ( Fig. 1 D and E
