Abstract: Objective: To investigate the protective effect of urolithin A (UroA) against liver fibrosis (LF) from the perspective of the gut-liver axis, with a focus on the potential mechanism by which UroA exerts its anti-LF effect by regulating the bile acid (BA)-gut microbiota axis. Methods: The therapeutic potential of UroA was evaluated using a mouse model of CCl4-induced LF and transforming growth factor-β1 (TGF-β1)-activated hepatic stellate cells (LX2). Histopathology, biochemical assays, quantitative real-time polymerase chain reaction (PCR), and Western blot were used to determine liver injury markers (alanine aminotransferase (ALT), aspartate aminotransferase (AST), and alkaline phosphatase (ALP)), fibrosis markers (α-smooth muscle actin (α-SMA), collagen type I (COL1A1), and TGF-β1), and inflammatory factors (tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and interleukin-1β (IL-1β)). 16S rDNA sequencing was performed to analyze the gut microbiota, immunohistochemistry was used to detect the expression of the intestinal tight junction proteins zonula occludens-1 (ZO-1) and occludin, and targeted metabolomics was used to profile BA metabolism. The expression of the key molecules in the farnesoid X receptor (FXR) signaling pathway—cytochrome P450 family 7 subfamily A member 1 (CYP7A1), cytochrome P450 family 27 subfamily A member 1 (CYP27A1), cytochrome P450 family 8 subfamily B member 1 (CYP8B1), bile salt export pump (BSEP), and multidrug resistance-associated protein 2 (MRP2)—was also examined. Results: UroA significantly reduced serum ALT, AST, and ALP levels, decreased hepatic hydroxyproline content, and mitigated liver injury and collagen deposition. Moreover, UroA downregulated the expression of fibrogenic markers. 16S rDNA sequencing revealed that UroA reshaped the gut microbiota composition, significantly increasing the abundance of beneficial genera such as Lawsonibacter, Paramuribaculum, Oscillibacter, Barnesiella, and Akkermansia, while reducing the abundance of the pro-inflammatory genus Allobaculum. Furthermore, UroA strengthened intestinal barrier function by upregulating the expression of tight junction proteins and restored BA homeostasis by reducing the level of the FXR antagonist tauro-β-muricholic acid (T-β-MCA). Mechanistic studies revealed that UroA activated the FXR pathway, thereby inhibiting the BA synthesis enzymes CYP7A1, CYP27A1, and CYP8B1 while promoting the expression of the BA efflux transporters BSEP and MRP2. Conclusion: UroA reshapes the gut microbiota, restores the intestinal mucosal barrier, and improves BA homeostasis via regulating the gut microbiota-BA-FXR axis, thereby mitigating LF. These findings provide new potential targets and a theoretical basis for LF treatment.

Key words: bile acid metabolism; farnesoid X receptor; gut microbiota; liver fibrosis; urolithin A