Non-Transferrin Iron Uptake by HeLa Cells Cultured in Serum-Free Media with Different Iron Sources1)

Dedicated to Prof. Dr. Erich Kaiser on the occasion of his 70th birthday Summary: HeLa cells cultured in defined serum-free media supplied with iron either in the form of diferric transferrin (transferrin-dependent cells), ferric citrate at 500 μιηοΐ/ΐ (high-iron dependent cells) or ferric citrate at 5 μπιοΐ/ΐ (low-iron dependent cells) accumulate iron from ferric citrate in different ways. The uptake rate in transfer-rin-dependent cells is always much lower than in the other two lines. In all three, the uptake rate rises almost linearly with the concentration of iron up to 10 μιηοΐ/l. In high-iron dependent cells, the uptake of radiolabelled iron is suppressed by a 100-fold excess of the iron complex, whereas this same excess stimulates iron uptake in the other two lines. The same concentrations of pure citrate completely inhibit iron uptake by all three types of cell. Only high-iron dependent cells take up citrate at measurable and reproducible rates. These rates are independent of the presence of iron, and the uptake is inhibited by an unlabelled surplus. The pH-dependence of iron uptake in high-iron dependent cells is also different from that of the other cells. Low-iron dependent cells transferred to medium containing 500 μιηοΐ/ΐ iron show increased uptake rates within 3 to 7 h, and after overnight maintenance in this medium they acquire the uptake characteristics of high-iron dependent cells. The special characteristics of iron uptake by high-iron dependent cells are paralleled by low binding activity o