Temporal analysis of hepatocyte differentiation by small hepatocyte-like progenitor cells during liver regeneration in retrorsine-exposed rats

Liver regeneration after two-thirds surgical partial hepatectomy (PH) in ra ts treated with the pyrrolizidine alkaloid retrorsine is accomplished throu gh the activation, expansion, and differentiation of a population of small hepatocyte-like progenitor cells (SHPCs). We have examined expression of th e major liver-enriched transcription factors, cytochrome P450 (CYP) enzymes , and other markers of hepatocytic differentiation in SHPCs during the prot racted period of liver regeneration after PH in retrorsine-exposed rats, Ea rly-appearing SHPCs (at 3-7 days after PH) express mRNAs for all of the maj or liver-enriched transcription factors at varying levels compared to fully differentiated hepatocytes. In addition, SHPCs lack (or have significantly reduced) expression of mRNA for hepatocyte markers tyrosine aminotransfera se and alpha-1 antitrypsin, but their expression levels of mRNA and/or prot ein for WT1 and alpha-fetoprotein. (AFP) are increased. With the exception of AFP expression, SHPCs resembled fully differentiated hepatocytes by 14 d ays after PH. Expression of AFP was maintained by most SHPCs through 14 day s after PH, gradually declined through 23 days after PH, and was essentiall y absent from SHPC progeny by 30 days after PH. Furthermore, early appearin g SHPCs lack (or have reduced expression) of hepatic CYP proteins known to be induced in rat livers after retrorsine exposure. The resistance of SHPCs to the mitoinhibitory effects of retrorsine may be directly related to a l ack of CYP enzymes required to metabolize retrorsine to its toxic derivativ es. These results suggest that SHPCs represent a unique parenchymal (less d ifferentiated) progenitor cell population of adult rodent liver that is phe notypically distinct from fully differentiated hepatocytes, biliary epithel ial cells, and (ductular) oval cells.