Iron overload and inhibition of heme synthesis restore iron-dependent Fer expression in differentiating mouse erythroblasts. Fer expression as determined by Western blotting in erythroid progenitors differentiating for 48 hours. (A) Cells were either incubated for 24 hours with Fe₂-Tf (6.3-100 μM; highest concentration corresponds to 8 times the physiologic level; top left panel), or Des (50 μM), Fe₂-Tf (12.5 μM), FAC (63 μM Fe) and SA (0.2 μM; inhibition of heme synthesis) (top right panel). FAC probably can enter the cells directly, bypassing Tf/TfR-mediated endocytosis and the assumed vectorial iron transport from endosomes into mitochondria, and may thus lead to direct cytosolic iron overload ERK1/2 (bottom panels), loading control; membranes restained with corresponding antibody. (B) Hemoglobin synthesis in the cells described in panel A.