Fig. 7. The PD98059 MEK1/2 inhibitor blocks DNA synthesis in response to EGF or HGF. A, NIH-3T3 cells grown in 24-well plates were growth-arrested by serum starvation and then stimulated with 100 ng/ml EGF. Two h later, PD98059 was added at 50, 100, or 200 µM, as indicated. Twelve h later, cells were labeled with [3H]thymidine for an additional 2 h, and acid precipitable radioactivity was determined. Results are expressed as the mean ± SE cpm of four wells. B, A549 cells grown in 24-well plates were growth-arrested by serum starvation and then stimulated with 40 ng/ml HGF. Where indicated, PD98059 (50 or 100 µM) was added. [3H]Thymidine labeling followed, and cells were processed as described above. Results represent the mean ± SE cpm from six wells.