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 X 10(6) atoms of Fe/cell per 30 min in uninduced cells, increased to 3 X 10(6) 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.

Erythroid differentiation and protoporphyrin IX down-regulate frataxin expression in Friend cells: characterization of frataxin expression compared to molecules involved in iron metabolism and hemoglobinization

“…The expression of Nramp2 and TfR mRNA was important to assess because their protein products act as partners involved in Fe uptake from Tf, 48,49 which is increased on erythroid differentiation. 40,50 In accordance with previous studies, 38 we found that the expression of ␤-globin mRNA increased markedly and significantly (P Ͻ .00005) during the incubation with DMSO (3 experiments), whereas it remained very low in the absence of this inducing agent ( Figure 6E). In fact, the normalized level of ␤-globin mRNA expression was 85-fold greater in induced compared to uninduced Friend cells at the 120-hour time point ( Figure 6E).…”

“…67 Interestingly, depletion of frataxin in FA patients does not result in marked defects in heme synthesis. 20 Because a reduction in frataxin expression occurs concurrently with an increase in Fe uptake, heme synthesis, and TfR-and ␤-globin mRNA expression 26,37,38,40,50,56,64 (Figures 1B, 2, and 6), this decrease in frataxin levels may facilitate these latter processes. Indeed, it is well known in erythroid cells that the limiting factor for heme synthesis is Fe acquisition.…”

“…37,38 The marked up-regulation of TfR expression results in a significant increase in Fe uptake ( Figure 1B) as demonstrated by others. 26,40,50,56,64 This Fe is used for heme synthesis within the mitochondrion ( Figure 1B). 26,40,50,56 The decrease in frataxin protein levels began after 24 hours of induction with DMSO, with the greatest decrease becoming obvious between 72 and 120 hours (Figure 3).…”

“…26,40,50,56,64 This Fe is used for heme synthesis within the mitochondrion ( Figure 1B). 26,40,50,56 The decrease in frataxin protein levels began after 24 hours of induction with DMSO, with the greatest decrease becoming obvious between 72 and 120 hours (Figure 3). The decreased frataxin expression is correlated with increased Fe uptake by the mitochondrion and the induction of heme biosynthesis.…”

“…These data examining Fe uptake are similar to those found by others using Friend cells. 40,50 To examine the possible role of frataxin in Fe or heme metabolism, experiments were designed to assess the effect on Frda messenger RNA (mRNA) levels by inducing erythroid differentiation in Friend 707 cells ( Figure 2). To further investigate the function of frataxin, we also assessed changes in expression of ␤-globin mRNA (Figures 2, 6, and 7), and TfR and Nramp2 mRNAs ( Figures 6 and 7).…”