Regulation of the transferrin-independent iron transport system in cultured cells

Mammalian cells accumulate iron via the binding of transferrin to high affinity surface receptors, or through a transferrin-independent pathway which involves the uptake of iron-organic anion chelates by a membrane-based transport system. Previously we determined that the transferrin-independent transport system was present on a wide variety of cultured cells (Sturrock, A., Alexander, J., Lamb, J., Craven, C. M., and Kaplan, J. (1990) J. Biol. Chem. 265, 3139-3145). In this communication we demonstrate that the transferrin-independent iron uptake system is regulated differently than the transferrin-mediated pathway. The activity of the transferrin-independent system was unaffected by changes in cellular growth rate, induction of DNA synthesis and cell division, or depletion of cellular iron. Exposure of cells to ferric or ferrous iron, however, resulted in a time-dependent increase in transport activity, due to a change in Vmax with no change in Km. Increased transport activity was seen in a variety of cultured cell types, occurred in the presence of cycloheximide, and persisted for hours after removal of iron. The ability of other transition metals to induce changes in transport, or to compete with iron for accumulation by the transferrin-independent uptake system, was critically dependent on the composition of the media in which the cells were incubated. Metals such as Cu2+ or Zn2+, but not Cd2+ or Mn2+, when dissolved in a balanced salt solution buffered with 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, induced changes in the transferrin-independent iron transport system. The same metals which induced changes in transport were ineffective in media containing amino acids, ascorbate, or N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine. The Vmax of the transferrin-independent iron transport system was also elevated by increases in intracellular Ca2+. The effect of iron on transport activity, however, did not result from an iron-induced release of intracellular Ca2+. These results suggest a novel form of regulation in which the presence of extracellular iron induces the appearance of previously cryptic transporters and thus accelerates the clearance of potentially toxic molecules.