Effect of pharmacologic inhibitors on erythroid differentiation. CD34⁺ cells were cultured for 6 days in the presence of EPO + IL-3 + SCF. At this time point, cultures were preincubated with vehicle (0.1% DMSO), PD98059 (PD, 10 μM), SB203580 (SB, 10 μM), or z-VAD-fmk (z-VAD, 20 μM) before treatment with TRAIL. (A) Surface glyophorin A expression, reported as MFI, was measured by flow cytometry. Data represent the means ± SDs of 4 different experiments; *P < .05, compared with untreated culture. (B) The level of phosphorylated ERK1/2 was analyzed in cell lysates at 5 minutes of TRAIL treatment. Equal loading was confirmed by tubulin staining. This experiment is representative of 3 independent experiments that gave similar results.