Song Bao-Liang, Wang Can-Hua, Yao Xiao-Min, Yang Li, Zhang Wen-Jing, Wang Zhen-Zhen, Zhao Xiao-Nan, Yang Jin-Bo, Qi Wei, Yang Xin-Ying, Inoue Kenji, Lin Zhi-Xin, Zhang Hui-Zhan, Kodama Tatsuhiko, Chang Catherine C Y, Liu Yin-Kun, Chang Ta-Yuan, Li Bo-Liang
State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China.
Biochem J. 2006 Mar 15;394(Pt 3):617-26. doi: 10.1042/BJ20051417.
Humans express two ACAT (acyl-CoA:cholesterol acyltransferase) genes, ACAT1 and ACAT2. ACAT1 is ubiquitously expressed, whereas ACAT2 is primarily expressed in intestinal mucosa and plays an important role in intestinal cholesterol absorption. To investigate the molecular mechanism(s) responsible for the tissue-specific expression of ACAT2, we identified five cis-elements within the human ACAT2 promoter, four for the intestinal-specific transcription factor CDX2 (caudal type homeobox transcription factor 2), and one for the transcription factor HNF1alpha (hepatocyte nuclear factor 1alpha). Results of luciferase reporter and electrophoretic mobility shift assays show that CDX2 and HNF1alpha exert a synergistic effect, enhancing the ACAT2 promoter activity through binding to these cis-elements. In undifferentiated Caco-2 cells, the ACAT2 expression is increased when exogenous CDX2 and/or HNF1alpha are expressed by co-transfection. In differentiated Caco-2 cells, the ACAT2 expression significantly decreases when the endogenous CDX2 or HNF1alpha expression is suppressed by using RNAi (RNA interference) technology. The expression levels of CDX2, HNF1alpha, and ACAT2 are all greatly increased when the Caco-2 cells differentiate to become intestinal-like cells. These results provide a molecular mechanism for the tissue-specific expression of ACAT2 in intestine. In normal adult human liver, CDX2 expression is not detectable and the ACAT2 expression is very low. In the hepatoma cell line HepG2 the CDX2 expression is elevated, accounting for its elevated ACAT2 expression. A high percentage (seven of fourteen) of liver samples from patients affected with hepatocellular carcinoma exhibited elevated ACAT2 expression. Thus, the elevated ACAT2 expression may serve as a new biomarker for certain form(s) of hepatocellular carcinoma.
人类表达两种ACAT（酰基辅酶A：胆固醇酰基转移酶）基因，即ACAT1和ACAT2。ACAT1在全身广泛表达，而ACAT2主要在肠黏膜中表达，并在肠道胆固醇吸收中起重要作用。为了研究负责ACAT2组织特异性表达的分子机制，我们在人类ACAT2启动子中鉴定出五个顺式元件，四个用于肠道特异性转录因子CDX2（尾型同源框转录因子2），一个用于转录因子HNF1α（肝细胞核因子1α）。荧光素酶报告基因和电泳迁移率变动分析结果表明，CDX2和HNF1α发挥协同作用，通过与这些顺式元件结合来增强ACAT2启动子活性。在未分化的Caco-2细胞中，通过共转染表达外源性CDX2和/或HNF1α时，ACAT2表达增加。在分化的Caco-2细胞中，使用RNAi（RNA干扰）技术抑制内源性CDX2或HNF1α表达时，ACAT2表达显著降低。当Caco-2细胞分化为肠样细胞时，CDX2、HNF1α和ACAT2的表达水平均大幅增加。这些结果为ACAT2在肠道中的组织特异性表达提供了分子机制。在正常成人肝脏中，检测不到CDX2表达，ACAT2表达也非常低。在肝癌细胞系HepG2中，CDX2表达升高，这解释了其ACAT2表达升高的原因。在肝细胞癌患者的肝脏样本中，高比例（十四分之七）表现出ACAT2表达升高。因此，ACAT2表达升高可能作为某些形式肝细胞癌的新生物标志物。
