SOX9-positive hepatocytes mediate the progression of ductular reaction in biliary atresia

Ductular reaction (DR) is seen in liver diseases as an abnormal proliferation of cells with a biliary phenotype; however, the mechanism underlying this condition is controversial. We have previously demonstrated ectopic expression of SOX9, a marker of biliary cells, on HepPar1-positive hepatocytes in biliary atresia (BA). Notch signaling has recently been reported to be involved in the biliary conversion of hepatocytes in vivo. The aims of this study, using BA livers, were to elucidate the involvement of ectopic SOX9 expression with DR and the association between Notch signaling and DR progression. Formalin-fixed paraffin embedded liver sections from 47 BA livers were used. Liver specimens from three living liver transplant donors were included as control. Two Alagille syndrome (AGS) livers were used for investigating the involvement of Notch signaling. The medical records of the patients were retrospectively reviewed. Image and data processing were performed using ImageJ software. Progressive cholestatic liver injury after Kasai portoenterostomy led to decreasing numbers of SOX9+HepPar1+ cells and an increasing CK19+ area. SOX9+HepPar1+ cell numbers were significantly negatively correlated with CK19+ area size (r=-0.6, P=0.002) at the time of liver transplant. Immunohistochemical analysis using AGS liver, which has defective Notch signaling, showed remarkable accumulation of SOX9+HepPar1+ cells and a reduction in the number of CK19+ cells. The progression of DR in BA liver, characterized by increasing CK19+ area, is mediated by SOX9+HepPar1+ cells, and Notch signaling may be required for the progression of DR.