Heme trafficking and its impact on systemic iron homeostasis

描述（由申请人提供）：本项目的目标是研究血红素运输如何影响全身铁稳态。由于每天需要25毫克的铁来维持红细胞的生成，只有1-2毫克/天从胃肠道吸收，1-2毫克/天通过脱落的胃肠道、皮肤和肾脏粘膜细胞流失，大多数铁被循环利用。传统的观点认为，当巨噬细胞吞噬衰老的红细胞时，它们的血红蛋白被降解为血红素，然后是铁，铁通过铁转运蛋白转运到转铁蛋白，运送到肝脏（储存）或骨髓（生产新的红细胞）。在先前的研究中，我们确定猫白血病病毒c受体FLVCR特异性地从细胞中输出血红素（Cell 118:757-66, 2004）。western分析显示，FLVCR在十二指肠、肝脏和巨噬细胞中高度表达，这些组织对铁的吸收和运输至关重要；和flvcr缺失的小鼠在这些部位有过量的铁，除了红细胞发育不全（Science 319:825-828, 2008）。这些观察结果，加上对细胞系和原代巨噬细胞的体外研究，使我们假设血红素，而不仅仅是铁，被贩运。这项资助将定义FLVCR的结构，并通过诱变分析、非洲爪蟾卵母细胞研究和电子晶体学来辨别它向血红素结合蛋白（如血凝素和白蛋白）输出血红素的机制。我们将确定铁如何调节FLVCR的细胞间定位，以帮助全身铁平衡。此外，我们将通过研究Flvcrflox/flox的巨噬细胞来确定红细胞被巨噬细胞吞噬后血红素的命运；体外和体内LysM-cre小鼠；通过研究Flvcrflox/flox来确定血红素递送到肝脏后的命运；alb-cre老鼠;确定肝细胞是否能通过FLVCR将血红素分泌到胆汁中，从而使铁排出体外；并探讨这些发现在溶血、慢性炎症和血色素沉着症模型中的意义。总之，这些研究应该表明，铁的生理调节比以前所认识的更复杂和复杂。

DESCRIPTION (provided by applicant): The goal of this project is to study how heme trafficking might impact systemic iron homeostasis. As 25 mg of iron is required daily to maintain red cell production, only 1-2 mg/d is absorbed from the GI tract, and 1-2 mg/d is lost through sloughed gastrointestinal, skin and kidney mucosal cells, most iron is recycled. The traditional view is that when macrophages engulf senescent red cells, their hemoglobin is degraded to heme then iron, which is exported via ferroportin to transferrin for delivery to the liver (for storage) or to the marrow (for new red cell production). In previous studies, we determined that the feline leukemia virus-C receptor, FLVCR, specifically exports heme from cells (Cell 118:757-66, 2004). On western analysis, FLVCR is highly expressed in the duodenum, liver and macrophage, tissues critical for iron absorption and trafficking; and Flvcr-deleted mice have excess iron at these sites, in addition to red cell aplasia (Science 319:825-828, 2008). These observations, plus studies of cell lines and primary macrophages in vitro, led us to hypothesize that heme, not only iron, is trafficked. This grant will define FLVCR structure and discern the mechanism by which it exports heme to heme-binding proteins, such as hemopexin and albumin, using mutagenesis analyses, Xenopus oocytes studies, and electron crystallography. We will determine how iron regulates the intercellular localization of FLVCR to aid systemic iron balance. In addition, we will determine the fate of heme from red cells after the cells are ingested by macrophages by studying macrophages from Flvcrflox/flox;LysM-cre mice in vitro and in vivo; determine the fate of heme once delivered to liver by studying Flvcrflox/flox;alb-cre mice; determine if hepatocytes can secrete heme via FLVCR into the bile permitting iron to exit the body; and explore the implications of these findings in models of hemolysis, chronic inflammation, and hemochromatosis. Together these studies should demonstrate that the physiologic regulation of iron is more intricate and complex than previously appreciated. PUBLIC HEALTH RELEVANCE: Systemic iron balance largely depends on macrophage iron recycling (20-25 mg/d in persons) and minimally on the ingestion of iron in the diet (1-2 mg/day). The current concept is that heme from red cells is converted to iron, then recycled to the marrow to make new red cells or transported to the liver for storage. Our data challenge this accepted paradigm; demonstrate that heme, and not only iron, is trafficked; and suggest that this trafficking could help regulate systemic iron balance and/or modify the clinical manifestations of hemolysis, chronic inflammation and hemochromatosis.