Heme trafficking and recycling in iron metabolism

本提案的长期目标是确定铁代谢中的血红素运输和再循环。缺铁是世界上最常见的营养失调。缺铁最常见的临床表现是由于红细胞（红细胞）血红蛋白受损引起的贫血。这是因为人体中超过70%的铁被用作血红蛋白生产中的血红素。在健康成年人中，大约90%的体内铁来自循环血红素铁。每天，网状内皮系统（RES）巨噬细胞通过每秒摄取近500万个衰老或受损的红细胞来回收血红素铁；骨髓利用这些回收的铁来产生新的红细胞。因此，巨噬细胞铁循环中的遗传缺陷导致贫血。尽管血红素在铁循环中的重要性，但RES巨噬细胞中负责血红素运输和运输的途径仍然知之甚少。在红细胞吞噬作用后，铁通过细胞质中的血红素加氧酶（HMOX1）从血红素中释放出来。这些铁可以储存在铁蛋白（FTH1）中，也可以通过铁转运蛋白（FPN1）输出到细胞外，在骨髓中重新利用产生新的红细胞。因此，HMOX1、FPN1和FTH1（血红素-铁循环途径中的步骤）的遗传破坏会导致小鼠胚胎死亡。我们发现了第一个真核血红素进口/转运蛋白HRG1，并表明它是巨噬细胞吞噬溶酶体将血红素转运到细胞质中所必需的。我们最近的研究表明，hrg1缺陷小鼠即使不能再循环血红素铁并在巨噬细胞内积累10倍的过量血红素，也能存活。小鼠对血红素毒性具有耐受性，因为它们将血红素隔离在溶酶体中，溶酶体比正常的大10-100倍，形成血红素-迄今为止仅在吸血生物中发现的大的多聚血红素晶体来解毒血红素。血红素耐受性需要一个完全运作的血红素降解途径，因为HMOX1单倍不足合并HRG1缺乏会导致围产期死亡，显示出一种合成的致死相互作用。我们令人兴奋的结果表明，在哺乳动物中，血红素解毒和耐受性存在一种以前未预料到的途径。本研究旨在揭示hrg1介导的血红素耐受的分子基础。我们将测试血红素耐受性是由HRG1对血红素运输的协调调节及其与血红素降解和血红素运输机制的其他成分的遗传和物理相互作用所赋予的假设。我们将利用(a)遗传学方法来阐明血红素形成和血红素耐受性的调控机制；(b)细胞生物学方法鉴定HRG1和HMOX1之间合成致死相互作用的机制。我们的目标是深入了解血红素运输和回收的分子基础及其在哺乳动物血红素耐受性中的作用。

The long-term goals of this proposal are to define heme transport and recycling in iron metabolism. Iron deficiency is the most common nutritional disorder in the world. The most common clinical manifestation of iron deficiency is anemia due to impaired hemoglobinization of red blood cells (RBCs). This is because over 70% of iron in the human body is used as heme in hemoglobin production. In healthy adults, about 90% of body iron is derived from recycled heme-iron. Each day, reticuloendothelial system (RES) macrophages recycle heme-iron by ingesting almost 5 million senescent or damaged RBCs per second; the bone marrow utilizes this recycled iron to produce new RBCs. As a consequence genetic defects in macrophage iron recycling result in anemia. Despite the importance of heme in iron recycling, the pathways responsible for heme transport and trafficking in RES macrophages remain poorly understood. After erythrophagocytosis, the iron is enzymatically released from heme by heme oxygenases (HMOX1) in the cytosol. This iron can either be stored in ferritin (FTH1) or exported out of the cell by ferroportin (FPN1) to be reutilized for new RBC production in the bone marrow. Consequently, genetic disruption in HMOX1, FPN1, and FTH1 – steps in the heme-iron recycling pathway – causes embryonic lethality in mice. We identified the first eukaryotic heme importer/transporter, HRG1 and showed that it is essential for transporting heme from the macrophage phagolysosome into the cytoplasm in macrophages. Our recent studies show that HRG1-deficient mice are viable even though they are incapable of recycling heme-iron and accumulate 10-fold excess heme within macrophages. The mice are tolerant to heme toxicity because they sequester heme inside lysosomes, which become 10-100 times larger than normal, by forming hemozoin - large multimeric heme crystals heretofore only identified in blood-feeding organisms to detoxify heme. Heme tolerance requires a fully-operational heme degradation pathway as haploinsufficiency of HMOX1 combined with HRG1 deficiency causes perinatal lethality, demonstrating a synthetic lethal interaction. Our exciting results suggest the existence of a previously unanticipated pathway for heme detoxification and tolerance in mammals. The studies in this proposal are designed to uncover the molecular basis of HRG1-mediated heme tolerance. We will test the hypotheses that heme tolerance is conferred by the coordinated regulation of heme transport by HRG1 and its genetic and physical interactions with heme degradation and additional components of the heme trafficking machinery. We will utilize (a) a genetic approach to elucidate the regulatory mechanisms of hemozoin formation and heme tolerance; and (b) a cell biological approach to identify the mechanisms for the synthetic lethal interactions between HRG1 and HMOX1. Our goals are to acquire a deep understanding for the molecular basis for heme trafficking and recycling and its role in heme tolerance in mammals.