Progressive induction of hepatocyte progenitor cells in chronically injured liver

Differentiated epithelial cells show substantial lineage plasticity upon severe tissue injuries. In chronically injured mouse livers, part of hepatocytes become Sry-HMG box containing 9 (Sox9) (+) epithelial cell adhesion molecule (-) hepatocyte nuclear factor 4 α (+) biphenotypic hepatocytes. However, it is not clear whether all Sox9⁺ hepatocytes uniformly possess cellular properties as hepatocyte progenitors. Here, we examined the microarray data comparing Sox9⁺ hepatocytes with mature hepatocytes and identified CD24 as a novel marker for biphenotypic hepatocytes. Immunohistochemical analyses showed that part of Sox9⁺ hepatocytes near expanded ductular structures expressed CD24 in the liver injured by 3,5-diethoxycarbonyl-1,4-dihydro-collidine (DDC) diet and by bile duct ligation. Indeed, Sox9⁺ hepatocytes could be separated into CD24^- and CD24⁺ cells by fluorescence activated cell sorting. The ratio of CD24⁺ cells against CD24^- ones in Sox9⁺ hepatocytes gradually increased while DDC-injury progressed and colony-forming capability mostly attributed to CD24⁺ cells. Although hepatocyte markers were remarkably downregulated in of Sox9⁺ CD24⁺ hepatocytes, they re-differentiated into mature hepatocytes in vitro and in vivo. Our current results demonstrate that the emergence of biphenotypic hepatocytes is a sequential event including the transition from CD24^- and CD24⁺ status, which may be a crucial step for hepatocytes to acquire progenitor properties.