Inhibition of uptake of transferrin-bound iron by human hepatoma cells by nontransferrin-bound iron

The liver acquires iron from transferrin by transferrin receptor-mediated (TR) and transferrin receptor-independent pathways (NTR) and from nontransferrin-bound iron (NTB-Fe). Iron uptake by the NTR processes involves an iron-carrier mediated step. Experiments, using human hepatoma cells (HuH7) transfected with TR antisense (sense for control) RNA expression vectors to suppress TR expression, were performed to examine the effect of unlabeled NTB-Fe as iron citrate on the uptake of 59Fe-125I-transferrin. This was to determine if the uptake of transferrin-bound iron (Tf-Fe) and NTB-Fe uptake is mediated by a common iron-carrier. Iron citrate inhibited the uptake of 59Fe-transferrin (2.5 micromol/L Fe) in a concentration-dependent manner with a maximum effect when the citrate-iron:Tf-Fe molar ratio was 10:1. Transferrin uptake was not affected. At a lower Tf-Fe concentration of (0.125 micromol/L) when uptake of iron is TR-mediated, a 10-fold molar excess of iron citrate had no effect on Tf-Fe uptake by HuH7 TR antisense and sense cells. However, at a higher Tf-Fe concentration (2.5 micromol/L), when uptake occurs mainly by the NTR-mediated process, there was a 40% reduction in the membrane-bound and intracellular uptake of iron. Iron citrate did not affect the maximum rate (Vmax) of Tf-Fe uptake but the Michaelis-Menten constant (Km) for Tf-Fe uptake by the NTR-mediated process was increased, indicating there was competitive inhibition of Tf-Fe uptake by iron citrate. These results suggest that the uptake of NTB-Fe and Tf-Fe by the NTR- mediated process occurs by the same cellular pathway, using a common iron-carrier.