Taketani S, Kohno H, Sawamura T, Tokunaga R
Department of Hygiene, Kansai Medical University, Osaka, Japan.
J Biol Chem. 1990 Aug 15;265(23):13981-5.
To investigate the regulation mechanism of the uptake of iron and heme iron by the cells and intracellular utilization of iron, we examined the interaction between iron uptake from transferrin and hemopexin-mediated uptake of heme by human leukemic U937 cells or HeLa cells. U937 cells exhibited about 40,000 hemopexin receptors/cell with a dissociation constant (Kd) of 1 nM. Heme bound in hemopexin was taken up by U937 cells or HeLa cells in a receptor-mediated manner. Treatment of both species of cells with hemopexin led to a rapid decrease in iron uptake from transferrin in a hemopexin dose-dependent manner, and the decrease seen in case of treatment with hemin was less than that seen with hemopexin. The decrease of iron uptake by hemopexin contributed to a decrease in cell surface transferrin receptors on hemopexin-treated cells. Immunoblot analysis of the transferrin receptors revealed that the cellular level of receptors in U937 cells did not vary during an 8-h incubation with hemopexin although the number of surface receptors as well as iron uptake decreased within the 2-h incubation. After 4 h of incubation of the cells with hemopexin, a decrease of the synthesis of the receptors occurred. Thus, the down-regulation of transferrin receptors by hemopexin can be attributed to at least two mechanisms. One is a rapid redistribution of the surface receptor into the interior of the cells, and the other is a decrease in the biosynthesis of the receptor. 59Fe from the internalized heme rapidly appeared in non-heme iron (ferritin) coincidently with the induction of heme oxygenase. The results suggest that iron released from heme down-regulates the expression of the transferrin receptors and iron uptake.
为了研究细胞对铁和血红素铁的摄取调控机制以及铁在细胞内的利用情况，我们检测了人白血病U937细胞或HeLa细胞中，转铁蛋白介导的铁摄取与血红素结合蛋白介导的血红素摄取之间的相互作用。U937细胞表现出约40,000个血红素结合蛋白受体/细胞，解离常数（Kd）为1 nM。与血红素结合蛋白结合的血红素以受体介导的方式被U937细胞或HeLa细胞摄取。用血红素结合蛋白处理这两种细胞，均导致转铁蛋白介导的铁摄取迅速下降，且呈血红素结合蛋白剂量依赖性，而用血红素处理时观察到的下降幅度小于用血红素结合蛋白处理的情况。血红素结合蛋白导致的铁摄取下降，使得经血红素结合蛋白处理的细胞表面转铁蛋白受体减少。对转铁蛋白受体的免疫印迹分析显示，尽管在2小时的孵育内表面受体数量和铁摄取量均下降，但在与血红素结合蛋白孵育8小时期间，U937细胞中受体的细胞水平并未发生变化。在用血红素结合蛋白孵育细胞4小时后，受体的合成出现下降。因此，血红素结合蛋白对转铁蛋白受体的下调至少可归因于两种机制。一种是表面受体迅速重新分布到细胞内部，另一种是受体生物合成减少。内化血红素中的59Fe迅速出现在非血红素铁（铁蛋白）中，同时伴有血红素加氧酶的诱导。结果表明，从血红素释放的铁下调了转铁蛋白受体的表达和铁摄取。
