Sry HMG Box Protein 9-positive (Sox9+) Epithelial Cell Adhesion Molecule-negative (EpCAM−) Biphenotypic Cells Derived from Hepatocytes Are Involved in Mouse Liver Regeneration*

Background: Biphenotypic hepatocytes expressing Sox9 emerge upon liver injuries associated with ductular reaction. Results: Sox9+ biphenotypic hepatocytes are derived from mature hepatocytes (MHs). Some of them are incorporated into ductular structures, whereas they efficiently differentiate to functional hepatocytes. Conclusion: Biphenotypic hepatocytes not only terminally convert to cholangiocytes but also differentiate back to MHs. Significance: Mature epithelial cells can show plasticity upon severe injuries and contribute to regeneration. It has been shown that mature hepatocytes compensate tissue damages not only by proliferation and/or hypertrophy but also by conversion into cholangiocyte-like cells. We found that Sry HMG box protein 9-positive (Sox9+) epithelial cell adhesion molecule-negative (EpCAM−) hepatocyte nuclear factor 4α-positive (HNF4α+) biphenotypic cells showing hepatocytic morphology appeared near EpCAM+ ductular structures in the livers of mice fed 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-containing diet. When Mx1-Cre:ROSA mice, which were injected with poly(I:C) to label mature hepatocytes, were fed with the DDC diet, we found LacZ+Sox9+ cells near ductular structures. Although Sox9+EpCAM− cells adjacent to expanding ducts likely further converted into ductular cells, the incidence was rare. To know the cellular characteristics of Sox9+EpCAM− cells, we isolated them as GFP+EpCAM− cells from DDC-injured livers of Sox9-EGFP mice. Sox9+EpCAM− cells proliferated and could differentiate to functional hepatocytes in vitro. In addition, Sox9+EpCAM− cells formed cysts with a small central lumen in collagen gels containing Matrigel® without expressing EpCAM. These results suggest that Sox9+EpCAM− cells maintaining biphenotypic status can establish cholangiocyte-type polarity. Interestingly, we found that some of the Sox9+ cells surrounded luminal spaces in DDC-injured liver while they expressed HNF4α. Taken together, we consider that in addition to converting to cholangiocyte-like cells, Sox9+EpCAM− cells provide luminal space near expanded ductular structures to prevent deterioration of the injuries and potentially supply new hepatocytes to repair damaged tissues.