背景:胆汁酸(BA)是类固醇衍生的分子,在消化中具有重要作用,维持宿主的新陈代谢,和免疫调节。主BA由主机合成,而次级BA是由肠道微生物组通过前者的转化产生的。对次级BA的微生物生产的调节还没有很好的理解,特别是7-脱羟基BA的生产,它们是宿主BA受体最有效的激动剂。胆酸(CA)的7-脱羟基作用已得到很好的建立,并且与负责该过程的胆汁酸诱导型(bai)操纵子的表达有关。然而,对于其他宿主衍生的BA,几乎没有7-脱羟基的报道(例如,鹅去氧胆酸,CDCA或熊去氧胆酸,UDCA)。
结果:这里,我们证明,当CA存在时,人分离梭菌scindens诱导CDCA和UDCA的7-脱羟基作用,表明CA依赖性转录调节是这些主要BA的大量7-脱羟基作用所必需的。这得到单独的UDCA不促进bai基因表达的发现的支持。CDCA上调bai基因的表达,但当CA存在时,表达更高。相比之下,鼠分离株Extibactermuris表现出明显的反应;CA没有诱导主要BAs的显着7-脱羟基化,而BA7-脱羟基作用在体外添加无菌小鼠盲肠含量后得到促进。然而,发现E.muris在体内7-去羟基化。
结论:菌株之间不同的表达反应表明bai基因受到不同的调控。CA在C.scindens菌株中促进bai操纵子基因表达和7-脱羟基活性。相反,仅在添加盲肠内容物后,E.muris的体外活性才得到促进,并且该分离株不会响应CA改变bai基因的表达。辅助基因baiJ仅在C.scindensATCC35704菌株中上调,暗示分离株之间的机械差异。有趣的是,人来源的C.scindens菌株也能够在有限的程度上对鼠胆汁酸(胞嘧啶酸)进行7-脱羟基。这项研究显示了由于CA的存在而在体外产生的新的7-脱羟基活性,并暗示了跨细菌物种的不同bai基因表达。
BACKGROUND: Bile acids (BAs) are steroid-derived molecules with important roles in digestion, the maintenance of host metabolism, and immunomodulation. Primary BAs are synthesized by the host, while secondary BAs are produced by the gut microbiome through transformation of the former. The regulation of microbial production of secondary BAs is not well understood, particularly the production of 7-dehydroxylated BAs, which are the most potent agonists for host BA receptors. The 7-dehydroxylation of cholic acid (CA) is well established and is linked to the expression of a bile acid-inducible (bai) operon responsible for this process. However, little to no 7-dehydroxylation has been reported for other host-derived BAs (e.g., chenodeoxycholic acid, CDCA or ursodeoxycholic acid, UDCA).
RESULTS: Here, we demonstrate that the 7-dehydroxylation of CDCA and UDCA by the human isolate Clostridium scindens is induced when CA is present, suggesting that CA-dependent transcriptional regulation is required for substantial 7-dehydroxylation of these primary BAs. This is supported by the finding that UDCA alone does not promote expression of bai genes. CDCA upregulates expression of the bai genes but the expression is greater when CA is present. In contrast, the murine isolate Extibacter muris exhibits a distinct response; CA did not induce significant 7-dehydroxylation of primary BAs, whereas BA 7-dehydroxylation was promoted upon addition of germ-free mouse cecal content in vitro. However, E. muris was found to 7-dehydroxylate in vivo.
CONCLUSIONS: The distinct expression responses amongst strains indicate that bai genes are regulated differently. CA promoted bai operon gene expression and the 7-dehydroxylating activity in C. scindens strains. Conversely, the in vitro activity of E. muris was promoted only after the addition of cecal content and the isolate did not alter bai gene expression in response to CA. The accessory gene baiJ was only upregulated in the C. scindens ATCC 35704 strain, implying mechanistic differences amongst isolates. Interestingly, the human-derived C. scindens strains were also capable of 7-dehydroxylating murine bile acids (muricholic acids) to a limited extent. This study shows novel 7-dehydroxylation activity in vitro resulting from the presence of CA and suggests distinct bai gene expression across bacterial species.