Fig. 8. Expression of Rb partially restores sensitivity to PD 98059. In A, H526-IC4 cells containing the "reverse" Tet repressor were transfected with the empty pTRE plasmid or with the pTRE plasmid containing the Rb cDNA. Individual subclones were isolated and analyzed for Rb expression in the absence and presence of doxycycline by Western blotting; shown is the Mr 110,000 region of the blot. In B, subclones expressing Rb or containing empty vector were cultured in serum-containing medium with doxycycline for 24 h to induce Rb expression, were serum-starved for 6 h, and then were placed in serum-free medium containing 100 ng/ml SCF in the presence of DMSO vehicle (control) or 80 µM PD 98059. The subclones were incubated for 12 h, at which time 1 µCi/ml [3H]thymidine was added, and the incubation was continued for an additional 3 h. The cells were then collected and [3H]thymidine incorporation was determined by scintillation counting. The data are expressed as the percent of thymidine incorporation by cultures containing PD 98059 relative to cultures containing vehicle; the data are representative of four independent experiments.