Regulatory Role of Transferrin in Erythropoiesis and Iron Metabolism

描述（由申请人提供）：铁超载是以无效红细胞生成为特征的贫血的发病和死亡的主要原因，包括地中海贫血。近年来，在阐明铁代谢的分子参与者方面取得了前所未有的进展，提供了新的潜在治疗方法。特别是，肝脏激素hepcidin已被确定为铁吸收和分布的中央调节剂。铁负荷性促红细胞生成性贫血的特点是尽管铁负荷，但hepcidin水平不适当地低。我们发现外源性转铁蛋白可以改善地中海贫血小鼠的贫血，减少循环中的非转铁蛋白结合铁，并增加hepcidin的表达，这表明转铁蛋白是红细胞生成和铁代谢之间交叉对话的重要调节因子。我们建议进一步探索转铁蛋白通过其在红细胞和肝细胞中的作用调节hepcidin表达的机制。最近，HFE和转铁蛋白受体2 (TfR2)，已知参与肝磷脂的肝脏调节的分子，被发现在红细胞前体中也具有重要的功能。然而，转铁蛋白对红细胞HFE和TfR2表达和信号传导的影响，以及HFE和TfR2如何调节红细胞生成，在很大程度上仍未明确。初步数据表明，外源性转铁蛋白（Tf）增加了单铁Tf的相对浓度，减少了铁进入红细胞前体的量。我们提出了一个模型，其中增加的单铁转铁蛋白改变了红细胞前体和肝细胞中HFE、TfR1和TfR2介导的铁摄取和信号传导。为了检验这一模型，我们提出了三个具体目标：1。比较在存在或不存在HFE的情况下，通过TfR1和TfR2从单铁和异铁转铁蛋白中摄取铁的结合动力学和吸收。2. 检查有效和无效红细胞生成过程中转铁蛋白浓度变化对红细胞前体的影响。3. 观察转铁蛋白浓度变化对肝细胞hepcidin表达的影响。转基因小鼠和细胞系将作为实验系统来测试以下假设：HFE调节单铁转铁蛋白与异铁转铁蛋白竞争与转铁蛋白受体结合的能力（目的1）。Tf结合通过调节HFE、TfR1和TfR2的表达和信号传导影响红细胞分化（Aim 2A）。Tf通过TfR1/ hfe介导的红细胞铁状态和小鼠铁动力学变化调节红细胞生成（Aim 2B）。monoferic - tf通过TfR2信号传导调节肝细胞中hepcidin的表达（Aim 3A）。升高的Tf减轻了非转铁蛋白结合铁对hepcidin表达的影响（Aim 3B）。给药Tf可减弱红细胞生成小鼠血清中的hepcidin下调因子（Aim 3C）。这些研究将阐明铁代谢和红细胞生成之间的交叉对话的生理学，并可能为并发贫血和铁超载疾病患者开发新的治疗方案提供基础。

DESCRIPTION (provided by applicant): Iron overload is the principal cause of morbidity and mortality in anemias characterized by ineffective erythropoiesis, including ¿-thalassemias. In recent years, unprecedented progress in elucidating the molecular participants in iron metabolism has provided novel potential therapeutic approaches. In particular, the hepatic hormone hepcidin has been identified as a central regulator of iron absorption and distribution. The iron loading dyserythropoietic anemias are characterized by inappropriately low hepcidin levels despite iron loading. We have shown that exogenous transferrin ameliorates anemia, reduces circulating non-transferrin bound iron, and increases hepcidin expression in ¿-thalassemic mice, suggesting that transferrin is an important regulator of the cross-talk between erythropoiesis and iron metabolism. We propose to further explore mechanisms by which transferrin regulates hepcidin expression via its roles in the erythron and in the hepatocyte. Recently HFE and transferrin receptor 2 (TfR2), molecules known to participate in the hepatic regulation of hepcidin, have been found to be also functionally important in erythroid precursors. However, the effects of transferrin on erythroid HFE and TfR2 expression and signaling, and how HFE and TfR2 modulate erythropoiesis remain largely uncharacterized. Preliminary data demonstrate that exogenous transferrin (Tf) increases the relative concentration of monoferric Tf and decreases iron entry into erythroid precursors. We propose a model in which increased monoferric transferrin alters HFE, TfR1, and TfR2 mediated iron uptake and signaling, in erythroid precursors and in hepatocytes. To test this model, we propose three specific aims: 1. Compare the binding kinetics and uptake of iron from monoferric and diferric transferrin via TfR1 and TfR2 in the presence or absence of HFE. 2. Examine the effects of changes in transferrin concentration on erythroid precursors during effective and ineffective erythropoiesis. 3. Examine the effects of changes in transferrin concentration on hepatocellular hepcidin expression. Genetically modified mice and cell lines will serve as experimental systems to test the following hypotheses: HFE modulates the ability of monoferric transferrin to compete with diferric transferrin for binding to transferrin receptors (Aim 1). Tf binding affects erythroid differentiaton through modulation of HFE, TfR1, and TfR2 expression and signaling (Aim 2A). Tf regulates erythropoiesis via TfR1/HFE-mediated changes in erythroid cellular iron status and mouse ferrokinetics (Aim 2B). Monoferric-Tf modulates hepcidin expression in hepatocytes via signaling through TfR2 (Aim 3A). Increased Tf mitigates the effects of non-transferrin bound iron on hepcidin expression (Aim 3B). Administration of Tf attenuates a hepcidin down-regulatory factor in serum of mice with dyserythropoiesis (Aim 3C). These studies will clarify the physiology of the cross-talk between iron metabolism and erythropoiesis, and potentially provide the basis for the development of novel therapeutic alternatives for patients with diseases of concurrent anemia and iron overload. PUBLIC HEALTH RELEVANCE: Iron overload disorders are common worldwide and can lead to severe heart and liver damage. They include certain iron loading anemias (for example, thalassemias, sickle cell anemia, and myelodysplastic syndrome) or inherited disorders of iron metabolism (hemochromatosis). We discovered that administering transferrin is an effective way of treating thalassemia in mice, and will investigate its mode of action and possible application to other forms of iron overload as well.