Iron uptake from transferrin and transferrin endocytic cycle in friend erythroleukemia cells

Abstract

Several aspects of iron metabolism were studied in cultured Friend erythroleukemia cells before and after induction of hemoglobin synthesis by dimethyl sulfoxide. The maximal rate of iron uptake from ⁵⁹Fe-labeled transferrin, 1.5 × 10⁶ atoms of Fe/cell per 30 min in uninduced cells, increased to 3 × 10⁶ atoms/cell after 5 days of induction. The increase in iron uptake was not accompanied by a proportional increase in the number of transferrin receptors detected by ¹²⁵I-labeled transferrin binding, suggesting a more efficient iron uptake by transferrin receptors in induced cells, with the rate of about 26 iron atoms per receptor per hour, compared to 15 atoms in uninduced cells. In agreement with this conclusion are results of the study of cellular ¹²⁵I or ⁵⁹Fe labeled transferrin kinetics. In the induced cells transferrin endocytosis and release proceeded with identical rates and all the endocytosed iron was retained inside the cell. On the other hand, transferrin release by uninduced cells was significantly slower and a substantial part of internalized ⁵⁹Fe was released. On the basis of these results, different efficiency of iron release from internalized transferrin, accompanied by changes in cellular transferrin kinetics, is proposed as one of the factors determining the rate of iron uptake by developing erythroid cells.