NON-TRANSFERRIN IRON UPTAKE BY HELA-CELLS CULTURED IN SERUM-FREE MEDIA WITH DIFFERENT IRON SOURCES

HeLa cells cultured in defined serum-free media supplied with iron eit her in the form of diferric transferrin (transferrin-dependent cells), ferric citrate at 500 mu mol/l (high-iron dependent cells) or ferric citrate at 5 mu mol/l (low-iron dependent cells) accumulate iron from ferric citrate in different ways. The uptake rate in transferrin-depen dent cells is always much lower than in the other two lines. In all th ree, the uptake rate rises almost linearly with the concentration of i ron up to 10 mu mol/l. In high-iron dependent cells, the uptake of rad iolabelled iron is suppressed by a 100-fold excess of the iron complex , whereas this same excess stimulates iron uptake in the other two lin es. The same concentrations of pure citrate completely inhibit iron up take by all three types of cell. Only high-iron dependent cells take u p citrate at measurable and reproducible rates. These rates are indepe ndent of the presence of iron, and the uptake is inhibited by an unlab elled surplus. The pi-I-dependence of iron uptake in high-iron depende nt cells is also different from that of the other cells. Low-iron depe ndent cells transferred to medium containing 500 mu mol/l iron show in creased uptake rates within 3 to 7 h, and after overnight maintenance in this medium they acquire the uptake characteristics of high-iron de pendent cells. The special characteristics of iron uptake by high-iron dependent cells are paralleled by low binding activity of iron-regula tory protein to iron-responsive elements of RNA. We conclude that low- iron dependent cells maintain their iron supply from the culture mediu m by unspecific uptake of oligomeric complexes, while cells in media w ith a high content of low-molecular weight iron induce a specific upta ke system which might have a protective function.