Activation of p38 and ERKs is required for Epo-dependent cell growth of FD-EPO cells. (A and B) Both SB203580 and PD98059 inhibit Epo-dependent cell growth. Various concentrations of SB203580 (A) or PD98059 (B) were mixed in FD-EPO cell culture, and the effect of the inhibitors on Epo-dependent cell growth was measured by MTT assay. (C) SB203580 blocked p38 activity. The p38 activity was assayed in the presence (+) or absence (−) of SB203580 (50 μmol/L) in Epo-stimulated FD-EPO cells. (D) PD98059 inhibited ERKs. The activity of ERKs was measured with (+) or without (−) PD98059 (200 μmol/L) in Epo-stimulated FD-EPO cells.