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Original Articles: Hepatology

Intestinal Inflammation Alters the Expression of Hepatic Bile Acid Receptors Causing Liver Impairment

Negroni, Anna; Fiaschini, Noemi†,‡; Palone, Francesca; Vitali, Roberta; Colantoni, Eleonora; Laudadio, Ilaria§; Oliva, Salvatore; Aloi, Marina; Cucchiara, Salvatore; Stronati, Laura§

Author Information
Journal of Pediatric Gastroenterology and Nutrition: August 2020 - Volume 71 - Issue 2 - p 189-196
doi: 10.1097/MPG.0000000000002759
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Abstract

What Is Known/What Is New

What Is Known

  • Gut-liver axis connects liver and intestine via bile acid metabolism.
  • Bile acid receptors, regulating lipid and glucose metabolism, are currently known to be also involved in immune and inflammatory response.

What Is New

  • Gut inflammation alters farnesoid-X-receptor, pregnane-X-receptor, and G-protein-coupled receptor (TGR5) levels and increases the production of pro-inflammatory cytokines and oxidative stress in the liver.
  • The amelioration of intestinal inflammation causes liver improvement as shown by the bile acid receptor level restitution.
  • Children with inflammatory bowel disease show altered farnesoid-X-receptor, pregnane-X-receptor, and TGR5 expression in the inflamed colon.

The gut and the liver are anatomically connected by portal circulation, and their functional unit realizes the gut-liver axis (GLA) with the integrity of intestinal barrier crucial for the maintenance of liver homeostasis (1–3). In this mutual relationship, the liver acts as a second firewall towards potentially harmful substances translocated from the gut, and in turn is implicated in the regulation of the mucosal barrier (4).

The GLA connects the liver with the intestine via bile acid metabolism (5–6). Bile acids (BAs) are amphipathic steroid acids, synthesized from cholesterol in the liver, that regulate lipid, glucose, and energy metabolism (7–9). Moreover, a specific role of BAs as immunomodulators is emerging (5,10–14).

The regulatory functions of BAs are predominantly mediated by the bile receptors, such as the nuclear receptors farnesoid-X-receptor (FXR) and pregnane-X-receptor (PXR) as well as the membrane G-protein-coupled receptor (GPCRs; TGR5) (15,16).

Nuclear receptors are ligand-dependent transcription factors that regulate a variety of physiological processes, such as homeostasis, reproduction, development, inflammation, and metabolism, by inducing the transcription of target genes (15). FXR is highly expressed in the liver and gut and affects lipid and glucose metabolism (17). Similarly, PXR is involved in the regulation of xenobiotic metabolism (18), although, recent evidences outline its role also in inflammatory response, cell proliferation, and migration (19).

TGR5 is a member of GPCRs, ubiquitously expressed in diverse tissues, including endocrine organs, muscle, adipose tissue, immune cells, and intestinal tract (20). Recent literature has extended its functions to more than metabolic regulation, which include inflammatory response, cancer, and liver regeneration (20–22). Upon binding with lithocholic acids (LCA) and taurolithocholic acid (TLCA), it is activated to transduce signal transduction into the nucleus and control diverse gene expression (23).

The enterohepatic circulation of BAs is governed by specific transporters expressed in the liver and the intestine and plays a critical role in the digestion of fats and oils. During this process, the majority of the BAs secreted from the liver is reabsorbed in intestinal epithelial cells via the apical sodium-dependent bile acid transporter (ASBT/SLC10A2) and then transported into the portal vein (24).

In this setting, BAs and their receptors are attractive candidates for therapeutic development in chronic diseases, such as type 2 diabetes, hypertriglyceridemia, obesity, nonalcoholic steatohepatitis, inflammatory bowel disease (IBD), and sclerosing cholangitis (25–29).

To deeper investigate the strict relationship between the liver and the gut, the present study aims to assess in vivo and in vitro, the impact of gut inflammation on liver health by focusing on the modulation of BA receptors FXR, PXR, and TGR5. The strategy to improve liver health by reducing gut inflammation is also considered. Moreover, given the role of BA receptors in controlling intestinal mucosal immunity and inflammation, a secondary goal is to analyze the expression pattern of FXR, PXR, and TGR5 BA receptors as well as the ASBT transporter in the inflamed colonic tissues of pediatric patients with IBD and in controls.

METHODS

Ethical Statement

This work has been approved by the Ethic Committee of the Umberto I Hospital, Sapienza University of Rome, Italy. All parents of patients entered into the study provided written informed consent. Experimental procedures on mice were previously approved by the Ministry of Health.

Cell Lines

The human colon adenocarcinoma cell line Caco2, the hepatic adenocarcinoma cell line HepG2, and the murine macrophage-like cell line RAW264.7 were purchased from ATCC (Rockville, MD). Inflammation was induced by cytomix, a combination of TNF-alpha (10 ng/mL; Sigma, St. Louis, MO) and Interferon-gamma (250 ng/mL).

Co-culture System

Cells will be seeded for differentiation on ThinCert cell culture polyethylene terephthalate (PET) capillary pore membranes (0.4 μm pore diameter; Greiner Bio-One International GmbH) and maintained in complete medium supplemented with 10% FBS in both apical (AP) and basolateral (BL) compartments. For co-culture experiments, Caco2 cells differentiated on filter inserts will be transferred to culture plates containing confluent HepG2 cells. Treatment with cytomix will be added to the AP compartment of Caco2 cells and the culture plates will be incubated at 37 °C for 24 and 48 hours. Alternatively, Caco2 cells were infected with LF82 strain at a MOI of 10 : 1 for 3 hours. Then medium with bacteria were removed and replaced with new medium for additional 3 hours.

Trans-epithelial Electric Resistance Assay

Caco2 cells were grown on polyethylene terephthalate membrane inserts, pore size 0.4 mm (Falcon, Becton Dickinson, Franklin Lakes, NJ). TEER values were measured using a Millicell-ERS voltohmmeter (Millipore, Billerica, MA) according to the Manufacturer's instructions.

Animal

C57BL/6 female mice (8--9 weeks of age) were purchased from the animal housing unit of Harlan Laboratories, SRL. Induction of colitis was performed through administration of dextran sodium sulphate (DSS, molecular mass, 36,000–50,000 Da, MP Biomedicals, Santa Ana, CA), 3% dissolved in autoclaved drinking water, for 7 days. Clinical score (CS) was assessed according to the criteria of Maxwell et al (30). On the seventh day, animals were euthanized. Distal colonic, and liver specimens were fixed immediately in a 10% (w/v) formalin solution for histological analysis and frozen in liquid nitrogen or for molecular analyses.

Patients

Ten patients with Crohn disease (CD) (median age: 13.0 years; range: 6–17 years), 10 with ulcerative colitis (UC) (median age: 12.9 years; range: 7–17 years), and 10 controls (median age: 11 years; range: 5–17 years), referred to the Maternal Infantile Department and Urological Sciences, at the Sapienza University of Rome and needed an ileocolonoscopy to reassess the intestinal disease, were included in this study. All selected patients had an established diagnosis of IBD and were in active phase of disease even under therapy. Patients were under treatment with immunomodulators (azathioprine or methotrexate), mesalamine, or oral corticosteroids at low doses. Activity in CD and UC was measured, respectively, by the PCDAI (Pediatric Crohn's Disease Activity Index) (31) and the PUCAI (Pediatric Ulcerative Colitis Activity Index) score (32). The intestinal inflammation was assessed at endoscopy by using the SES-CD score (33) and the endoscopic Mayo subscore (34) in CD and UC patients, respectively. Children with incapacitating functional gastrointestinal disorders requiring extensive investigation, having normal endoscopy and histology, served as controls.

Biopsy Treatment

Colonic mucosal specimens were immediately snap frozen in liquid nitrogen for RNA analysis or fixed immediately in a 10% (w/v) formalin solution for histology and immunohistochemistry.

Real-time Polymerase Chain Reaction

Total RNA was isolated from cells and biopsies using the RNeasy kit (QiaGen, Hilden, Germany). Total RNA (1 μg) was reverse-transcribed to cDNA by a High Capacity cDNA Reverse Transcription Kit (Applied Biosystems). Real-time PCR amplification was done with an ABI PRISM 7300 Sequence Detection System using the SYBR Green kit (Applied Biosystems). The quantity of each mRNA to a reference gene was calculated by the 2−ΔΔCT method.

Immunohistochemistry

Sections (4 μm) of paraffin-embedded intestinal inflamed tissues were prepared following standard protocol. Anti-FXR and anti-TGR5 (Abcam, Cambridge, UK) antibody were used. Finally, sections were stained with hematoxylin and eosin.

Statistics

A minimum of 5 mice per group were included in all experiments. Statistical analysis for significance was determined using the GraphPad InStat software. The Kolmogorov-Smirnov test showed significant departures from the normal distribution for some of the analyzed parameters. Therefore, comparison of the group was performed by the Mann-Whitney U test. For in vitro experiments, the Student t-test was used. Experiments were repeated 3 times. Data were presented as mean ± SD. Differences were noted as significant P < 0.05, and ∗∗P < 0.01.

RESULTS

Farnesoid-X-receptor, Pregnane-X-receptor, and TGR5 Levels are Significantly Altered in the Inflamed Colon as well as in the Liver of Mice With Dextran Sodium Sulphate-induced Colitis

We used the 3% DSS-treated mice as a model of severe colitis. The occurrence of gut inflammation was confirmed by histology, which showed the loss of crypt structure, extensive inflammatory cell infiltration and thickening of the mucosa with abundant edema (Fig. 1A). Interestingly, increased inflammatory cells were also detected in the hepatic tissue of DSS mice as compared with controls (Fig. 1B). Moreover, as expected, inflamed colonic tissues showed increased levels of pro-inflammatory cytokines, TNF-α, IL-6, and IL-1β (P < 0.01); however, IL-6, and IL-1β were strongly increased also in the liver (P < 0.01) (Fig. 1C).

F1
FIGURE 1:
(A and B) Histological sections of colon and liver in DSS mice and control. (C) mRNA expression of IL-6, TNF-α and IL-1β in colon and liver and (D) mRNA level of FXR, PXR, and TGR5 in colon of DSS mice and control. (E) Immunohistochemistry showing FXR and TGR5 in colon of DSS mice and control. (F) mRNA expression of FXR, PXR, and TGR5 in Caco2 and HepG2 and TGR5 level in RAW 264.7. (G and H) mRNA analysis for FXR, PXR, and TGR5 and for iNOS and COX2 in liver of DSS mice and control. P < 0.05; ∗∗ P < 0.01. FXR = farnesoid-X-receptor; PXR = pregnane-X-receptor.

FXR and PXR mRNA expression was significantly decreased in the inflamed colon of DSS-treated mice (P < 0.05), whereas an increased expression of TGR5 was found (P < 0.05) (Fig. 1D). This finding could be explained by recent data reporting the occurrence of higher concentration of TGR5 in macrophages that are highly involved in the tissue inflammation processes (35). To deeper investigate this point, we performed immunohistochemistry on mucosal colonic samples of mice and found that whereas FXR was mainly localized in the nuclei of enterocytes, TGR5 was largely confined to phagocytes (Fig. 1E). Then, we used the murine macrophages RAW 264.7 to confirm in vitro that inflammation enhances the levels of TGR5 receptor. RAW 264.7 cells were exposed to a mix of pro-inflammatory cytokines (cytomix: TNFα+INFγ) for 6 or 24 hours. Results showed a significant increase of TGR5 (P < 0.01), whereas FXR and PXR were very poorly expressed (data not shown). Accordingly, intestinal epithelial cells Caco2 and liver cells HepG2 exposed to the same inflammatory agents showed a decrease of all BA receptors, including TGR5 (P < 0.05) (Fig. 1F).

Remarkably, we observed a significant decrease of FXR and PXR in the liver of mice (P < 0.05) and a substantially unchanged level of TGR5 as well (Fig. 1G). As it is increasingly recognized that FXR has antioxidant activities, in order to better characterize the liver impairment induced by intestinal inflammation, we also analyzed in hepatic tissues, the gene expression of nitric oxide synthase (iNOS) and cyclooxygenase (COX2), that have been shown to play pivotal roles in the development of inflammatory diseases, including cancer. We found that both enzymes were importantly up-regulated (P < 0.01) in the liver of DSS-treated mice (Fig. 1H).

Co-cultures of Intestinal and Hepatic Cells Show that Gut Inflammation Directly Affects Liver Cells by Altering Bile Acid Receptor Expression

We set up a co-culture system with differentiated and oriented Caco2 (above) and HepG2 (below) (Fig. 2A). The full confluence of Caco2 to form a whole intestinal barrier was proven by the transepithelial electric resistance (TEER) assay (Fig. 2B). Intriguingly, we observed upon exposure of only Caco2 to the cytomix (24 and 48 hours) inflammation in HepG2 cells occurred, as shown by the increase of IL-8 and IL1-β (P < 0.01), and in the reduction of BA receptor expression (P < 0.05) (Fig. 2C).

F2
FIGURE 2:
(A) Schematic representation of the co-culture model. (B) TEER measurement to control the integrity of epithelial barrier. (C) mRNA expression of IL-8 and IL-1β cytokines and FXR, PXR, and TGR5 receptors in HepG2 after 24 and 48 hours of treatment of Caco2 with cytomix. (D) mRNA expression of IL-8, IL-1β and FXR, PXR, TGR5 in HepG2 after 3 hours of LF82 infection and after 3 hours post-infection. P < 0.05; ∗∗ P < 0.01. FXR = farnesoid-X-receptor; PXR = pregnane-X-receptor; TEER = transepithelial electric resistance.

As the persisting intestinal inflammation is usually associated to bacterial dysbiosis that is characterized by a substantial increase of bacterial groups with higher pro-inflammatory potential, such as the adherent-invasive Escherichia coli (AIEC) strains, we repeated the above experiment using the AIEC prototype, LF82, as inflammatory agent. Thus, Caco2 were challenged with LF82 (MOI10:1) for 3 hours. Again, this exposure resulted in the induction of inflammation in HepG2 cells, as shown by the increase of cytokines (P < 0.01), and in the reduction of BA receptors (P < 0.05) that returned to normal levels after LF82 removal (Fig. 2D).

Reducing Gut Inflammation Through the Anti-inflammatory Agent Dipotassium Glycyrrhizate Protects Liver Health by Improving Farnesoid-X-receptor and Pregnane-X-receptor Expression

DPG has been already studied and used in vitro and in vivo in our laboratory for its potent anti-inflammatory effects on cells from different tissues, including the gut. In this study, only Caco2 were exposed to cytomix for 48 hours or co-exposed to cytomix and DPG (150 and 300 μmol/L). We observed that DPG strongly reduced inflammation in HepG2, as shown by the decrease of IL-8 (P < 0.01), and significantly increased FXR and PXR expression dose proportionally (P < 0.05) (Fig. 3).

F3
FIGURE 3:
Farnesoid-X-receptor, pregnane-X-receptor, and IL-8 expression in HepG2 cells after a combined treatment of Caco2 with cytomix and 2 concentrations of dipotassium glycyrrhizate (DPG) (150 and 300 μmol/L) in a co-culture system. P < 0.05; ∗∗ P < 0.01.

The mRNA Expression of Farnesoid-X-receptor, Pregnane-X-receptor, and TGR5 as well as Apical Sodium-dependent Bile Acid Transporter is Notably Altered in the Inflamed Colon of Children With Inflammatory Bowel Disease

There is a growing body of interest about a supposed role of BAs in mucosal immunity and inflammation. To assess a possible contribution of BA receptors in chronic inflammation, we analyzed the expression pattern of FXR, PXR, and TGR5 in inflamed colonic areas from children with CD, UC, and in age-matched controls. Moreover, we analyzed the expression of ASBT transporter that represents the first step in bile acid reabsorption to the liver from the intestine. We found a strong decrease of FXR, PXR (P < 0.05), and ABST (P < 0.01) in the inflamed mucosa of all patients as compared with controls. We also found an increased expression of TGR5 (P < 0.05) in the same patient samples as compared with controls (Fig. 4A and B), in agreement with results previously obtained in murine inflamed colonic tissues (Fig. 1D). Accordingly, immunohistochemistry showed that FXR was mainly concentrated in enterocytes and TGR5 in phagocytes (Fig. 4C).

F4
FIGURE 4:
(A and B) Real-time PCR for the expression of FXR, PXR, TGR5, and IL-8 in inflamed colon of Crohn disease and ulcerative colitis pediatric patients. (C) mRNA level of ASBT in inflamed colon of Crohn disease and ulcerative colitis pediatric patients. (D) Immunohistochemistry for FXR and TGR5 in inflamed colon of a UC patient. CD, Crohn disease; UC, ulcerative colitis. P < 0.05; ∗∗ P < 0.01. FXR = farnesoid-X-receptor; PXR = pregnane-X-receptor.

DISCUSSION

Behind their role in nutrients absorption, Bas not only act as signaling molecules, activating several receptors that regulate central metabolic pathways but also may modulate inflammation. Accumulating data identify BAs and their receptors as pleiotropic-signaling molecules that control gut-liver crosstalk (36).

In this study, we first focused on the relationship between the gut and the liver, a close functional and vascular association also known as GLA, examining whether the occurrence of intestinal inflammation might affect healthy hepatic cells. Hence, we analyzed the liver of mice with a DSS-induced colitis and found that intestinal inflammation alters the BA receptor expression both in the inflamed colon and in the apparently normal hepatic tissue. Indeed, the cytokine analysis has proven the presence of a certain grade of inflammation also in the liver, as confirmed by histology. Interestingly, the nuclear receptors FXR and PXR were markedly down-regulated both in the gut and liver samples, whereas the trans-membrane receptor TGR5 significantly increased in the gut but not in the liver of mice.

Recent data reported that TGR5 is highly expressed in monocytes/macrophages (35,37,38), members of the mononuclear phagocyte system that circulate through the blood and extravasate into inflamed tissues. We performed immunohistochemistry and confirmed that TGR5 is principally localized in the macrophage infiltrate of murine colonic samples, whereas FXR is mainly found in the nuclei of enterocytes (in uninflamed tissue). To better investigate this different cell response, we induced in vitro inflammation in intestinal epithelial cells, hepatic cells, and macrophages and found that TGR5 was significantly increased only in the latter. This confirms that TGR5 is primarily expressed by macrophages and explains its marked increase in the inflamed gut, harboring the largest pool of macrophages.

As it is recognized that FXR has also antioxidant activities (39,40), we believe that the reduction of FXR in liver cells of DSS mice may also contribute to raise the oxidative stress level, as shown by the increased expression of COX2 and iNOS.

In summary, we found that gut inflammation induced by DSS alters BA receptor expression and causes liver inflammation in mice. It is, however, conceivable to attribute a share of damage to a direct action of the DSS that reaches the liver through the portal system. Therefore, in order to assess the extent of liver damage caused by intestinal inflammation as such, we used co-cultures of intestinal and hepatic cells. Intestinal cells, grown as a confluent monolayer mimicking the gut barrier were exposed to a mix of cytokines (TNF-α, INF-γ) as inflammatory agents. We found that the hepatic cells grown below showed increased levels of pro-inflammatory cytokines (IL-1β, IL-8) and a down-regulation of BA receptor expression, confirming that gut inflammation is able to elicit liver impairment.

Furthermore, there is a current widely agreed view that the gut microbiota (GM) plays a central role in BA host metabolism by regulating their deconjugation, dehydroxylation, dehydrogenation, and that BAs and GM reciprocally control each other's compositions (41,42). Additionally, it is known that intestinal inflammation is often associated with GM alterations leading to intestinal dysbiosis with a prevalence of adherent-invasive E coli (AIEC) pathotypes with pro-inflammatory properties (43). Hence, we used the AIEC prototype LF82 as an alternative agent to induce inflammation in intestinal cells and observed the same effects on liver cells as above. These results confirm that the onset and development of gut inflammation, featured by circulation of pro-inflammatory cytokines and abundance of AIEC strains, is also turned into a liver damage that should be seriously taken into consideration, given the chronic nature of some inflammatory processes, such as those characterizing the IBD.

Providing further evidence, we exposed intestinal cells to DPG, a salt of the glycoconjugated triterpene glycyrrhizin that exhibits potent anti-inflammatory, antiviral and antiallergic effects (44,45) and was previously used by our group to counteract gut inflammation (46). Remarkably, we observed that decreasing intestinal inflammation caused the reduction of liver inflammation and the normalization of BA receptor expression, strengthening the close gut-liver relationship.

BAs interact directly with a variety of transmembrane and nuclear receptors, in particular, FXR and TGR5 contribute to maintain the tolerogenic state of the liver and intestine immunity, indeed, they are highly expressed in innate immunity cells including intestinal and liver macrophages. Accordingly, perturbed BA circulation and/or metabolism seem to be implicated in the pathogenesis of primary sclerosing cholangitis, metabolic syndrome, colon cancer, and IBD (5,14). Thus, we analyzed the expression pattern of FXR, PXR, and TGR5 in inflamed colonic tissues of CD and UC children and found that FXR and PXR were significantly decreased, while TGR5 increased, in patients as compared with age-matched controls, in agreement with results in mice. Furthermore, as BAs secreted from the liver are mostly reabsorbed in intestinal epithelial cells via ASBT, we analyzed ASBT expression in the same samples and found that it was strongly decreased. We speculate that FXR, PXR, and ASBT decrease may result in intestinal BA accumulation worsening gut inflammation and causing liver distressing.

CONCLUSION

This study clearly demonstrates that gut inflammation, featured by the decrease of intestinal FXR, PXR, and ASBT expression and increase of TGR5, is able to affect hepatic cells by altering BA receptor levels and increasing the production of pro-inflammatory cytokines and oxidative stress. Hence, reducing gut inflammation is mandatory not only to improve the intestinal disease but also to protect the liver.

Future work will be addressed to understand whether gut inflammation is involved in the progression of liver diseases.

REFERENCES

1. Albillos A, de Gottardi A, Rescigno M. The gut-liver axis in liver disease: pathophysiological basis for therapy. J Hepatol 2020; 72:558–577.
2. Milosevic I, Vujovic A, Barac A, et al. Gut-liver axis, gut microbiota, and its modulation in the management of liver diseases: a review of the literature. Int J Mol Sci 2019; 20:395.
3. Brandl K, Kumar V, Eckmann L. Gut-liver axis at the frontier of host-microbial interactions. Am J Physiol Gastrointest Liver Physiol 2017; 312:G413–G419.
4. Vancamelbeke M, Vermeire S. The intestinal barrier: a fundamental role in health and disease. Expert Rev Gastroenterol Hepatol 2017; 11:821–834.
5. Li T, Chiang JY. Bile acids as metabolic regulators. Curr Opin Gastroenterol 2015; 31:159–165.
6. Ahmad TR, Haeusler RA. Bile acids in glucose metabolism and insulin signalling- mechanisms and research needs. Nat Rev Endocrinol 2019; 15:701–712.
7. Theiler-Schwetz V, Zaufel A, Schlager H, et al. Bile acids and glucocorticoid metabolism in health and disease. Biochim Biophys Acta Mol Basis Dis 2019; 1865:243–251.
8. Kiriyama Y, Nochi H. The biosynthesis, signaling, and neurological functions of bile acids. Biomolecules 2019; 9:232.
9. Chiang JYL, Ferrell JM. Bile acid metabolism in liver pathobiology. Gene Expr 2018; 18:71–87.
10. Wang G, Huang S, Wang Y, et al. Bridging intestinal immunity and gut microbiota by metabolites. Cell Mol Life Sci 2019; 76:3917–3937.
11. Chen ML, Takeda K, Sundrud MS. Emerging roles of bile acids in mucosal immunity and inflammation. Mucosal Immunol 2019; 12:851–861.
12. Biagioli M, Carino A. Signaling from intestine to the host: how bile acids regulate intestinal and liver immunity. Handb Exp Pharmacol 2019; 256:95–108.
13. Sipka S, Bruckner G. The immunomodulatory role of bile acids. Int Arch Allergy Immunol 2014; 165:1–8.
14. Garcia M, Thirouard L, Sedès L, et al. Nuclear receptor metabolism of bile acids and xenobiotics: a coordinated detoxification system with impact on health and diseases. Int J Mol Sci 2018; 19:3630.
15. Shin DJ, Wang L. Bile acid-activated receptors: a review on FXR and other nuclear receptors. Handb Exp Pharmacol 2019; 256:51–72.
16. Kliewer SA, Mangelsdorf DJ. Bile acids as hormones: the FXR-FGF15/19 pathway. Dig Dis 2015; 33:327–331.
17. Matsubara T, Li F, Gonzalez FJ. FXR signaling in the enterohepatic system. Mol Cell Endocrinol 2013; 368:17–29.
18. Buchman CD, Chai SC, Chen T. A current structural perspective on PXR and CAR in drug metabolism. Expert Opin Drug Metab Toxicol 2018; 14:635–647.
19. Oladimeji PO, Chen T. PXR: more than just a master xenobiotic receptor. Mol Pharmacol 2018; 93:119–127.
20. Guo C, Chen WD, Wang YD. TGR5, not only a metabolic regulator. Front Physiol 2016; 26:646.
21. Keitel V, Häussinger D. Role of TGR5 (GPBAR1) in liver disease. Semin Liver Dis 2018; 38:333–339.
22. Kim H, Fang S. Crosstalk between FXR and TGR5 controls glucagon-like peptide 1 secretion to maintain glycemic homeostasis. Lab Anim Res 2018; 34:140–146.
23. Meadows V, Kennedy L, Kundu D, et al. Bile acid receptor therapeutics effects on chronic liver diseases. Front Med (Lausanne) 2020; 7:15.
24. Xiao L, Pan G. An important intestinal transporter that regulates the enterohepatic circulation of bile acids and cholesterol homeostasis: the apical sodium-dependent bile acid transporter (SLC10A2/ASBT). Clin Res Hepatol Gastroenterol 2017; 41:509–515.
25. Ashby K, Navarro Almario EE, Tong W, et al. Review article: therapeutic bile acids and the risks for hepatotoxicity. Aliment Pharmacol Ther 2018; 47:1623–1638.
26. Schaap FG, Trauner M, Jansen PL. Bile acid receptors as targets for drug development. Nat Rev Gastroenterol Hepatol 2014; 11:55–67.
27. Arab JP, Karpen SJ, Dawson PA, et al. Bile acids and nonalcoholic fatty liver disease: molecular insights and therapeutic perspectives. Hepatology 2017; 65:350–362.
28. Trauner M, Fuchs CD, Halilbasic E, et al. New therapeutic concepts in bile acid transport and signaling for management of cholestasis. Hepatology 2017; 65:1393–1404.
29. Gadaleta RM, van Erpecum KJ, Oldenburg B, et al. Farnesoid X receptor activation inhibits inflammation and preserves the intestinal barrier in inflammatory bowel disease. Gut 2011; 60:463–472.
30. Maxwell JR, Brown WA, Smith CL, et al. Methods of inducing inflammatory bowel disease in mice. Curr Protoc Pharmacol; Chapter 5: Unit 558 2009; doi: 10.1002/0471141755.ph0558s47.
31. Hyams JS, Ferry GD, Mandel FS, et al. Development and validation of a pediatric Crohn's disease activity index. J Pediatr Gastroenterol Nutr 1991; 12:439–447.
32. Turner D, Otley AR, Mack D, et al. Development, validation, and evaluation of a paediatric ulcerative colitis activity index: a prospective multicenter study. Gastroenterology 2007; 133:423–432.
33. Daperno M, D’Haens G, Van Assche G, et al. Development and validation of a new, simplified endoscopic activity score for Crohn's disease: the SES-CD. Gastrointest Endosc 2004; 60:505–512.
34. Schroeder KW, Tremaine WJ, Ilstrup DM. Coated oral 5-amino- salicylic acid therapy for mildly to moderately active ulcerative colitis. A randomized study. N Engl J Med 1987; 317:1625–1629.
35. Biagioli M, Carino A, Cipriani S, et al. The bile acid receptor GPBAR1 regulates the M1/M2 phenotype of intestinal macrophages and activation of GPBAR1 rescues mice from murine colitis. J Immunol 2017; 199:718–733.
36. Schneider KM, Albers S, Trautwein C. Role of bile acids in the gut liver axis. J Hepatol 2018; 68:1083–1085.
37. Islam Z, Horikawa A, Inui T, et al. Datasets of microarray analysis to identify Gpr137b-dependent interleukin-4-responsive genes in the mouse macrophage cell line RAW264. Data Brief 2019; 23:103669.
38. Perino A, Schoonjans K. TGR5 and immunometabolism: insights from physiology and pharmacology. Trends Pharmacol Sci 2015; 36:847–857.
39. Zhu JB, Xu S, Li J, et al. Farnesoid X receptor agonist obeticholic acid inhibits renal inflammation and oxidative stress during lipopolysaccharide-induced acute kidney injury. Eur J Pharmacol 2018; 5:60–68.
40. Vavassori P, Mencarelli A, Renga B, et al. The bile acid receptor FXR is a modulator of intestinal innate immunity. Immunol 2009; 15:6251–6261.
41. Kegami T, Honda A. Reciprocal interactions between bile acids and gut microbiota in human liver diseases. Hepatol Res 2018; 48:15–27.
42. Tripathi A, Debelius J, Brenner DA, et al. The gut-liver axis and the intersection with the microbiome. Nat Rev Gastroenterol Hepatol 2018; 15:785.
43. Shaler CR, Elhenawy W, Coombes BK. The unique lifestyle of Crohn's disease-associated adherent-invasive Escherichia coli. J Mol Biol 2019; 431:2970–2981.
44. Selyutina OY, Polyakov NE. Glycyrrhizic acid as a multifunctional drug carrier - from physicochemical properties to biomedical applications: a modern insight on the ancient drug. Int J Pharm 2019; 559:271–279.
45. Ming LJ, Yin AC. Therapeutic effects of glycyrrhizic acid. Nat Prod Commun 2013; 8:415–418.
46. Vitali R, Palone F, Cucchiara S, et al. Dipotassium glycyrrhizate inhibits HMGB1-dependent inflammation and ameliorates colitis in mice. PLoS One 2013; 8:e66527.
Keywords:

bile acid receptors; gut; inflammation; inflammatory bowel disease; liver

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