Quercetin Represses Apolipoprotein B Expression by Inhibiting the Transcriptional Activity of C/EBPβ

Quercetin is one of the most abundant polyphenolic flavonoids found in fruits and vegetables and has anti-oxidative and anti-obesity effects. Because the small intestine is a major absorptive organ of dietary nutrients, it is likely that highly concentrated food constituents, including polyphenols, are present in the small intestinal epithelial cells, suggesting that food factors may have a profound effect in this tissue. To identify novel targets of quercetin in the intestinal enterocytes, mRNA profiling using human intestinal epithelial Caco-2 cells was performed. We found that mRNA levels of some apolipoproteins, particularly apolipoprotein B (apoB), are downregulated in the presence of quercetin. On the exposure of Caco-2 cells to quercetin, both mRNA and protein levels of apoB were decreased. Promoter analysis of the human apoB revealed that quercetin response element is localized at the 5′-proximal promoter region, which contains a conserved CCAAT enhancer-binding protein (C/EBP)-response element. We found that quercetin reduces the promoter activity of apoB, driven by the enforced expression of C/EBPβ. Quercetin had no effect on either mRNA or protein levels of C/EBPβ. In contrast, we found that quercetin inhibits the transcriptional activity of C/EBPβ but not its recruitment to the apoB promoter. On the exposure of Caco-2 cells to quercetin 3-O-glucuronide, which is in a cell-impermeable form, no notable change in apoB mRNA was observed, suggesting an intracellular action of quercetin. In vitro interaction experiments using quercetin-conjugated beads revealed that quercetin binds to C/EBPβ. Our results describe a novel regulatory mechanism of transcription of apolipoprotein genes by quercetin in the intestinal enterocytes.

槲皮素（Quercetin）是果蔬中含量最丰富的多酚类黄酮（polyphenolic flavonoids）之一，具备抗氧化与抗肥胖活性。由于小肠是膳食营养素的主要吸收器官，小肠上皮细胞中大概率存在包括多酚在内的高浓度食物成分，提示食物因子可对该组织产生显著影响。为鉴定肠上皮细胞中槲皮素的新型作用靶点，本研究采用人小肠上皮Caco-2细胞进行了mRNA谱分析（mRNA profiling）。结果发现，部分载脂蛋白（apolipoproteins）的mRNA水平，尤其是载脂蛋白B（apolipoprotein B，apoB），在槲皮素处理后出现下调。将Caco-2细胞暴露于槲皮素后，apoB的mRNA与蛋白水平均有所降低。对人apoB的启动子分析（promoter analysis）显示，槲皮素应答元件定位于5′近端启动子区域（5′-proximal promoter region），该区域包含保守的CCAAT增强子结合蛋白（CCAAT enhancer-binding protein，C/EBP）应答元件。研究发现，槲皮素可降低由C/EBPβ强制表达（enforced expression）驱动的apoB启动子活性（promoter activity）。槲皮素对C/EBPβ的mRNA及蛋白水平均无影响。与之相反，槲皮素可抑制C/EBPβ的转录活性（transcriptional activity），但不影响其向apoB启动子的募集。将Caco-2细胞暴露于细胞不可通透形式（cell-impermeable form）的槲皮素3-O-葡萄糖醛酸苷（quercetin 3-O-glucuronide）后，未观察到apoB mRNA出现显著变化，提示槲皮素通过细胞内途径发挥作用。采用槲皮素偶联磁珠（quercetin-conjugated beads）进行的体外相互作用实验（in vitro interaction experiments）显示，槲皮素可与C/EBPβ结合。本研究结果阐明了槲皮素在肠上皮细胞中调控载脂蛋白基因转录的新型分子机制。