Mast cells promote biliary proliferation and hepatic fibrosis in normal and HDC −/− mice by interacting with cholangiocytes and hepatic stellate cells via TGF‐β1 signaling

Background In normal, homeostatic conditions, cholangiocytes are mitotically dormant, hepatic stellate cells (HSCs) are quiescent and mast cell (MC) numbers are relatively low. Upon injury resulting in hepatic fibrosis, there is an activation of HSCs and the release of pro‐inflammatory factors like TGF‐β1 from proliferating cholangiocytes. MCs infiltrate the liver following injury and promote biliary hyperplasia by releasing factors like histamine. In mice lacking the histidine decarboxylase gene (HDC −/− mice), MCs are morphologically altered and also low in number. We tested the hypothesis that MCs promote fibrosis in normal and HDC −/− mice by increasing HSC activation, cholangiocyte proliferation and TGF‐β1 release. Methods Normal, wild‐type (WT) mice and HDC −/− mice were injected with 1xPBS or cultured MCs derived from fetal mouse liver (5 × 10 6 ) via tail vein delivery. After 6 days, mice were sacrificed and liver blocks, isolated cholangiocytes and serum was obtained. Biliary mass and proliferation was evaluated by immunohistochemistry for CK‐19 and PCNA, respectively, and hepatic fibrosis and collagen deposition was measured by Masson's Trichrome and Fast Green/Sirius Red staining. Real‐time PCR was performed in total liver and isolated cholangiocytes for fibrotic markers, α‐SMA, collagen‐type 1a and fibronectin. HSC activation was measured by immunofluorescence and real‐time PCR in total liver for synaptophysin‐9 (SYP‐9). TGF‐β1 secretion was measured by EIA in serum from all mice and by real‐time PCR in total liver and isolated cholangiocytes. In vitro , cultured HSCs and cholangiocytes were treated with cultured MCs and fibrosis markers and TGF‐β1 expression/secretion was measured. Results MC injection increased biliary mass, proliferation and hepatic fibrosis in both WT and HDC −/− mice compared to controls. HSC activation was also enhanced in mice injected with MC compared to controls. TGF‐β1 secretion in serum and gene expression was increased following MC injections in both WT and HDC −/− mice compared to controls. In vitro , co‐culture with MCs increased cholangiocyte and HSC proliferation, activation and fibrotic marker gene expression. Conclusion In normal conditions, MCs are low in number, however; when the liver is damaged, MCs infiltrate, release profibrotic factors and induce further damage. Our results demonstrate that MCs promote biliary proliferation, HSC activation and hepatic fibrosis in normal and HDC −/− mice via increased TGF‐β1 signaling. MCs are a pro‐proliferative and pro‐fibrogenic component of hepatic fibrosis. Targeting MCs may be a key therapeutic in fibrotic‐related liver pathologies when MC numbers are increased. Support or Funding Information VA Career Development Award