MEK inhibitor PD98059 enhances EPO-induced phosphorylation of MEK. (A) BaF3-EpoR cells were treated with specified concentrations of PD98059 for 10 minutes, then treated with EPO for 5 minutes. Cytoplasmic extracts were subjected to Western blotting with anti–phospho-MEK antibody. (B) The same membrane was stripped and reprobed with anti–phospho-ERK1/2 antibody. (C) Cells were treated with EPO alone, inhibitor alone, or inhibitor then with EPO for 5 minutes. Cytoplasmic extracts were subjected to Western blotting with anti–phospho-MEK antibody.