The surface of rat hepatocytes can transfer iron from stable chelates to external acceptors

Abstract

The chelator diethylenetriaminepentaacetate (DTPA) forms a stable complex with iron that does not donate iron to transferrin under physiological conditions, i.e., pH above 7 and isotonic milieu. It does, however, deliver iron to hepatocytes. This uptake is initiated by a mobilization of the metal from the complex by the cell surface. When an external chelator is added simultaneously, it can bind the iron and inhibit its accumulation by the cells. This is shown here with the impermeant siderophore conjugate hydroxyethyl-starch coupled desferrioxamine, as well as with apotransferrin. We also demonstrate exchange of iron between DTPA and holo-transferrin, or at least movement from the chelator to the protein, which may have lost its iron to the cell in advance, providing new binding sites for mobilized iron. The efficient hepatocyte iron donor lactoferrin greatly stimulates iron uptake from DTPA, apparently by binding iron and transferring it into the cells by endocytosis. Ferritin is unable to do this; therefore, the mobilization of iron is not caused by a reducing activity at the cell surface, because iron is readily transferred from DTPA to ferritin by the reductant ascorbic acid. The transfer process is dependent on the temperature, the time, and the amount of cells present, and is partly inhibited by sulfhydryl reagents. We conclude that this activity represents a hitherto unidentified first step in the movement of iron through the cell membrane and may be relevant for transferrin-bound, as well as for non-transferrin-bound, iron uptake by hepatocytes. (HEPATOLOGY 1998;27:1075-1080.)The uptake of iron by mammalian cells occurs largely by receptor-mediated endocytosis. The parenchymal cells of the liver, among others, have been shown to possess additional pathways for extracting iron from the plasma protein transferrin without the need of specific receptor binding. [1][2][3][4] The mechanism of this uptake is largely unknown, although binding to low-affinity sites and adsorptive endocytosis may explain iron accumulation. However, it is not known why and how the iron is released from transferrin without the aid of the receptor. When diferric transferrin bound to the receptor enters the endocytic compartment on its recycling pathway, the receptor itself facilitates the release of iron at the lowered pH and thus makes possible the cellular accumulation of the iron within the very short cycling times of transferrin. 5 In the absence of the receptor, iron release is much slower. Previous evidence suggests the existence of a plasma-membrane site for release of iron from stable chelate complexes and transfer to the cell interior, 6-9 which might also be used by transferrin bound to non-receptor-binding sites. 3 This hypothesis is strengthened in this study, in which a competition between various iron-binding and chelating agents and the hepatocyte surface is demonstrated. A putative iron-binding or -mobilizing activity is able to mediate the transfer of iron from an extracellular chelate to another chelator or into th...

Differential expression of peroxiredoxin subtypes in human brain cell types

“…Even heterozygotes for this mutation, which have 50-70% of normal plasma Tf concentrations at weaning, have increased hepatic Fe concentrations at weaning (Malecki et al, 1998b). Hepatocytes can take up Fe from the N-terminal fragment of Tf, which contains the iron-binding domain, but not the receptor-docking domain (Thorstensen et al, 1995), and from other Fe chelates (Scheiber and Goldenberg, 1998). Thus, there is a high-affinity Fe uptake mechanism on these cells independent of the Tf receptor.…”

Expression of SFT (Stimulator of Fe Transport) Is Enhanced by Iron Chelation in HeLa Cells and by Hemochromatosis in Liver

“…Cells were iron-loaded with addition of 65 M Fe-NTA (1:1) to the culture medium for the times shown. These conditions are not necessarily selective for nontransferrin-bound iron uptake, but transferrin-independent iron transport has been demonstrated in studies employing Fe-NTA (16 -18), as well as Fe-citrate (18,19), Fe-ascorbate (19 -23), and Fe-diethylenetriaminepentaacetic acid (24). Fe-NTA was chosen for this investigation, because NTA is membrane-impermeable and provides a stably chelated form of Fe(III); others have shown a 4-fold increase in cellular iron content under these experimental conditions (25).…”