Activation of p38 is required for Epo-induced erythroid differentiation. (A, B, and C) DAF-staining of the hemoglobinized SKT6 cells with (B and C) or without (A) Epo stimulation in the presence (C) or absence (A and B) of 5 μmol/L of p38-specific inhibitor SB203580. (D) SKT6 cells were cultured in the presence (+) or absence (−) of Epo (0.5 U/mL) and SB203580 (10 μmol/L) dissolved in 0.1% DMSO, and the hemoglobinized cells were stained with DAF. The percentage of hemoglobinized cells in the presence of Epo is defined as 100% as a control, and the inhibition or enhancement of SKT6 cell differentiation in the presence of the inhibitor is shown as a relative value against the control. Values shown are the means of five experiments. (E) Dose-dependency of SB203580 in SKT6 cell differentiation in the presence (•) or absence (○) of Epo. (F) Dose-dependency of SB203580 in SKT6 cell growth. The cell growth was measured by MTT assay. (G) The p38 activity was measured in the presence (+) or absence (−) of SB203580 (10 μmol/L) 5 minutes after Epo stimulation.