Concentration-dependent uptake of iron from⁵⁹Fe-diferric transferrin in HtTA-HFE cells. Cells were grown for 4 days in the presence (•) or absence (○) of 1 μg/mL doxycycline. Uptake of ⁵⁹Fe from diferric transferrin was determined as outlined in Materials and Methods. Cells were incubated for 30 minutes at 37°C. Transferrin concentrations are indicated. Data are expressed as means of triplicate samples.