Time-course analyses of ERK1/2, p38/MAPK, and JNK1 phosphorylation in erythroid cells in response to TRAIL. CD34⁺ cells were cultured in the presence of EPO + IL-3 + SCF for 6 days and then were exposed to TRAIL for the indicated times. Equal amounts of cell lysates were analyzed for ERK1/2 (A), JNK1, and p38/MAPK (B) phosphorylation by Western blot analyses using antibodies specific for the native form of the kinases and for residues that are phosphorylated in each kinase upon activation. Tubulin staining is also shown as loading control. In panel A, protein bands were quantified by densitometry and level of P-ERK1/2 was calculated for each time point, after normalization to ERK1/2 in the same sample. Unstimulated basal expression was set as unity. A representative of 3 separate experiments is shown.