Gut microbiota-mediated bile acid metabolism aggravates biliary injury after liver transplantation through mitochondrial apoptosis

Ischemic-type biliary lesions (ITBL) are a major cause of graft loss and even mortality after liver transplantation (LT). The underlying cellular mechanisms for ITBL remain unclear. Gut microbiota has been found to be closely related to complications after LT. Here, using gut microbiome compositions, we found patients with ITBL had a higher abundance of bacteria associated with bile salt metabolism. These bacteria are reported to convert cholic acid (CA) into deoxycholic acid (DCA), consistent with our data that there were higher DCA concentrations and DCA/CA ratio in patients with ITBL than patients without ITBL. Using an in vitro model, human intrahepatic biliary epithelial cells (HIBEC) subjected to DCA showed a higher apoptosis rate, lower viability, and higher levels of cleaved-caspase3 than CA at the same concentration. DCA also changed the morphology of mitochondria and farnesoid X receptor (FXR) expression. Interestingly, DCA-induced apoptosis rate was significantly reduced in HIBEC when the FXR or BAX gene was knocked down, suggesting that DCA-induced apoptosis was dependent on FXR-mitochondrial pathway. Furthermore, increasing DCA/CA ratio in a bile acid-feeding mouse model resulted in cholangiocyte apoptosis and impaired liver function. The patients with ITBL also showed an increased proportion of TUNEL-positive biliary epithelial cells than those without ITBL. These suggest that changes in the gut microbiota following LT may enhance the conversion of CA to DCA, and may contribute to biliary damage via FXR-mitochondrial apoptosis pathway, providing new ideas for the early monitoring and treatment of ITBL.