Studies of gene expression and physiological roles of cellular retinol-binding protein, type II in the small intestine.

我们报道了细胞视黄醇结合蛋白II型（CRBPII）的基因表达受膳食脂肪酸的转录调节，该蛋白在小肠细胞维生素A的摄取、转运及其代谢中起重要作用。在本研究中，我们进一步研究了CRBPII基因的转录调控机制，以及脂肪酸对小肠维生素A摄取和代谢的影响。我们在共转染实验中证明，在PPAR配体如亚油酸和花生四烯酸存在的情况下，过氧化物酶体增殖物激活受体α (PPARα)和类视黄醇X受体（RXR）的异源二聚体通过与两种假定的核受体反应元件（RXRE和RE3）结合，诱导大鼠CRBPII基因启动子的交互活性。在大鼠小肠中，PPARα和PPARδ转录本都大量表达。因此，在下一步中，我们检测了PPARα和PPARδ在人小肠样细胞系Caco-2中配体响应性的差异。在单杂交实验中，与PPARδ相比，ppar α介导的交易性表现出广泛而强烈的配体反应性。此外，PPARα而不是PPARδ优先与在大鼠小肠中大量表达的共激活因子p300相互作用。我们进一步发现，脂肪酸增加小肠CRBPII基因表达水平导致小肠维生素A的细胞摄取和代谢增加。这些结果表明，小肠中包括CRBPII在内的ppar依赖基因的表达可能受到PPARα和PPARδ的协调调节。

We have reported that the gene expression of cellular retinol-binding protein, type II (CRBPII), which plays important roles in vitamin A uptake, transport and their metabolisms in small intestinal cells, is transcriptionally regulated by dietary fatty acids. In the present study, we further investigated the mechanism of transcriptional regulation of CRBPII gene and the effects on the intestinal vitamin A uptake and metabolisms modulated by fatty acids in the small intestine. We demonstrated in the cotransfection assay that the heterodimer of peroxisome proliferator-activated receptor α (PPARα) and retinoid X receptor (RXR) induced the rat CRBPII gene promoter's transactivity by binding to two putative nuclear receptor response elements (RXRE and RE3) in the presence of PPAR ligands such as linoleic acid and arachidonic acid. In the rat small intestine, not only PPARα but also PPARδ trasncripts was abundantly expressed. Therefore, in the next step, we examined the differences of ligand responsiveness between PPARα and PPARδ in the human small intestine-like cell lines Caco-2. In the one-hybrid assay, broad and great ligand responsiveness was seen in PPARα-mediated transactivity compared to PPARδ. Moreover, PPARα but not PPARδ preferentially interacted with coactivator p300, which was abundantly expressed in the rat small intestine. We further revealed that the increase of small intestinal CRBPII gene expression level by fatty acids led to an increase in the cellular uptake and metabolisms of vitamin A in the small intestine. These results suggest that the expressions of PPAR-dependent genes in the small intestine including CRBPII may be coordinately regulated by PPARα and PPARδ.