Transferrin receptors, iron transport and ferritin metabolism in friend erythroleukemia cells

Abstract

Abstract Four aspects of iron metabolism were studied in cultured Friend erythroleukemia cells before and after induction of erythroid differentiation by dimethyl sulfoxide. (1) The binding of 125I-labeled transferrin was determined over a range of transferrin concentrations from 0.5 to 15 μM. Scatchard analysis of the binding curves demonstrated equivalent numbers of transferrin binding sites per cell: 7.78 ± 2.41 · 105 in non-induced cells and 9.28 ± 1.57 · 105 after 4 days of exposure to dimethyl sulfoxide. (2) The rate of iron transport was determined by measuring iron uptake from 59Fe-labeled transferrin. Iron uptake in non-induced cells was approx. 17 000 molecules of iron/cell per min; 24 h after addition of dimethyl sulfoxide it increased to 38 000, and it rose to maximal levels of approx. 130 000 at 72 h. (3) Heme synthesis, assayed qualitatively by benzidine staining and measured quantitatively by incorporation of 59Fe or [2-14C]glycine into cyclohexanone-extracted or crystallized heme, was not detected until 3 days after addition of dimethyl sulfoxide, when 12% of the cells were stained by benzidine and 6 pmol 59Fe and 32 pmol [2-14C]glycine were incorporated into heme per 108 cells/h. After 4 days, 60% of the cells were benzidine positive and 34 pmol 59Fe and 90 pmol [2-14C]glycine were incorporated into heme per 108 cells/h. (4) The rate of incorporation of 59Fe into ferritin, measured by immunoprecipitation of ferritin by specific antimouse ferritin immunoglobulin G, rose from 4.4 ± 0.6 cells to 18.4 ± 1.3 pmol 59Fe/h per 108 cells 3 days after addition of dimethyl sulfoxide, and then fell to 11.6 ± 3.1 pmol 4 days after dimethyl sulfoxide when heme synthesis was maximal. These studies indicate that one or more steps in cellular iron transport distal to transferrin binding is induced early by dimethyl sulfoxide and that ferritin may play an active role in iron delivery for heme synthesis.