Erythropoietin induction of PKD phosphorylation. (A) Erythropoietin–deprived G1ER cells were stimulated with erythropoietin, followed by immunoblotting for phospho- and total PKD. (B) Cytokine-starved human erythroid progenitors were stimulated with erythropoietin and analyzed as in panel A. (C) Impact of erythropoietin dosage and duration on steady-state PKD phosphorylation. Left: Human progenitors cultured 4 days with the indicated doses of erythropoietin. Right: Human progenitors cultured for the indicated durations with 4.5 U/mL erythropoietin. (D) Impact of iron availability on steady-state PKD phosphorylation. Human progenitors were cultured in erythroid medium under either iron replete (100% transferrin saturation) or iron restricted (15% transferrin saturation) conditions.