Dose-response curve of IRE-binding activity of IRP upon treatment with Epo. THP-1 or K562 cells were treated with increasing concentrations of human recombinant Epo (Epo) for 24 hours. After this treatment, detergent cell extracts were prepared and RNA-protein interaction was assayed by gel retardation assay as described in the Materials and Methods. The IRE/IRP complexes were then densitometrically quantified and related to the amount of RNA-protein interaction in untreated cells (control), which was set at 1 (ie, 100%). The results are plotted as means ± SD for four (K562) or three (THP-1) independent experiments. The calculation of statistical differences from the control was performed using the Student's t-test. #P < .01; *P < .05; +P < .05, not significant.