The hepatic uptake of transferrin-bound iron by a
Iron is required for the synthesis of many heme and nontransferrin receptor (NTR)-mediated process was non-heme proteins and enzymes necessary for cellular investigated using the human hepatoma cell line HuH7. growth and metabolism. Most cells take up iron from Because HuH7 cells also acquire iron from transferrin serum transferrin by a transferrin receptor (TR)-mediby a receptor (TR)-mediated process, TR expression was ated process. Hepatocytes, in addition to their own inhibited by transfecting the cells with a plasmid conneed for iron, are also an important site of iron storage. taining human TR complementary DNA in antisense ori-They acquire iron by many pathways, including TRentation relative to a human cytomegalovirus promoter/ mediated uptake of iron from transferrin, 1 iron taken enhancer element. Cell clones were obtained that exup by from desialylated transferrin through the asiapressed a 50% to 60% reduction in cell surface TR, leadloglycoprotein receptor, 2 iron carried by ferritin, 3,4 and ing to a corresponding decrease in transferrin and iron uptake compared with wild-type cells. Uptake of trans-transferrin-iron secured by a non-transferrin receptorferrin by a second process was nonsaturable and not mediated process 5 (NTR). The TR-mediated uptake of inhibited by a 100-fold excess of unlabeled transferrin. transferrin-iron has been studied extensively, particu-The amounts of transferrin taken up by the wild-type larly in the developing erthyroid cell. 6 The receptorand antisense cells by this process were similar, showing mediated process involves the binding of transferrin to that it did not involve TR. The proteolytic enzyme Proa cell surface TR, internalization of the transferrin-TR nase reduced the uptake of transferrin, suggesting that complex into an endocytotic vesicle, the release of iron the NTR-mediated process entailed the nonsaturable from transferrin, and transport into the cell cytosol and binding of transferrin to plasma membrane proteins.