Characterization of a transferrin-independent uptake system for iron in HeLa cells.
HeLa cells incubated in serum-free medium accumulated 59Fe ("non-transferrin iron") when incubated with either 59Fe-citrate, 59Fe-nitrilotriacetate, or 59Fe dissolved in Tricine ascorbate. Accumulation of iron was time-, concentration-, and Ca2+-dependent and was saturable. Uptake of non-transferrin (non-Tf) iron was transferrin-independent because of the fact that uptake occurred at pH 5.5, a pH at which transferrin binds iron poorly and at which transferrin is not internalized by cells. Uptake of non-Tf iron was less affected than uptake of transferrin iron by 1) exposure of cells to trypsin, a maneuver that cleaves Tf receptors, or 2) incubation of cells with phenylarsine oxide, an agent that inhibits both fluid- and receptor-mediated internalization. After exposure of cells to non-Tf iron at 37 degrees C, most of the cell-associated radioactivity was recovered in heme and ferritin, demonstrating that iron gained access to intracellular compartments and was not simply adsorbed to the cell surface. Uptake of non-Tf iron could be partially blocked by Cu2+ in a dose-dependent manner, while the accumulation of transferrin-bound iron was unaffected by Cu2+. Other transition metals, such as Zn2+, Cd2+, and Mn2+ were able to inhibit the uptake of non-Tf iron to different degrees. The accumulation of 109Cd was inhibited by incubation of cells with non-Tf iron, Cu2+, or Mn2+. The extent of inhibition was concentration- and metal-dependent. A number of cultured cell lines including HeLa, human skin fibroblasts, and Chinese hamster ovary cells demonstrated uptake of non-Tf iron and 109Cd. Additionally, an endosome acidification mutant of Chinese hamster ovary cells, which exhibited an increase in non-Tf iron uptake, also exhibited an increase in the uptake of Cd2+. These observations suggest that the characteristics of the non-Tf iron transport system in HeLa cells are similar if not identical to those reported for perfused rat liver (Wright, T. L., Brissot, P., Ma, W.-L., and Weisiger, P. A. (1986) J. Biol. Chem. 261, 10909-10914) and suggest the existence of a family of transition metal transport systems, each with a different metal specificity.
Sturrock
A
A
Department of Pathology, University of Utah School of Medicine, Salt Lake City 84132.
Alexander
J
J
Lamb
J
J
Craven
C M
CM
Kaplan
J
J
D001152:Arsenicals / Q000494:pharmacology; D001667:Binding, Competitive; D001692:Biological Transport; D002104:Cadmium / Q000378:metabolism; D004533:Egtazic Acid / Q000494:pharmacology; D006367:HeLa Cells / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000378:metabolism*; D007700:Kinetics; D013696:Temperature; D014168:Transferrin / Q000378:metabolism; D014357:Trypsin / Q000494:pharmacology
D001152:Arsenicals; D014168:Transferrin; D002104:Cadmium; C029341:oxophenylarsine; D004533:Egtazic Acid; D007501:Iron; D014357:Trypsin