Cholestasis is characterized by increased total bile acid (TBA) levels, which are regulated by farnesoid X receptor (FXR)/FGF15. Patients with primary sclerosing cholangitis (PSC) typically present with inflammatory bowel disease (IBD). Mast cells (MCs) (i) express FXR and (ii) infiltrate the liver during cholestasis promoting liver fibrosis. In bile‐duct‐ligated (BDL) MC‐deficient mice (B6.Cg‐Kit^W‐sh/HNihrJaeBsmJ [Kit^W‐sh]), ductular reaction (DR) and liver fibrosis decrease compared with BDL wild type, and MC injection exacerbates liver damage in normal mice.

In this study, we demonstrated that MC‐FXR regulates biliary FXR/FGF15, DR, and hepatic fibrosis and alters intestinal FXR/FGF15. We found increased MC number and biliary FXR expression in patients with liver injury compared with control. Histamine and FGF19 serum levels and small heterodimer partner expression increase in patients PSC and PSC‐IBD compared with healthy controls. MC injection increased liver damage, DR, inflammation, biliary senescence/senescence‐associated secretory phenotype (SASP), fibrosis, and histamine in Kit^W‐sh mice. Inhibition of MC‐FXR before injection reduced these parameters. BDL and Kit^W‐sh mice injected with MCs displayed increased TBA content, biliary FXR/FGF15, and intestinal inflammation, which decreased in BDL Kit^W‐sh and Kit^W‐sh mice injected with MC‐FXR. MCs increased ileal FXR/FGF15 expression in Kit^W‐sh mice that was reduced following FXR inhibition. BDL and multidrug resistance 2/ATP‐binding cassette family 2 member 4 knockout (Mdr2^−/−) mice, models of PSC, displayed increased intestinal MC infiltration and FXR/FGF15 expression. These were reduced following MC stabilization with cromolyn sodium in Mdr2^−/− mice. In vitro, MC‐FXR inhibition decreased biliary proliferation/SASP/FGF and hepatic stellate cell activation.

Our studies demonstrate that MC‐FXR plays a key role in liver damage and DR, including TBA regulation through alteration of intestinal and biliary FXR/FGF15 signaling.