Cellular and molecular changes in the early stages of chemical hepatocarcinogenesis in the rat

The early cellular and molecular changes in the Solt-Farber model of hepatocarcinogenesis with and without initiation was studied by using histochemical, immunohistochemical, and in situ hybridization techniques. Increased cellularity was observed in the periductal space in both models 32 to 56 h after partial hepatectomy. These periductal cells and Ito cells were the only cells that became labeled with tritiated thymidine in the uninitiated liver model. Forty-five to 60% of the labeled periductal cells were positive for gamma-glutamyltranspeptidase. From the periductal area the cells that were positive for antibody raised against oval cells (OV-6) infiltrated into liver parenchyma and were followed by desmin-positive Ito cells. The number of Ito cells in the uninitiated model 6 days after partial hepatectomy was 3.5 times higher in the area occupied by oval cells than elsewhere in the liver. The first alpha-fetoprotein (AFP)-positive cells appeared either as individual cells or as pseudoductal formations 32 or 56 h after partial hepatectomy at the periphery of the periductal space in both initiated and uninitiated animals. A combination of in situ and immunohistochemistry revealed that the OV-6-positive cells were AFP positive, whereas desmin-positive cells were AFP negative. Glutathione S-transferase P (GST-P) transcripts could be found mainly in OV-6-positive oval cells. Bile duct cells were positive for GST-P and negative for transforming growth factor beta 1, whereas cells in the periductal space were positive for both of these transcripts. The GST-P-positive early preneoplastic lesions showed a similar distribution pattern as that of oval cells; the preexisting hepatocytes became trapped between small basophilic hepatocytes that showed either irregular or pseudoalveolar arrangement. This raises the question as to whether cells which are stem cell-like are among the target cells in the Solt-Farber model of hepatocarcinogenesis. Proliferation of transforming growth factor beta 1-producing, desmin-positive cells (Ito cells) and multipotent oval cells in a close proximity to each other indicates an intricate relationship between Ito cells and oval cells in liver that warrants further investigation.