Trinder D, Zak O, Aisen P
Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
Hepatology. 1996 Jun;23(6):1512-20. doi: 10.1053/jhep.1996.v23.pm0008675172.
The hepatic uptake of transferrin-bound iron by a nontransferrin receptor (NTR)-mediated process was investigated using the human hepatoma cell line HuH7. Because HuH7 cells also acquire iron from transferrin by a receptor (TR)-mediated process, TR expression was inhibited by transfecting the cells with a plasmid containing human TR complementary DNA in antisense orientation relative to a human cytomegalovirus promoter/enhancer element. Cell clones were obtained that expressed a 50% to 60% reduction in cell surface TR, leading to a corresponding decrease in transferrin and iron uptake compared with wild-type cells. Uptake of transferrin by a second process was nonsaturable and not inhibited by a 100-fold excess of unlabeled transferrin. The amounts of transferrin taken up by the wild-type and antisense cells by this process were similar, showing that it did not involve TR. The proteolytic enzyme Pronase reduced the uptake of transferrin, suggesting that the NTR-mediated process entailed the nonsaturable binding of transferrin to plasma membrane proteins. This process, like the TR-mediated one, involved the internalization and recycling of transferrin, leading to accumulation of iron with time. Iron uptake mediated by NTR process was saturable and displaced by 100-fold excess unlabeled transferrin and reduced by weak bases and metabolic inhibitors. Therefore, the NTR-mediated process entailed transferrin adsorption to membrane-bound proteins, internalization, and release of iron from transferrin by a pH-dependent step followed by the intracellular transport of iron into ferritin and heme by a saturable carrier-mediated mechanism.
利用人肝癌细胞系HuH7研究了非转铁蛋白受体（NTR）介导的转铁蛋白结合铁的肝脏摄取过程。由于HuH7细胞也通过受体（TR）介导的过程从转铁蛋白获取铁，因此通过用含有相对于人巨细胞病毒启动子/增强子元件呈反义方向的人TR互补DNA的质粒转染细胞来抑制TR表达。获得了细胞表面TR表达降低50%至60%的细胞克隆，与野生型细胞相比，转铁蛋白和铁摄取相应减少。通过第二种过程摄取转铁蛋白是不饱和的，并且不受100倍过量未标记转铁蛋白的抑制。野生型细胞和反义细胞通过该过程摄取的转铁蛋白量相似，表明它不涉及TR。蛋白水解酶链霉蛋白酶降低了转铁蛋白的摄取，提示NTR介导的过程需要转铁蛋白与质膜蛋白的不饱和结合。该过程与TR介导的过程一样，涉及转铁蛋白的内化和再循环，导致铁随时间积累。由NTR过程介导的铁摄取是可饱和的，被100倍过量未标记转铁蛋白取代，并被弱碱和代谢抑制剂降低。因此，NTR介导的过程需要转铁蛋白吸附到膜结合蛋白上，内化，并通过pH依赖性步骤从转铁蛋白释放铁，随后通过可饱和载体介导的机制将铁细胞内转运到铁蛋白和血红素中。
