In patients with iron overload disorders, increasing number of reports of renal dysfunction and renal iron deposition support an association between increased iron exposure and renal injury. In systemic iron overload, elevated circulating levels of transferrin-bound (TBI) and non-transferrin-bound iron (NTBI) are filtered to the renal proximal tubules, where they may cause injury. However, the mechanisms of tubular iron handling remain elusive. To unravel molecular renal proximal tubular NTBI and TBI handling, human conditionally immortalized proximal tubular epithelial cells (ciPTECs) were incubated with 55Fe as NTBI and fluorescently labeled holo-transferrin as TBI. Ferrous iron importers ZIP8 and ZIP14 were localized in the ciPTEC plasma membrane. Whereas silencing of either ZIP8 or ZIP14 alone did not affect 55Fe uptake, combined silencing significantly reduced 55Fe uptake compared to control (p < 0.05). Furthermore, transferrin receptor 1 (TfR1) and ZIP14, but not ZIP8, colocalized with early endosome antigen 1 (EEA1). TfR1 and ZIP14 also colocalized with uptake of fluorescently labeled transferrin. Furthermore, ZIP14 silencing decreased 55Fe uptake after 55Fe-Transferrin exposure (p < 0.05), suggesting ZIP14 could be involved in early endosomal transport of TBI-derived iron into the cytosol. Our data suggest that human proximal tubular epithelial cells take up TBI and NTBI, where ZIP8 and ZIP14 are both involved in NTBI uptake, but ZIP14, not ZIP8, mediates TBI-derived iron uptake. This knowledge provides more insights in the mechanisms of renal iron handling and suggests that ZIP8 and ZIP14 could be potential targets for limiting renal iron reabsorption and enhancing urinary iron excretion in systemic iron overload disorders.

中文翻译：

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ZIP8和ZIP14在人近端肾小管上皮细胞中摄取铁。

在铁超负荷疾病患者中，越来越多的肾功能不全和肾铁沉积的报道支持铁暴露增加与肾损伤之间的关联。在全身性铁超负荷中，转铁蛋白结合（TBI）和非转铁蛋白结合铁（NTBI）的循环水平升高被过滤到肾脏近端小管，这可能会造成伤害。然而，管状铁处理的机制仍然难以捉摸。为了阐明分子肾近端肾小管NTBI和TBI的处理方法，将人类有条件永生化的近端肾小管上皮细胞（ciPTEC）与55Fe作为NTBI孵育，并用荧光标记的全运铁蛋白作为TBI。黑色金属进口商ZIP8和ZIP14位于ciPTEC质膜中。尽管单独对ZIP8或ZIP14进行沉默都不会影响55Fe的吸收，与对照相比，联合沉默显着降低了55Fe摄取（p <0.05）。此外，转铁蛋白受体1（TfR1）和ZIP14，而不是ZIP8，与早期内体抗原1（EEA1）共定位。TfR1和ZIP14也与荧光标记的转铁蛋白的摄取共定位。此外，ZIP14沉默可降低55Fe-转铁蛋白暴露后的55Fe摄取（p <0.05），这表明ZIP14可能参与了TBI衍生铁向胞质溶胶的早期内体转运。我们的数据表明，人近端肾小管上皮细胞吸收TBI和NTBI，其中ZIP8和ZIP14均参与NTBI摄取，但ZIP14（而非ZIP8）介导TBI衍生的铁摄取。