Effect of the MEK1 inhibitor, PD 098059, on CD34⁺ cells. (A) Cells cultured for 8 days in IL-3, IL-6, and SCF (day-8 cells) were incubated for 18 hours in the absence of growth factors to allow re-expression of growth factor receptors. Cells were then preincubated with 30 μmol/L PD 098059 or DMSO diluent for 30 minutes before stimulation with GM-CSF for 7 minutes, and p42/ERK2 activation was measured by gel retardation assay. Data from 2 separate experiments are shown. (B) Effect of PD 098059 on PLA₂activity of primary CD34⁺ cells (day-1 cells, n = 5) or day-8 cultured cells (n = 3). Cells were incubated for 30 minutes with 30 μmol/L PD 098059 or DMSO diluent, and arachidonate release was measured after 10 minutes of priming with 100 ng/mL SCF, 10 ng/mL GM-CSF, or 500 ng/mL TPA as indicated, followed by activation with 1 μmol/L A23187 for 20 minutes. Basal release of arachidonate in unstimulated samples was subtracted from the A23187-stimulated values, and the data are expressed as the percentage of incorporated cellular radioactivity. The statistical significance of the difference between DMSO and PD-treated cells is shown: *.05 > P > .01; **.01 > P > .001 (Student’s paired t-test).