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L‐Tryptophan and Bacterial Modulation of Intestinal and Hepatic Gene Expression

Host and microbial metabolites of L‐Tryptophan (Trp) have been shown in vitro to activate aryl hydrocarbon receptor (AhR) and upregulate its prototypical target genes: Cytochrome P450 (CYP) 1A1 and 1A2. Additionally, the Trp microbial metabolite indole‐3‐propionic acid (IPA) produced by Clostridium sporogenes has been shown to be an activator of pregnane X receptor (PXR) in mouse intestine to maintain mucosal integrity. However, very little is known regarding the effect of Trp and its metabolites on host xenobiotic biotransformation in liver and intestine. The objective of this study was to determine the potential crosstalk between microbial Trp metabolism and host xenobiotic biotransformation. To investigate the effect of Trp on the host expression of drug‐processing genes (DPGs), 60‐day‐old conventional (CV) male C57BL/6J mice were orally gavaged 0, 10, 20, or 40 mg/kg L‐Trp once daily for 6 days. Bacterial 16S rRNA qPCR showed that Trp did not alter the abundance of 5 known Trp‐metabolizing bacteria in intestine of CV mice; however, Trp at all 3 doses down‐regulated Bifidobacterium bifidum, which has been shown to be involved in bile acid metabolism. Unexpectedly, Trp did not up‐regulate Cyp1a1 mRNA in any sections of the intestine; in fact, all 3 doses of Trp decreased Cyp1a1 mRNA in jejunum. Conversely, Trp dose‐dependently up‐regulated the PXR‐target gene Cyp3a11, and the constitutive androstane receptor (CAR)‐target gene Cyp2b10 in duodenum at all doses, as well as the PXR‐target gene multidrug resistance transporter 1a (Mdr1a) at the highest dose. In liver, Trp oral treatment did not affect the mRNAs of the prototypical targeted DPGs for AhR, CAR, PXR, or peroxisome proliferator‐activated receptor α (PPARα), although the CAR mRNA was downregulated at the medium and high doses. Furthermore, control germ‐free (GF) mice have lower expression of Cyp3a11. To test that replacing the Trp microbial metabolite IPA in GF mice normalizes the expression of PXR‐targeted DPGs in liver and intestine, 90‐day‐old male GF C57BL/6 mice were administered 0 or 40 mg/kg IPA p.o. or i.p. once daily for 4 days. Neither route of IPA exposure markedly affected Cyp3a11 mRNA expression in intestine or liver. However, Mdr1a was up‐regulated by IPA in jejunum (p.o.) and large intestine (i.p.). The PPARα‐target gene Cyp4a14 was down‐regulated by IPA (i.p.) in duodenum, whereas Cyp1a2 mRNA was up‐regulated by IPA (p.o.) in liver. To recapitulate the effect of Trp oral exposure on human liver, differentiated HepaRG cells were treated with Trp or IPA at a concentration range from 10 to 250 μM for 24h. IPA, but not Trp, dose‐dependently up‐regulated CYP1A1, 2B6, and 3A4 in HepaRG cells, suggesting that the microbial metabolism of Trp is necessary to modulate a hepatic transcriptional response in DPGs. In conclusion, oral exposure to Trp has minimal effect on DPG expression in mouse liver, and preferably up‐regulates the PXR‐ and CAR‐targeted but not the AhR‐targeted DPGs in mouse intestine. The Trp microbial metabolite IPA does not appear to be a major inducer for PXR‐targeted DPGs in liver or intestine of mice, but it may be important for activating AhR, CAR, and PXR pathways in human hepatocytes.