Contribution of mature hepatocytes to small hepatocyte-like progenitor cells in retrorsine-exposed rats with chimeric livers.

The potential lineage relationship between hepatic oval cells, small hepatocyte-like progenitor cells (SHPCs), and hepatocytes in liver regeneration is debated. To test whether mature hepatocytes can give rise to SHPCs, rats with dipeptidyl peptidase IV (DPPIV) chimeric livers, which harbored endogenous DPPIV-deficient hepatocytes and transplanted DPPIV-positive hepatocytes, were subjected to retrorsine treatment followed by partial hepatectomy (PH). DPPIV-positive hepatocytes comprised about half of the DPPIV chimeric liver mass. Tissues from DPPIV chimeric livers after retrorsine/PH treatment showed large numbers of SHPC clusters. None of the SHPC clusters were stained positive for DPPIV in any analyzed samples. Furthermore, serial sections stained for gamma-glutamyl-transpeptidase (GGT, a marker of fetal hepatoblasts) and glucose-6-phosphatase (G6Pase, a marker of mature hepatocytes) showed inverse expression of the two enzymes and a staining pattern consistent with a lineage that begins with GGT(+)/G6Pase(-) to GGT(-)/G6Pase(+) within a single SHPC cluster. Using double immunofluorescence staining for markers specific for hepatic oval cells and hepatocytes in serial sections, oval cell proliferations with CK-19(+)/laminin(+) and OV-6(+)/C/EBP-α(-) were shown to extend from periportal areas into the SPHC clusters, differentiating into hepatic lineage by progressive loss of CK-19/laminin expression and appearance of C/EBP-α expression towards the cluster side. Cells in the epithelial cell adhesion molecule (EpCAM(+)) SHPC clusters showed membranous EpCAM(+)/HNF-4α(+) (hepatocyte nuclear factor-4α) staining and were contiguous to the surrounding cytoplasmic EpCAM(+)/HNF-4α(-) ductular oval cells. Extensive elimination of oval cell response by repeated administration of 4,4'-methylenedianiline (DAPM) to retrorsine-exposed rats impaired the emergence of SHPC clusters. These findings highly suggest the hepatic oval cells but not mature hepatocytes as the origin of SHPC clusters in retrorsine-exposed rats.