In plasma, iron is normally bound to transferrin, the principal protein in blood responsible for binding and transporting iron throughout the body. However, in conditions of iron overload when the iron-binding capacity of transferrin is exceeded, non-transferrin-bound iron (NTBI) appears in plasma. NTBI is taken up by hepatocytes and other parenchymal cells via NTBI transporters and can cause cellular damage by promoting the generation of reactive oxygen species. However, how NTBI affects endothelial cells, the most proximal cell type exposed to circulating NTBI, has not been explored. We modeled in vitro the effects of systemic iron overload on endothelial cells by treating primary human umbilical vein endothelial cells (HUVECs) with NTBI (ferric ammonium citrate [FAC]). We showed by RNA-Seq that iron loading alters lipid homeostasis in HUVECs by inducing sterol regulatory element-binding protein 2-mediated cholesterol biosynthesis. We also determined that FAC increased the susceptibility of HUVECs to apoptosis induced by tumor necrosis factor-α (TNFα). Moreover, we showed that cholesterol biosynthesis contributes to iron-potentiated apoptosis. Treating HUVECs with a cholesterol chelator hydroxypropyl-β-cyclodextrin demonstrated that depletion of cholesterol was sufficient to rescue HUVECs from TNFα-induced apoptosis, even in the presence of FAC. Finally, we showed that FAC or cholesterol treatment modulated the TNFα pathway by inducing novel proteolytic processing of TNFR1 to a short isoform that localizes to lipid rafts. Our study raises the possibility that iron-mediated toxicity in human iron overload disorders is at least in part dependent on alterations in cholesterol metabolism in endothelial cells, increasing their susceptibility to apoptosis.

中文翻译：

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铁负荷诱导胆固醇合成并使内皮细胞对 TNFα 介导的细胞凋亡敏感。

在血浆中，铁通常与转铁蛋白结合，转铁蛋白是血液中负责在全身结合和运输铁的主要蛋白质。然而，在铁过载的情况下，当超过转铁蛋白的铁结合能力时，血浆中会出现非转铁蛋白结合铁（NTBI）。NTBI 通过 NTBI 转运蛋白被肝细胞和其他实质细胞吸收，可通过促进活性氧的产生而引起细胞损伤。然而，尚未探索 NTBI 如何影响内皮细胞，这是暴露于循环 NTBI 的最接近的细胞类型。我们通过用 NTBI（柠檬酸铁铵 [FAC]）处理原代人脐静脉内皮细胞 (HUVEC) 在体外模拟全身性铁过载对内皮细胞的影响。我们通过 RNA-Seq 表明，铁负载通过诱导甾醇调节元件结合蛋白 2 介导的胆固醇生物合成来改变 HUVEC 中的脂质稳态。我们还确定 FAC 增加了 HUVEC 对由肿瘤坏死因子-α (TNFα) 诱导的细胞凋亡的敏感性。此外，我们发现胆固醇生物合成有助于铁增强的细胞凋亡。用胆固醇螯合剂羟丙基-β-环糊精处理 HUVECs 表明，即使在 FAC 存在的情况下，胆固醇的消耗也足以挽救 HUVECs 免受 TNFα 诱导的细胞凋亡。最后，我们发现 FAC 或胆固醇处理通过诱导 TNFR1 的新蛋白水解加工为定位于脂筏的短同种型来调节 TNFα 途径。