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,. 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 gM) 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]p1. 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.

Modulation of tyrosine, serine, and threonine phosphorylation and intracellular processing of the epidermal growth factor receptor by antireceptor monoclonal antibody

“…Studies on the transferrin receptor (May et al, 1984;Cox et al, 1985) and on the type I1 insulin-like growth factor receptor (Corvera and Czech, 1985) have suggested that increased phosphorylation of those receptors might affect the process of internalization. We have shown, however, that a n anti-EGF receptor mAb elicited rapid receptor internalization into endosomes without stimulating receptor phosphorylation (Sunada et al, 1986).…”

A general map of iron metabolism and tissue-specific subnetworks

“…First, ALAS2, the erythroid-specific form of ALAS, contains an IRE in the 5′ UTR258, 259 and neither its synthesis nor its activity is inhibited by heme260. In reticulocytes, heme inhibits the internalization of transferrin by TfR1261, 262. The non-IRE form of DMT1 dominates in reticulocytes and is found on the membranes of endosomes containing TfR1263.…”

Haem inhibits iron uptake subsequent to endocytosis of transferrin in reticulocytes

“…It was proposed that haem inhibits either iron (Ponka et al, 1974) or bicarbonate (Schulman et al, 1974b) release from transferrin; however, the release of both are probably inhibited by the same mechanism, since iron release from transferrin is accompanied by the release of bicarbonate, and vice versa (Schulman et al, 1974b;Martinez-Medellin & Schulman, 1973). More recently it has been suggested that haem inhibits the internalization of transferrin by reticulocytes (Tacopetta & Morgan, 1984;Cox et al, 1985), but the results of the present study indicate that this is probably not the primary site of haem action. We demonstrated that, at low concentrations, haem inhibits cellular iron uptake and even more its incorporation into haem, but does not inhibit the initial rate of transferrin uptake or the internalization of transferrin by the cells.…”

“…It has long been recognized that haem is a feedback regulator of haem synthesis in erythroid cells (London , 1964;Ponka & Neuwirt, 1969), and although there is no doubt that haem inhibits haem synthesis in reticulocytes (Ponka et al, 1974;Schulman et al, 1974b;Morgan, 1981;Iacopetta & Morgan, 1984;Cox et al, 1985;Ponka & Schulman, 1985a), in which it can affect only post-transcriptional processes, there has been controversy conceming the mechanism involved. The original (London et al, 1964) and widely held view (Ibraham et al, 1983) is that haem feedback-inhibits the first enzyme in the haem-biosynthetic pathway, namely 6-aminolaevulinic acid synthase.…”

“…Originally it was proposed that haem inhibits either iron (Ponka et al, 1974) or bicarbonate (Schulman et al., 1 974b) release from transferrin, but more recently (lacopetta & Morgan, 1984;Cox et al, 1985) it was suggested that haem inhibits the internalization of transferrin by immature erythroid cells. In the present study we re-examined the effect of haem on Fe-transferrin interaction with reticulocytes.…”