Figure 4.

Raf-induced cell cycle arrest is irreversible. (A) GFPΔRaf-1_[YY]:ER- and (B) GFPΔRaf-1_[DD]:ER-expressing IMR-90 cells and (C) control cells (Babe) plated on Cytostar T plates were rendered quiescent in serum-free medium in the absence or presence of 1 μm 4-HT for 24 hr. At that time cells in the absence of 4-HT were stimulated with media containing 10% vol/vol FCS. Cells in the presence of 4-HT were either maintained in the continuous presence of 4-HT (4-HT continuous) or were washed extensively to remove the 4-HT (4-HT washed) prior to stimulation with media containing 10% vol/vol FCS. Cell proliferation was then measured over the following 6 days by the incorporation of methyl-[¹⁴C]thymidine into DNA. (□) 10% FCS; (▵) 4-HT continuous; (○) 4-HT washed.