Sortilin Deficiency Reduces Ductular Reaction, Hepatocyte Apoptosis, and Liver Fibrosis in Cholestatic-Induced Liver Injury

Sortilin, a member of the vacuolar protein sorting 10 domain receptor family, traffics newly synthesized proteins from the trans-Golgi network to secretory pathways, endosomes, and cell surface. Sortilin-trafficked molecules, including IL-6 and acid sphingomyelinase (aSMase), mediate cholangiocyte proliferation and liver inflammation, hepatic stellate cell activation, hepatocyte apoptosis, and fibrosis. Based on these sortilin-regulated functions, we investigated its role in biliary damage leading to hepatocellular injury and fibrosis. Sortilin^−/− mice displayed impaired inflammation and ductular reaction 3 days after bile duct ligation (BDL), as demonstrated by reduced cholangiocyte proliferation and activation and reduced serum IL-6. Interestingly, liver fibrosis was reduced in Sortilin^−/− mice after both BDL and carbon tetrachloride treatment, in line with attenuated in vitro activation of Sortilin^−/− hepatic stellate cells. Sortilin^−/− hepatic aSMase activity was reduced in the BDL and carbon tetrachloride models and accompanied by reduced in vivo hepatocyte apoptosis. In addition, wild type (WT), but not Sortilin^−/− hepatocytes, had increased aSMase-dependent susceptibility to bile acid–induced apoptosis in vitro. Mechanistically, short-term IL-6 neutralization in bile duct–ligated WT mice decreased hepatic inflammation and reactive cholangiocyte-derived cytokines and chemokines, without affecting fibrosis, whereas pharmacological inhibition of aSMase activity was not sufficient to attenuate hepatic fibrosis. Only combined IL-6 and aSMase inhibition significantly reduced fibrosis in bile duct–ligated WT mice. We conclude that sortilin regulates cholestatic liver damage and fibrosis via effects on both aSMase activity and serum IL-6.