Kaplan J, Jordan I, Sturrock A
Department of Pathology, University of Utah College of Medicine, Salt Lake City 84132.
J Biol Chem. 1991 Feb 15;266(5):2997-3004.
Mammalian cells accumulate iron via the binding of transferrin to high affinity surface receptors, or through a transferrin-independent pathway which involves the uptake of iron-organic anion chelates by a membrane-based transport system. Previously we determined that the transferrin-independent transport system was present on a wide variety of cultured cells (Sturrock, A., Alexander, J., Lamb, J., Craven, C. M., and Kaplan, J. (1990) J. Biol. Chem. 265, 3139-3145). In this communication we demonstrate that the transferrin-independent iron uptake system is regulated differently than the transferrin-mediated pathway. The activity of the transferrin-independent system was unaffected by changes in cellular growth rate, induction of DNA synthesis and cell division, or depletion of cellular iron. Exposure of cells to ferric or ferrous iron, however, resulted in a time-dependent increase in transport activity, due to a change in Vmax with no change in Km. Increased transport activity was seen in a variety of cultured cell types, occurred in the presence of cycloheximide, and persisted for hours after removal of iron. The ability of other transition metals to induce changes in transport, or to compete with iron for accumulation by the transferrin-independent uptake system, was critically dependent on the composition of the media in which the cells were incubated. Metals such as Cu2+ or Zn2+, but not Cd2+ or Mn2+, when dissolved in a balanced salt solution buffered with 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, induced changes in the transferrin-independent iron transport system. The same metals which induced changes in transport were ineffective in media containing amino acids, ascorbate, or N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine. The Vmax of the transferrin-independent iron transport system was also elevated by increases in intracellular Ca2+. The effect of iron on transport activity, however, did not result from an iron-induced release of intracellular Ca2+. These results suggest a novel form of regulation in which the presence of extracellular iron induces the appearance of previously cryptic transporters and thus accelerates the clearance of potentially toxic molecules.
哺乳动物细胞通过转铁蛋白与高亲和力表面受体结合来积累铁，或者通过一条不依赖转铁蛋白的途径来积累铁，该途径涉及基于膜的转运系统摄取铁 - 有机阴离子螯合物。此前我们确定不依赖转铁蛋白的转运系统存在于多种培养细胞中（斯特罗克，A.，亚历山大，J.，兰姆，J.，克雷文，C. M.，以及卡普兰，J.（1990年）《生物化学杂志》265卷，3139 - 3145页）。在本通讯中，我们证明不依赖转铁蛋白的铁摄取系统的调节方式与转铁蛋白介导的途径不同。不依赖转铁蛋白的系统的活性不受细胞生长速率变化、DNA合成和细胞分裂的诱导或细胞内铁耗尽的影响。然而，将细胞暴露于三价铁或二价铁会导致转运活性随时间增加，这是由于Vmax发生变化而Km不变。在多种培养细胞类型中都观察到转运活性增加，在存在环己酰亚胺的情况下也会发生，并且在去除铁后持续数小时。其他过渡金属诱导转运变化或与铁竞争通过不依赖转铁蛋白的摄取系统积累的能力，严重依赖于细胞培养所用培养基的成分。当溶解在以4 - （2 - 羟乙基）- 1 - 哌嗪乙磺酸缓冲的平衡盐溶液中时，诸如Cu2 +或Zn2 +等金属，但不是Cd2 +或Mn2 +，会诱导不依赖转铁蛋白的铁转运系统发生变化。在含有氨基酸、抗坏血酸或N - [2 - 羟基 - 1,1 - 双（羟甲基）乙基]甘氨酸的培养基中诱导转运变化的相同金属却没有效果。细胞内Ca2 +的增加也会提高不依赖转铁蛋白的铁转运系统的Vmax。然而，铁对转运活性的影响并非由铁诱导的细胞内Ca2 +释放所致。这些结果提示了一种新的调节形式，即细胞外铁的存在诱导先前隐藏的转运体出现，从而加速潜在有毒分子的清除。
