Gut-liver axis on biotransformation of environmental chemicals polybrominated diphenyl ethers (PBDEs)
Polybrominated diphenyl ethers (PBDEs) are flame retardants that are widely used in plastics, rubbers, textiles, furniture and electronic devices. Worrisome levels of PBDEs have been detected in fatty food, household dust, human blood and breast milk due to their lipophilic and bio-accumulative features. Animal studies, with limited supporting human epidemiology data, have found associations between PBDE exposure and multiple detrimental effects including neurotoxicity, thyroid hormone disorders, and hepatotoxicity such as oxidative stress and cancer. In liver, which is the major organ for xenobiotic biotransformation and nutrient homeostasis, certain PBDE congeners are oxidized by cytochrome P450s (Cyps) to form more toxic metabolites. The gut microbiome has been increasingly recognized as a novel frontier for xenobiotic metabolism, and previous work from our laboratory has demonstrated that lack of a gut microbiome profoundly altered the expression of xenobiotic-processing genes (XPGs) in the liver. However, very little is known regarding the interactions between PBDEs and the gut microbiome, and how such interactions modify the hepatic xenobiotic biotransformation and intermediary metabolism pathways. Therefore, the goal of my dissertation is to utilize multidisciplinary approaches, including germ-free mice, second generation sequencing (RNA-Seq and microbial 16S rRNA sequencing), targeted proteomics (LC-MS), targeted metabolomics (GC-MS for PBDE phenolics and UPLC- MS/MS for bile acids), as well as bioinformatics, to strategically investigate the effect of PBDEs on the gut microbiome in vivo, and further determine how the gut microbiome modulates PBDE-mediated regulation of the hepatic transcriptome and the biotransformation and disposition of PBDEs. To achieve this goal, I exposed nine- week-old male C57BL/6 conventional (CV) and germ-free (GF) mice with vehicle (corn oil, p.o. 10 ml/kg) and the most predominant PBDE congeners, BDE-47 or BDE-99 (100 μmol/kg, p.o.), for four consecutive days. Gut microbiomes were quantified using 16S rRNA sequencing of the large intestinal content (LIC) of CV mice. Both BDE-47 and BDE-99 profoundly decreased the bacterial richness (i.e. α diversity). PBDEs differentially regulated 45 bacterial species, especially an up-regulation of Akkermansia muciniphila and Erysipelotrichacea allobaculum, which are known to have anti- inflammatory and anti-obesity properties. In livers of GF mice, the major BDE-47 hydroxylated metabolite, 5-OH-BDE-47, was higher as compared to livers of CV mice; whereas 4 minor BDE-47 metabolites and 4 minor BDE-99 metabolites were lower. RNA-Seq of the hepatic transcriptome in CV and GF mice showed that the interactions between gut microbiome and PBDEs not only regulated the protein-coding genes (PCGs), but also long non-coding RNAs (lncRNAs), which are increasingly recognized as novel biomarkers for toxicological responses. The absence of gut microbiome sensitized the liver to BDE-99-mediated transcriptional regulation of both PCGs and lncRNAs in GF mice. Besides xenobiotic metabolism, the gut microbiome is also important for endogenous functions such as bile acid (BA) homeostasis, which regulates nutrient absorption, and may contribute to obesity, inflammation, and cancer. Targeted metabolomics of 56 BAs in serum, liver, small intestinal content (SIC) and LIC of CV and GF mice showed that PBDEs, especially BDE-99, markedly increased many unconjugated BAs in multiple bio-compartments in a gut microbiota-dependent manner. Therefore, these BAs may serve as secondary signaling molecules to relay toxicological response following PBDE exposure. Taken together, the present study revealed for the first time that there is a novel interaction between the gut microbiome and PBDEs, and this markedly impacts both xenobiotic biotransformation and intermediary metabolism pathways in the host liver. Targeting the “gut-liver axis” may lead to the design of novel probiotic therapies to reduce the toxicities of PBDEs in vulnerable populations.
- Environmental health