Martínez-Jiménez Celia P, Gómez-Lechón M José, Castell José V, Jover Ramiro
Unidad de Hepatología Experimental, Centro de Investigación, Hospital Universitario La Fe, 46009 Valencia, Spain.
J Biol Chem. 2006 Oct 6;281(40):29840-9. doi: 10.1074/jbc.M604046200. Epub 2006 Aug 4.
Hepatocyte nuclear factor 4alpha (HNF4alpha) plays critical roles during liver development and in the transcriptional regulation of many hepatic genes in adult liver. Here we have demonstrated that in human hepatoma HepG2 cells, HNF4alpha is expressed at levels as high as in human liver but its activity on target genes is very low or absent. We have discovered that the low expression of key coactivators (PGC1alpha, SRC1, SRC2, and PCAF) might account for the lack of function of HNF4alpha in HepG2 cells. Among them, PGC1alpha and SRC1 are the two most important HNF4alpha coactivators as revealed by reporter assays with an Apo-CIII promoter construct. Moreover, the expression of these two coactivators was found to be down-regulated in all human hepatomas investigated. Overexpression of SRC1 and PGC1alpha by recombinant adenoviruses led to a significant up-regulation of well characterized HNF4alpha-dependent genes (ApoCIII, ApoAV, PEPCK, AldoB, OTC, and CYP7A1) and forced HepG2 cells toward a more differentiated phenotype as demonstrated by increased ureogenic rate. The positive effect of PGC1alpha was seen to be dependent on HNF4alpha. Finally, insulin treatment of human hepatocytes and HepG2 cells caused repression of PGC1alpha and a concomitant down-regulation of ApoCIII, PEPCK, AldoB, and OTC. Altogether, our results suggest that SRC1, and notably PGC1alpha, are key coactivators for the proper function of HNF4alpha in human liver and for an integrative control of multiple hepatic genes involved in metabolism and homeostasis. The down-regulation of key HNF4alpha coactivators could be a determinant factor for the dedifferentiation of human hepatomas.
肝细胞核因子4α（HNF4α）在肝脏发育过程以及成年肝脏中许多肝脏基因的转录调控中发挥着关键作用。在此，我们证明在人肝癌HepG2细胞中，HNF4α的表达水平与人肝脏中的一样高，但其对靶基因的活性非常低或不存在。我们发现关键共激活因子（PGC1α、SRC1、SRC2和PCAF）的低表达可能是HepG2细胞中HNF4α功能缺失的原因。其中，PGC1α和SRC1是两个最重要的HNF4α共激活因子，这是通过使用载脂蛋白CIII启动子构建体的报告基因检测所揭示的。此外，在所有研究的人肝癌中发现这两种共激活因子的表达下调。通过重组腺病毒过表达SRC1和PGC1α导致了特征明确的HNF4α依赖性基因（ApoCIII、ApoAV、磷酸烯醇式丙酮酸羧激酶、醛缩酶B、鸟氨酸转氨甲酰酶和CYP7A1）的显著上调，并使HepG2细胞向更分化的表型转变，这通过尿素生成率的增加得以证明。PGC1α的积极作用被认为依赖于HNF4α。最后，对人肝细胞和HepG2细胞进行胰岛素处理导致PGC1α受到抑制，同时ApoCIII、磷酸烯醇式丙酮酸羧激酶、醛缩酶B和鸟氨酸转氨甲酰酶下调。总之，我们的结果表明SRC1，尤其是PGC1α，是HNF4α在人肝脏中正常功能以及对参与代谢和体内平衡的多个肝脏基因进行综合调控的关键共激活因子。关键HNF4α共激活因子的下调可能是人肝癌去分化的一个决定性因素。
