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

我们已经报道，细胞视黄醇结合蛋白II(CRBPII)的基因表达受膳食脂肪酸的转录调控，CRBPII在小肠细胞对维生素A的吸收、运输及其代谢中起着重要作用。在本研究中，我们进一步研究了CRBPII基因转录调控的机制以及小肠脂肪酸对肠道维生素A摄取和代谢的影响。在共转染实验中，我们证明了过氧化体增殖物激活受体α(PPARα)和维甲酸X受体(RXR)的异源二聚体在亚油酸和花生四烯酸等PPAR配体存在的情况下，通过与两个可能的核受体反应元件(RXRE和RE3)结合而诱导了大鼠CRBPII基因启动子的反式反应。在大鼠小肠中，PPARα和PPARδ转录物均有丰富的表达。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中的配体反应性的差异。在单杂交试验中，与PPARα相比，PPARδ介导的反式反应具有广泛和更大的配基反应性。此外，PPARα而不是PPARδ优先与辅活化子p300相互作用，p300在大鼠小肠中大量表达。我们进一步揭示，脂肪酸提高了小肠CRBPII基因的表达水平，导致细胞对维生素A的摄取和代谢增加。这些结果表明，包括CRBPⅡ在内的PPAR依赖基因在小肠中的表达可能受到PPARα和PPARδ的协同调控。

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.