Lineage plasticity of cholangiocytes and hepatocytes in liver regeneration

The liver contains two types of epithelial cells, hepatocytes and cholangiocytes. When the self-duplicating capability of hepatocytes or cholangiocytes is impaired, it has been suggested that liver stem/progenitor cells are activated and supply new hepatocytes and/or cholangiocytes. In addition, lineage conversions between hepatocytes and cholangiocytes have been observed in chronically injured livers of human as well as rodents. However, it is still unclear how lineage conversion contributes to liver regeneration and how the cellular plasticity is regulated. We isolated EpCAM(+) cholangiocytes from neonatal and adult livers and induced hepatocytic differentiation. We found that neonatal, but not adult, cholangiocytes differentiated into hepatocytes in vitro. We identified that a transcription factor Grhl2 is a key factor limiting differentiation potential of cholangiocytes. We also examined lineage plasticity of hepatocytes. When mature hepatocytes were labeled with LacZ in Mx1Cre:ROSA mice by peritoneal injection of poly(I:C), LacZ(+)Sox9(+)EpCAM(-) biphenotypic hepatocytes emerged near expanded ductular structures upon chronic liver injury induced by feeding mice with 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) diet or by bile duct ligation. Those biphenotypic cells were isolated as GFP(+)EpCAM(-) cells from Sox9-EGFP mice, which efficiently differentiated to functional hepatocytes in vitro. We also found that part of biphenotypic cells further converted to cholangiocyte-like cells, but the incident was rare. Molecular mechanisms regulating lineage plasticity of cholangiocytes and physiological significance of hepatocytes with biliary phenotypes will be discussed.