Department of Structural and Molecular Biology, Division of Biosciences, University College London, Gower Street, London WC1E 6BT, UK.
J Biomed Sci. 2012 Sep 21;19(1):83. doi: 10.1186/1423-0127-19-83.
Iron homeostasis is chiefly regulated by hepcidin whose expression is tightly controlled by inflammation, iron stores, and hypoxia. Hemojuvelin (HJV) is a bone morphogenetic protein co-receptor that has been identified as a main upstream regulator of hepcidin expression; HJV mutations are associated with a severe form of iron overload (Juvenile haemochromatosis). Currently however, there is no information on how HJV is regulated by inflammation.
To study the regulation of Hjv expression by inflammation and whether Hfe has a role in that regulation, control and LPS-injected wild type and Hfe KO mice were used. Moreover, human hepatoma cells (HuH7) were used to study the effect of IL-6 and TNF-α on HJV mRNA expression.
Here we show that LPS repressed hepatic Hjv and BMPs, while it induced hepcidin 1 expression in wild-type and Hfe KO mice with no effect on hepatic pSMAD 1, 5, 8 protein levels. In addition, exogenous TNF-α (20 ng/mL) decreased HJV mRNA and protein expression to 40% of control with no effect on hepcidin mRNA expression in 24 hours. On the other hand, IL-6 induced hepcidin mRNA and protein expression with no effect on HJV mRNA expression levels. Moreover, using the HJV promoter-luciferase reporter fusion construct (HJVP1.2-luc), we showed that the basal luciferase activity of HJVP1.2-luc was inhibited by 33% following TNF-α treatment of HuH7 transfected cells suggesting that the TNF-α down-regulation is exerted at the transcriptional level. Additionally, mutation of a canonical TNF- alpha responsive element (TNFRE) within HJVP1.2-luc abolished TNF-α response suggesting that this TNFRE is functional.
From these results, we conclude that TNF-α suppresses HJV transcription possibly via a novel TNFRE within the HJV promoter. In addition, the results suggest that the proposed link between inflammation and BMP-SMAD signalling is independent of HJV and BMP ligands.
铁稳态主要由铁调素调节，其表达受炎症、铁储存和缺氧的严格控制。 珠蛋白（HJV）是骨形态发生蛋白的共同受体，已被确定为铁调素表达的主要上游调节剂; HJV 突变与严重的铁过载（青少年血色病）有关。 然而，目前尚无关于 HJV 如何受炎症调节的信息。
为了研究炎症对 Hjv 表达的调节作用以及 Hfe 是否在该调节中起作用，使用了对照和 LPS 注射的野生型和 Hfe KO 小鼠。此外，还使用人肝癌细胞（HuH7）研究了 IL-6 和 TNF-α对 HJV mRNA 表达的影响。
我们发现 LPS 抑制了野生型和 Hfe KO 小鼠肝脏中的 Hjv 和 BMPs，同时诱导了铁调素 1 的表达，但对肝脏 pSMAD 1、5、8 蛋白水平没有影响。此外，外源性 TNF-α（20ng/ml）将 HJV mRNA 和蛋白表达降低至对照的 40%，而对铁调素 mRNA 表达无影响 24 小时。另一方面，IL-6 诱导了铁调素 mRNA 和蛋白的表达，但对 HJV mRNA 表达水平没有影响。此外，使用 HJV 启动子-荧光素酶报告融合构建体（HJVP1.2-luc），我们发现 TNF-α 处理转染 HuH7 细胞的细胞后，HJVP1.2-luc 的基础荧光素酶活性抑制了 33%，表明 TNF-α 的下调是在转录水平上进行的。此外，在 HJVP1.2-luc 内突变一个典型的 TNF-α 反应元件（TNFRE）使 TNF-α 反应丧失功能，表明该 TNFRE 是功能性的。
从这些结果中，我们得出结论，TNF-α 通过 HJV 启动子内的新 TNFRE 抑制 HJV 转录。此外，研究结果表明，炎症与 BMP-SMAD 信号之间的拟议联系独立于 HJV 和 BMP 配体。
