Abstract
Cells grown in the presence of ferric ammonium citrate or hemin exhibited a concentration and time-dependent decrease in 125I-transferrin (Trf) binding. In contrast, cells grown in the presence of protoporphyrin IX or picolinic acid (an iron chelator) exhibited a marked increase in Trf binding. The decrease or increase in binding activity observed under these different conditions of culture reflected, respectively, a reduction or increase in receptor number rather than an alteration in ligand receptor affinity. Growth of the cells in the presence of saturating concentrations of apotransferrin only induced a slight reduction in receptor number. Investigation of the Trf receptors' turnover and biosynthesis clearly showed that iron and hemin decreased the synthesis of Trf receptors without any modification of the receptor turnover; in contrast, protoporphyrin IX and picolinic acid markedly increased the synthesis of Trf receptors. Our results suggest that hemin, iron, and protoporphyrin IX may represent the main molecules involved in the regulation of Trf receptors.
Highlights
Regulation of Transferrin Receptors in Human Hematopoietic Cell Lines*Transferrin Receptor Assay-Purified human transferrin was conjugated with ''I by the solid-phase lactoperoxidase method (New England Nuclear, radioiodination system)as previously described (Testa etal., 1982).The binding reaction was performed in polypropylene tubes (12 X 75 mm) in RPMI 1640 medium containing 0.1% bovine serum albumin (Sigma, Fraction V)
It was recentlysuggested that polypeptide receptorson mammalianplasmamembranescan beclassified into two different categories on the basis of function (Kaplan, 1981)
K 562 cells were grown with ferric ammonium citrate (10 Fg/ml), heme (0.1 mM), picolinic acid (2 mM), apotransferrin (50 Fg/ml), iron-saturated transferrin (50 gg/ml), or protoporphyrin IX (10 pM) for 3 days before assay of Trf-binding activity
Summary
Transferrin Receptor Assay-Purified human transferrin was conjugated with ''I by the solid-phase lactoperoxidase method (New England Nuclear, radioiodination system)as previously described (Testa etal., 1982).The binding reaction was performed in polypropylene tubes (12 X 75 mm) in RPMI 1640 medium containing 0.1% bovine serum albumin (Sigma, Fraction V). The number of washes did not modify the transferrin-binding capacity of the cells since they were grownin fetal calf serum, and bovine transferrin had a very low affinity for humantransferrin receptors, as previously reported by other investigators (Ward et al, 198213)and by us (Titeux et al, 1984). Investigation of the Biosynthesis of Trf Receptors-10 X lo cells were washed three times in Dulbecco's methionine-free medium and incubated in the same medium containing 10% fetal calf serum dialyzed overnight against Hanks' medium and 100pCi of [35S] methionine (The Radiochemical Centre, Amersham, England). Proteinconcentration was determined by the dye-binding method (Bio-Rad)
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