Immunohistochemical characterization of hepatoblastomas in B6C3F1 mice treated with diethylnitrosamine and sodium phenobarbital.

Hepatoblastomas (HBs) were induced in B6C3F1 male mice by diethylnitrosamine (DEN) and sodium phenobarbital (PB). Six-week-old mice received a single intraperitoneal dose of DEN followed by a continuous treatment with PB in diet at a concentration of 0 (group 1) or 500 (group 2) ppm for 50 weeks. HBs were observed in 13 of 21 (62%) group 2 mice, with typical histologic features as reported previously, while no such tumors were observed in group 1. Seven of 13 (54%) HBs were found in and/or adjacent to hepatocellular adenomas (HCAs) or hepatocellular carcinomas (HCCs). Immunohistochemically, all HBs were positive for S-100 protein but negative for keratin, alpha-fetoprotein (AFP), albumin (ALB) and vimentin, while HCC cells occasionally reacted positively for AFP with a mosaic pattern. HCC and HCA cells were occasionally positive for ALB. Non-neoplastic hepatocytes and normal bile ducts were positively stained for ALB and keratin/S-100 protein, respectively. S-100 protein is known to be expressed in many mesenchymal tissues and neoplasms including neuroectodermal elements but negative in cells of the hepatic lineage. Thus, the present immunohistochemical results suggested that mesenchymal differentiation occurs in mouse HB cells as observed in human HBs, one of the most frequent infant liver tumors in humans. Although the susceptibility of mouse HBs to PB-promotion suggests a hepatocytic histogenesis, the present immunohistochemical results support the hypothesis that the mouse HB is derived from pluripotent endodermal stem-like cells.