Matsuo Shunsuke, Ogawa Masayuki, Muckenthaler Martina U, Mizui Yumiko, Sasaki Shota, Fujimura Takafumi, Takizawa Masayuki, Ariga Nagayuki, Ozaki Hiroaki, Sakaguchi Masakiyo, Gonzalez Frank J, Inoue Yusuke
From the Division of Molecular Science, Graduate School of Science and Technology, Gunma University, Kiryu, Gunma 376-8515, Japan.
the Department of Pediatric Oncology, Hematology and Immunology, University of Heidelberg, 69120 Heidelberg, Germany.
J Biol Chem. 2015 Dec 25;290(52):30855-65. doi: 10.1074/jbc.M115.694414. Epub 2015 Nov 2.
Iron is an essential element in biological systems, but excess iron promotes the formation of reactive oxygen species, resulting in cellular toxicity. Several iron-related genes are highly expressed in the liver, a tissue in which hepatocyte nuclear factor 4α (HNF4α) plays a critical role in controlling gene expression. Therefore, the role of hepatic HNF4α in iron homeostasis was examined using liver-specific HNF4α-null mice (Hnf4a(ΔH) mice). Hnf4a(ΔH) mice exhibit hypoferremia and a significant change in hepatic gene expression. Notably, the expression of transferrin receptor 2 (Tfr2) mRNA was markedly decreased in Hnf4a(ΔH) mice. Promoter analysis of the Tfr2 gene showed that the basal promoter was located at a GC-rich region upstream of the transcription start site, a region that can be transactivated in an HNF4α-independent manner. HNF4α-dependent expression of Tfr2 was mediated by a proximal promoter containing two HNF4α-binding sites located between the transcription start site and the translation start site. Both the GC-rich region of the basal promoter and the HNF4α-binding sites were required for maximal transactivation. Moreover, siRNA knockdown of HNF4α suppressed TFR2 expression in human HCC cells. These results suggest that Tfr2 is a novel target gene for HNF4α, and hepatic HNF4α plays a critical role in iron homeostasis.
铁是生物系统中的一种必需元素，但过量的铁会促进活性氧的形成，从而导致细胞毒性。几个与铁相关的基因在肝脏中高度表达，在肝脏组织中，肝细胞核因子4α（HNF4α）在控制基因表达方面发挥着关键作用。因此，利用肝脏特异性HNF4α基因敲除小鼠（Hnf4a(ΔH)小鼠）研究了肝脏HNF4α在铁稳态中的作用。Hnf4a(ΔH)小鼠表现出低铁血症和肝脏基因表达的显著变化。值得注意的是，转铁蛋白受体2（Tfr2）mRNA在Hnf4a(ΔH)小鼠中的表达明显降低。对Tfr2基因的启动子分析表明，基础启动子位于转录起始位点上游富含GC的区域，该区域可以以不依赖HNF4α的方式被反式激活。Tfr2的HNF4α依赖性表达由一个近端启动子介导，该近端启动子包含位于转录起始位点和翻译起始位点之间的两个HNF4α结合位点。基础启动子的富含GC区域和HNF4α结合位点都是最大反式激活所必需的。此外，HNF4α的siRNA敲低抑制了人肝癌细胞中TFR2的表达。这些结果表明，Tfr2是HNF4α的一个新的靶基因，肝脏HNF4α在铁稳态中起关键作用。
