转铁蛋白受体TFR1（TFRC）：铁稳态关键成员，贫血、神经退行性疾病、癌症新锐靶点！
日期：2024-10-23 16:01:00
转铁蛋白受体TFR1是介导铁离子进入细胞通道的关键成员之一，在调节细胞铁代谢和维持铁平衡中发挥关键作用。癌细胞为了快速增殖需要大量铁，导致细胞表面转铁蛋白受体1（TfR1）显著上调，TfR1通过与携铁蛋白转铁蛋白结合来介导铁的摄取。利用这一现象和 TfR1 的快速内吞速率，美国丹娜-法伯癌症研究所Xin Zhou课题组开发了转铁蛋白受体靶向嵌合体（TransTAC），这是一种用于膜蛋白降解的异双特异性抗体模式 [1]。TransTAC被设计用于驱动感兴趣的靶蛋白与TfR1从细胞表面共同内吞，并促使靶蛋白进入溶酶体降解途径。这一研究于2024年9月25日发表在Nature上，TransTAC代表了一类有前景的新型双功能抗体家族，可用于精确调控膜蛋白和靶向癌症治疗。
现有研究证实，TFR1在许多肿瘤细胞中高表达，是潜在的肿瘤标志物，且针对TFR1进行治疗可以有效地抑制肿瘤生长和转移。此外，TFR1还与其它疾病如贫血、铁代谢障碍性疾病等有关。因此，以TFR1为靶点的治疗策略来靶向调节细胞内铁水平，可以在相关疾病的临床应用中发挥重要作用。
1. 什么是TFR1？
1.1 TFR1的结构
转铁蛋白受体1（Transferrin receptor protein 1，TFR1）也被称为CD71或TFRC。TFR1/TFRC是一种II型跨膜蛋白，是调节细胞内铁元素转运过程的最重要膜蛋白。TFR1是由两个同源二聚体的亚基通过二硫键交联而成。每个单体包含一个大的胞外C端区域，一个单跨膜区域及一个短的N端区域。C端区域作为外功能区，包含了与转铁蛋白（Transferrin，Tf）相结合的位点（图1） [2-4]。目前已发现两种转铁蛋白受体，分别是TFR1和TFR2，它们在结构和功能上都比较相似。在正常生理条件下，TFR1与转铁蛋白Tf发生相互作用，促进铁的吸收。这种结合形式是血液中铁的主要存在方式 [5-6]。
图1. TFR1的结构 [2]
1.2 TFR1的表达
TFR1是一种广泛表达于人体几乎所有细胞和组织类型中的膜蛋白。TFR1在人体需要铁元素时发挥作用，介导铁离子的转运和代谢。TFR1的表达广泛分布于免疫系统、造血系统（如骨髓干细胞、红细胞和白细胞）、神经系统（如神经元和神经胶质细胞）、生殖系统、心脏、肝脏、肾脏等各种组织和细胞类型。TFR1的表达水平受多种因素影响，包括细胞内铁含量、细胞分化状态、激素调节以及炎症状态等 [7-9]。
1.3 TFR1的功能
TFR1最主要的生理功能是与转铁蛋白（transferrin，Tf）结合，通过内吞方式介导细胞对铁的摄取。因此，Tf-TFR1系统被认为是机体获取铁离子的重要途径。具体而言，首先，Tf和铁离子（Fe3+或Fe2+形式）结合后，其空间结构随之发生相应变化，将铁离子包入蛋白内形成Tf-Fe2+。其次，TFR1在生理pH下与Tf-Fe2+结合，Tf-TFR1复合物被网格蛋白（Clathrin）通过小窝的内吞作用内部化（图2） [10-12]。
随之，胞内Tf-TFR1复合物被运输至内体酸化，TFR1和Tf的氨基酸残基相互作用，引起构象改变促使铁离子释放，TFR1通过高尔基复合体循环至细胞表面完成铁离子运输。总之，TFR1在细胞和组织中扮演着重要角色，通过TFR1的调节来平衡细胞内铁含量，维持人体铁稳态是保证人体各项生理机能正常运作的必要条件 [10-12]。
图2. Tf-TFR1系统平衡细胞内铁含量 [2]
2. TFR1相关的调控机制
TFR1是细胞最重要的铁元素摄取因子。TFR1表达量下降或异常会导致细胞缺铁，而过多的铁则可能催化活性氧（ROSs）并损伤生物大分子。为了确保充足的铁元素同时避免其毒性，细胞已经发展出多种机制来调控TFR1表达水平。尽管TFR1异常表达在多种疾病中发挥作用，但其分子机制和作用仍未完全明确。因此，还需要更深入地研究和探索。
TFR1的表达受多种刺激条件调控。在转录水平中，当细胞发生缺氧时，缺氧诱导因子（HIF）和其他转录因子如c-Myc、GATA1、Ets-1以及促红细胞生成素Stat5可以促进其转录。TFR1的转录后水平主要由IRP1和IRP2调控，它们与TFR1 mRNA中的IRE结合来影响基因表达 [13-15]。
在翻译后水平，CD133（PROM1）是TFR1转运铁元素过程中的负调节因子，同时EGF受体、c-Abl分子和MARCH8分子可能也参与其中。如下图所示，一项研究揭示FLCN有可能在翻译、或者翻译后水平调控TFR1的表达，即Tf-TFR1复合物可与含有Rab11蛋白的循环内体相结合，回到细胞膜上（图3）[13-17]。
TFR1在疾病中发挥调控作用。例如，在胶质瘤中，TFR1通过炎症反应，细胞周期，DNA损失及DNA甲基化等机制参与胶质瘤的发生发展。此外，PD1信号，如IL17，IL18，NF-kβ，FOXM1，FOCAL及JAK-STAT信号可能是TFR1调控的关键信号通路 [18-20]。在神经干细胞中敲除TFR1发现，条件性敲除小鼠有癫痫的症状。并且在此研究中还发现GluA2在海马神经元突触上表达增加，其中突触前的神经递质释放能力下降，突触后长时程增强（LTP）受到一定的抑制 [21-23]。
图3. FLCN通过Rab11A调控Tf-TFR1蛋白的回收运输 [17]
3. TFR1在肿瘤、神经退行性等疾病中的作用
正常人体铁代谢处于平衡状态，当其中的某一调控环节出现异常导致平衡被打破时，会影响细胞内自由基形成并加速氧化应激反应进展，同时也可导致肿瘤发生和发展。TFR1作为细胞摄取铁元素过程中最重要的调控受体，在多种疾病的发生发展过程中起到重要作用。
3.1 TFR1和肿瘤
现有研究表明TFR1在甲状腺癌 [24]、食管鳞状细胞癌 [25]、乳腺癌 [26]、肝癌 [27]、结肠癌 [28]、白血病 [29]、肺癌 [30]、胰腺癌 [31]、鼻咽癌 [31]等恶性肿瘤中显著表达。但在部分恶性肿瘤中，TFR1表达情况尚不明确，其中包括：前列腺癌、睾丸癌等。例如，在肝癌中，TFR1在肝癌中的表达与甲胎蛋白和血清凝血酶原的浓度有关 [27]；在乳腺癌中，敲降IRP2表达，可提高铁蛋白重链的表达，并下调TFR1蛋白表达，从而抑制乳腺癌细胞的生长 [26, 32]；在结肠癌中，TFR1的高表达可激活IL-6/IL-11-Stat3信号通路，促进结肠上皮细胞的增殖和凋亡，从而加重结肠黏膜的损伤并导致结肠癌的发生 [28, 33]。
3.2 TFR1和神经退行性疾病
铁代谢的紊乱是引发神经退行性疾病的病理生理机制之一。铁在大脑中的蓄积，与阿尔茨海默病、帕金森病、肌萎缩侧索硬化等神经退行性疾病有关 [34]。阿尔茨海默病是最常见神经退行性疾病之一，其主要以淀粉样斑块的积聚以及某些神经元的丢失为发病特征。有研究表明，抑制阿尔兹海默模型小鼠大脑颞叶皮层中的铁摄取蛋白TFR1、TF以及DMT1的表达，可有效缓解铁过载状态 [35-36]。
3.3 TFR1和贫血
TFR1与Tf复合物的结合，对红细胞生成过程中细胞获取铁元素有着重要意义。当人体内发生缺铁或者红细胞生成增多时，TFR1的表达将被反应性上调。临床研究证实，地中海贫血小鼠体内的溶性转铁蛋白受体（sTFR1）和TF的水平都显著高于正常值。进一步研究揭示，TFR1在β-地中海红系前体细胞中异常高表达，降低TFR1的表达可有效调节贫血小鼠中无效红细胞的生成并改善小鼠的贫血以及铁过载情况 [7, 37]。
3.4 TFR1和其它疾病
TFR1不仅参与细胞铁离子运输，有研究提示TFR1还可作为多种病毒受体介导HCV与宿主细胞膜融合，在HCV入胞过程中发挥重要作用。丙型肝炎病毒（HCV）是导致慢性化肝炎、原发性肝癌的主要病原体。因此，TFR1作为HCV抗病毒靶点的潜在可能性值得关注 [38-40]。在神经元中，研究发现TFR1对mGlul的转运起到重要的调节作用，并且可能参与了mGlul信号通路，且对小脑的运动协调能力起到影响 [41-42]。
4. TFR1的临床研究进展
目前已有多款针对转铁蛋白受体1(TFR1)的临床药物正在研发中，药物类型涵盖抗体核酸偶联药物、双体、单体、抗体融合蛋白、ADC药物等；适应症涉及非小细胞肺癌、食管癌、黏多醣贮积症型、肌强直性营养不良、阿尔茨海默症、杜氏肌营养不良症等。其中，JCR Pharmaceuticals Co., Ltd.的Pabinafusp Alfa，一款靶向IDS和TfR1的抗体融合蛋白已于2021年日本上市。近年来，基于TFR1的靶向治疗策略在不断发展。有研究利用TFR1提高抗体跨越血脑屏障的转运能力，并与抗β-淀粉样肽单抗相结合形成特异性复合抗体，以提高阿尔茨海默病患者的治疗效果 [43-44]。
同时，抗TFRC的抗体JST-TFR09和抗TFRC单克隆抗体A24分别可抑制肿瘤细胞对铁元素的摄取，以及诱导T系细胞白血病中恶性细胞的凋亡 [45-46]。这些研究提示，TFR1可成为有效的靶标分子参与到多种疾病的临床治疗。未来，随着TFR1相关研究的不断深入和完善，有望为患者提供更加精准、有效的治疗，为他们带来更多的获益和新希望。
5. 华美生物TFR1相关产品
● TFR1重组蛋白
Recombinant Human Transferrin receptor protein 1(TFRC),partial (Active) (Code: CSB-MP3648HU)
High specificity was validated by SDS-PAGE. SDS-PAGE (reduced) with 5% enrichment gel and 15% separation gel.
Immobilized Human TFRC at 2μg/mL can bind Anti-TFRC recombinant antibody (CSB-RA023441MA1HU), the EC50 is 3.305-8.220 ng/mL.
● TFR1抗体
| 产品名称 | 适应种属 | 应用 | 货号 |
|---|---|---|---|
| TFRC Recombinant Monoclonal Antibody | Human | ELISA | CSB-RA023441MA1HU |
| TFRC Antibody | Human | ELISA, IHC, IF | CSB-PA07219A0Rb |
| TFRC Antibody | Human, Mouse | ELISA, WB, IHC | CSB-PA001477 |
| TFRC Antibody | Human | ELISA, WB | CSB-PA005607 |
| Phospho-TFRC (S24) Antibody | Human, Mouse | ELISA, WB, IHC | CSB-PA010092 |
| TFRC Antibody | Human, Mouse, Rat | ELISA, WB, IHC | CSB-PA023441GA01HU |
| TFRC Antibody | Human, Mouse, Rat | ELISA, WB, IHC | CSB-PA104885 |
| TFRC Antibody | Human, Mouse, Rat | ELISA, WB, IHC | CSB-PA834425 |
● TFR1 ELISA试剂盒
| 产品名称 | 检测样本 | 检测范围 | 货号 |
|---|---|---|---|
| Mouse transferrin receptor,TFR ELISA Kit | serum, plasma, tissue homogenates | 0.625 ng/mL-40 ng/mL | CSB-E08389m |
| Human soluble transferrin receptor,sTfR ELISA Kit | serum, plasma | Request Information | CSB-E09100h |
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