INHIBITION OF UPTAKE OF TRANSFERRIN-BOUND IRON BY HUMAN HEPATOMA-CELLS BY NONTRANSFERRIN-BOUND IRON

The liver acquires iron from transferrin by transferrin receptor-media ted (TR) and transferrin receptor-independent pathways (NTR) and from nontransferrin-bound iron (NTB-Fe). Iron uptake by the NTR processes i nvolves an iron-carrier mediated step. Experiments, using human hepato ma cells (HuH7) transfected with TR antisense (sense for control) RNA expression vectors to suppress TR expression, were performed to examin e the effect of unlabeled NTB-Fe as iron citrate on the uptake of Fe-5 9-I-125-transferrin. This was to determine if the uptake of transferri n-bound iron (Tf-Fe) and NTB-Fe uptake is mediated by a common iron-ca rrier. Iron citrate inhibited the uptake of Fe-59-transferrin (2.5 mu mol/L Fe) in a concentration-dependent manner with a maximum effect wh en the citrate-iron:Tf-Fe molar ratio was 10:1. Transferrin uptake was not affected, At a lower Tf-Fe concentration of (0.125; mu mol/L) whe n uptake of iron is TR-mediated, a 10-fold molar excess of iron citrat e had no effect on Tf-Fe uptake by HuH7 TR antisense and sense cells. However, at a higher Tf-Fe concentration (2.5 mu mol/L), when uptake o ccurs 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 (V-max) of Tf-Fe uptake but the Michaelis -Menten constant (K-m) for Tf-Fe uptake by the NTR-mediated process wa s increased, indicating there was competitive inhibition of Tf-Fe upta ke by iron citrate. These results suggest that the uptake of NTB-Fe an d Tf-Fe by the NTR-mediated process occurs by the same cellular pathwa y, using a common iron-carrier.