Cytochalasin D inhibits the progression from the Go to S phase at the mid-prereplicative stage in GC-7 cells stimulated with serum.

When the growth of serum-arrested GC-7 cells, a clone from African green monkey kidney, was induced by the addition of 10% calf serum, they began to enter S phase after 15-16 h. When stimulated cells were cultured in the presence of 0.6 micrograms/ml of cytochalasin D, the entrance into S phase was inhibited. Treatment of cells with cytochalasin D during the period earlier than 8 h or later than 11 h after the serum stimulation showed no or little inhibitory effect on the entrance of cells into S phase. Inhibition of the entrance into S phase was observed only when stimulated cells were treated with cytochalasin D during the periods including 9-10 h after stimulation. A rapid increase in protein synthesis occurred 9-12 h after the serum stimulation and was inhibited in the presence of cytochalasin D. These and other results suggested that in the course of the prereplicative process from Go through S phase only the stage around 9-10 h after the start of the cell cycle was sensitive to cytochalasin D and that the block of the cycle was correlated with the inhibition of protein synthesis at this stage.