Center for Inhalation Toxicology, West Virginia University School of Medicine, Morgantown, WV, USA; Department of Physiology and Pharmacology, Health Sciences Center, West Virginia University, Morgantown, WV, USA.
Department of Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, PA, USA.
Redox Biol. 2023 Nov;67:102866. doi: 10.1016/j.redox.2023.102866. Epub 2023 Sep 4.
We recently reported a previously unknown salutary role for xanthine oxidoreductase (XOR) in intravascular heme overload whereby hepatocellular export of XOR to the circulation was identified as a seminal step in affording protection. However, the cellular signaling and export mechanisms underpinning this process were not identified. Here, we present novel data showing hepatocytes upregulate XOR expression/protein abundance and actively release it to the extracellular compartment following exposure to hemopexin-bound hemin, hemin or free iron. For example, murine (AML-12 cells) hepatocytes treated with hemin (10 μM) exported XOR to the medium in the absence of cell death or loss of membrane integrity (2.0 ± 1.0 vs 16 ± 9 μU/mL p < 0.0001). The path of exocytosis was found to be noncanonical as pretreatment of the hepatocytes with Vaculin-1, a lysosomal trafficking inhibitor, and not Brefeldin A inhibited XOR release and promoted intracellular XOR accumulation (84 ± 17 vs 24 ± 8 hemin vs 5 ± 3 control μU/mg). Interestingly, free iron (Fe and Fe) induced similar upregulation and release of XOR compared to hemin. Conversely, concomitant treatment with hemin and the classic transition metal chelator DTPA (20 μM) or uric acid completely blocked XOR release (p < 0.01). Our previously published time course showed XOR release from hepatocytes likely required transcriptional upregulation. As such, we determined that both Sp1 and NF-kB were acutely activated by hemin treatment (∼2-fold > controls for both, p < 0.05) and that silencing either or TLR4 with siRNA prevented hemin-induced XOR upregulation (p < 0.01). Finally, to confirm direct action of these transcription factors on the Xdh gene, chromatin immunoprecipitation was performed indicating that hemin significantly enriched (∼5-fold) both Sp1 and NF-kB near the transcription start site. In summary, our study identified a previously unknown pathway by which XOR is upregulated via SP1/NF-kB and subsequently exported to the extracellular environment. This is, to our knowledge, the very first study to demonstrate mechanistically that XOR can be specifically targeted for export as the seminal step in a compensatory response to heme/Fe overload.
我们最近报道了黄嘌呤氧化还原酶(XOR)在血管内血红素过载中以前未知的有益作用,其中肝细胞将 XOR 输出到循环系统被确定为提供保护的重要步骤。然而,这一过程的细胞信号和输出机制尚未确定。在这里,我们提供了新的数据,表明肝细胞在暴露于结合血红素的血影蛋白、血红素或游离铁后,会上调 XOR 的表达/蛋白丰度,并将其主动释放到细胞外隔室。例如,用血红素(10 μM)处理的鼠(AML-12 细胞)肝细胞将 XOR 分泌到培养基中,而没有细胞死亡或膜完整性丧失(2.0 ± 1.0 与 16 ± 9 μU/mL 相比,p < 0.0001)。发现外排途径是非典型的,因为用溶酶体转运抑制剂 Vaculin-1 预处理肝细胞,而不是布雷菲德菌素 A 抑制 XOR 释放并促进细胞内 XOR 积累(84 ± 17 与 24 ± 8 血红素与 5 ± 3 对照 μU/mg)。有趣的是,与血红素相比,游离铁(Fe 和 Fe)也诱导类似的 XOR 上调和释放。相反,同时用血红素和经典过渡金属螯合剂 DTPA(20 μM)或尿酸处理完全阻断了 XOR 的释放(p < 0.01)。我们之前发表的时间过程表明,肝细胞中 XOR 的释放可能需要转录上调。因此,我们确定 Sp1 和 NF-kB 都被血红素处理急性激活(两者均为对照的约 2 倍,p < 0.05),并且用 siRNA 沉默 TLR4 或 Sp1 可阻止血红素诱导的 XOR 上调(p < 0.01)。最后,为了确认这些转录因子对 Xdh 基因的直接作用,进行了染色质免疫沉淀,表明血红素显著富集(约 5 倍)Sp1 和 NF-kB 靠近转录起始位点。总之,我们的研究确定了以前未知的途径,通过该途径 XOR 通过 SP1/NF-kB 上调,然后输出到细胞外环境。据我们所知,这是第一个证明机制上 XOR 可以被专门靶向用于输出作为血红素/Fe 过载代偿反应的重要步骤的研究。