title: Hemin inhibits internalization of transferrin by reticulocytes and promotes phosphorylation of the membrane transferrin receptor.
abstract: Addition of hemin to reticulocytes inhibits incorporation of iron from transferrin [Ponka, P. & Neuwirt, J. (1969) Blood 33, 609-707]. Heme also regulates protein synthesis in immature erythroid cells through its effects on phosphorylation of the initiation factor eIF-2. We have therefore examined its effects on endocytosis of iron-transferrin and phosphorylation of the transferrin receptor. Hemin (10-50 microM) reduced iron transport but increased cell-associated transferrin. When intracellular iron delivery was inhibited by NH4Cl, no such increase in cell-associated transferrin was seen. During uptake of 125I-labeled transferrin in the steady state, the use of a washing technique to dissociate bound transferrin on the cell membrane showed that radioligand accumulated on the surface of hemin-treated cells. Hemin reduced the initial influx of transferrin, thereby diminishing incorporation of iron. Receptor phosphorylation was investigated by immunoprecipitation of reticulocyte extracts after metabolic labeling with [32P]Pi. In the absence of ligand, phosphorylated receptor was chiefly localized on cell stroma. Exposure to transferrin increased cytosolic phosphorylated receptor from 15-30% to approximately 50% of the total, an effect overcome by hemin treatment. Addition of hemin in the presence of transferrin enhanced net phosphorylated receptor in the reticulocyte in association with a redistribution of phosphorylated receptor to stromal membranes. The findings suggest a possible relationship of phosphorylation to endocytosis of the transferrin receptor in reticulocytes.
authors.forename: T M
authors.initials: TM
authors.lastname: Cox
authors.forename: M W
authors.initials: MW
authors.lastname: O'Donnell
authors.forename: P
authors.initials: P
authors.lastname: Aisen
authors.forename: I M
authors.initials: IM
authors.lastname: London
mesh_terms: D000818:Animals; D004705:Endocytosis; D006418:Heme / Q000031:analogs & derivatives*; D006427:Hemin / Q000502:physiology*; D007501:Iron / Q000378:metabolism; D007700:Kinetics; D008970:Molecular Weight; D010766:Phosphorylation; D011817:Rabbits; D011956:Receptors, Cell Surface / Q000378:metabolism*; D011990:Receptors, Transferrin; D012156:Reticulocytes / Q000502:physiology*; D014168:Transferrin / Q000378:metabolism*
chemical_list: D011956:Receptors, Cell Surface; D011990:Receptors, Transferrin; D014168:Transferrin; D006418:Heme; D006427:Hemin; D007501:Iron