The gut and the liver are anatomically related 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.
The GLA connects the liver with the intestine via bile acid metabolism. Bile acids (BAs) are amphipathic steroid acids, synthesized from cholesterol in the liver, that regulate lipid, glucose and energy metabolism. Moreover, a specific role of BAs as immunomodulators is emerging.
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 TGR5.
Recent findings suggest that the occurrence of gut inflammation, featured by altered epithelial and vascular permeability that causes the major translocation of bacterial antigens, may affect the healthy liver as well as worsen the severity of liver diseases, in particular of the non-alcholic fatty liver disease (NAFLD) and its progressive form, the non- alcoholic steatohepatitis (NASH).
Aims:
Accordingly, the whole purpose of this project is to assess the impact of gut inflammation on liver health and disease
For this purpose, in a first part of the thesis we aimed to:
1.Investigate in vitro and in vivo the possibility that gut inflammation affects the healthy liver by altering BA receptors FXR, PXR and TGR5 and increasing the production of inflammatory and oxidative stress molecules;
2.analyze ex vivo the expression pattern of BA receptors and the apical sodium- dependent bile acid transporter (ASBT) in the inflamed colonic tissues of a group of pediatric patients with inflammatory bowel disease (IBD) and age-matched controls.
In a second part of the thesis, we aimed to:
1. develop an animal model of hepatic steatosis, displaying the disease both in its early (NAFLD) and late (NASH) phase, in which an important intestinal inflammation was also induced;
2. use this model to assess that gut inflammation significantly contributes to the progression of the liver disease (from NAFLD to NASH) by altering BA receptor expression and increasing inflammatory (IL-6, TNF, NLRP3, TLR4, MCP-1, HMGB1) as well as fibrotic (TGF-, -SMA) mediator expression;
3. evaluate the potential of the anti-inflammatory molecule, the dipotassium glycyrrhizate (DPG), to improve the liver disease by reducing gut inflammation.
Results:
Results and conclusions of the first part of experimentation are reported in the original paper published in Journal of Pediatric Gastroenterology and Nutrition (Negroni A, Fiaschini N, Palone F, Vitali R, Colantoni E, Laudadio I, Oliva S, Aloi M, Cucchiara S, Stronati L. “Intestinal inflammation alters the expression of hepatic bile acid receptors causing liver impairment” J Pediatr Gastroenterol Nutr. 2020 Aug;71(2):189-196) that has been attached to the thesis.
Results of the second part of experimentation are fully described in the thesis. We have developed an animal model with intestinal inflammation and liver steatosis/steatohepatitis by treating C57BL/6J mice with dextran sodium sulphate (DSS) to induce colitis and high fat diet (HFD) with high glucose/fructose for different times to induce NAFLD/NASH. Mice with NAFLD/NASH without colitis served as control group. A further group of NAFLD/NASH-DSS-mice were also treated with DPG.
Results show that gut inflammation, assessed by the release of the alarmin HMGB1 in the stools, and consequent altered intestinal epithelial and vascular permeability, confirmed by a reduced expression of the tight junction protein zonulin-1 (ZO-1) and an increased level of the endothelial cell-specific protein plasmalemma vesicle-associated protein 1 (PV1), correlate with altered BAs receptor expression (TGR5 and PXR), increase of inflammatory marker expression (IL-6, TNF, NLRP3, TLR4, MCP-1, HMGB1) and inflammatory infiltrate in the steatotic liver of NASH-DSS mice. Moreover, the latter showed a significant rise of collagen fiber deposition and increased fibrotic marker (-
SMA and TGF-) expression as compared to DSS-mice. The administration of DPG to DSS- NASH mice significantly reduced these effects.
Conclusions:
These data confirm our hypothesis that the presence of gut inflammation causes liver injury and accelerates fibrosis in a steatotic liver, contributing to the progression of NAFLD towards NASH. We also suggest that reducing gut inflammation by using DPG could represent an interesting novel strategy for the management of the hepatic disease
