Guo Dongqing, Lu Ming, Hu Xihan, Xu Jiajia, Hu Guangjing, Zhu Ming, Zhang Xiaowei, Li Qin, Chang Catherine C Y, Chang Tayuan, Song Baoliang, Xiong Ying, Li Boliang
Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China.
Department of General Surgery, Huashan Hospital, Fudan University, Shanghai 200040, China.
Acta Biochim Biophys Sin (Shanghai). 2016 Nov;48(11):980-989. doi: 10.1093/abbs/gmw091. Epub 2016 Sep 29.
Acyl-coenzyme A:cholesterol acyltransferases (ACATs) are the exclusive intracellular enzymes that catalyze the formation of cholesteryl/steryl esters (CE/SE). In our previous work, we found that the high-level expression of human ACAT2 gene with the CpG hypomethylation of its whole promoter was synergistically regulated by two transcription factors Cdx2 and HNF1α in the intestine and fetal liver. Here, we first observed that the specific CpG-hypomethylated promoter was correlated with the low expression of human ACAT2 gene in monocytic cell line THP-1. Then, two CCAAT/enhancer binding protein (C/EBP) elements within the activation domain in the specific CpG-hypomethylation promoter region were identified, and the expression of ACAT2 in THP-1 cells was evidently decreased when the C/EBP transcription factors were knock-downed using RNAi technology. Furthermore, ChIP assay confirmed that C/EBPs directly bind to their elements for low-level expression of human ACAT2 gene in THP-1 cells. Significantly, the increased expressions of ACAT2 and C/EBPs were also found in macrophages differentiated from both ATRA-treated THP-1 cells and cultured human blood monocytes. These results demonstrate that the low-level expression of human ACAT2 gene with specific CpG-hypomethylated promoter is regulated by the C/EBP transcription factors in monocytic cells, and imply that the lowly expressed ACAT2 catalyzes the synthesis of certain CE/SE that are assembled into lipoproteins for the secretion.
酰基辅酶A：胆固醇酰基转移酶（ACATs）是唯一催化胆固醇酯/甾醇酯（CE/SE）形成的细胞内酶。在我们之前的研究中，我们发现人ACAT2基因在肠道和胎儿肝脏中通过两个转录因子Cdx2和HNF1α协同调控其整个启动子的CpG低甲基化的高水平表达。在此，我们首先观察到特定的CpG低甲基化启动子与单核细胞系THP-1中人ACAT2基因的低表达相关。然后，在特定的CpG低甲基化启动子区域的激活域内鉴定出两个CCAAT/增强子结合蛋白（C/EBP）元件，当使用RNAi技术敲低C/EBP转录因子时，THP-1细胞中ACAT2的表达明显降低。此外，染色质免疫沉淀分析证实C/EBPs直接结合其元件以在THP-1细胞中人ACAT2基因低水平表达。值得注意的是，在经全反式维甲酸（ATRA）处理的THP-1细胞和培养的人血单核细胞分化而来的巨噬细胞中也发现了ACAT2和C/EBPs表达增加。这些结果表明，具有特定CpG低甲基化启动子的人ACAT2基因的低水平表达在单核细胞中受C/EBP转录因子调控，并暗示低表达的ACAT2催化某些CE/SE的合成，这些CE/SE组装成脂蛋白用于分泌。
