Identification and characterization of a rat hepatic oncofetal membrane glycoprotein.

A major interest of our laboratory is to delineate the pathways leading to experimentally induced liver cancer in the rat. Although the cellular progenitors of primary hepatocellular carcinoma remain controversial, current findings suggest that proliferation of chemically initiated liver epithelial cells gives rise to hepatic nodules, a rare population of which eventually progress to carcinoma. Presently, the availability of cell surface markers that are closely associated with malignant progression is needed for the identification, isolation, and further characterization of these rare malignant cells. In this paper, we describe two new monoclonal antibodies (MAbs), MAb 324.5 and MAb 324.9, that recognize a novel oncofetal membrane glycoprotein, designated TuAg1. MAbs 324.5 and 324.9 were produced using three different transplantable hepatocellular carcinoma cell lines during immunization and screening. MAb 324.5 and MAb 324.9 were shown to be reactive with different epitopes on TuAg1 by competitive immunoprecipitation assays combined with results from immunodepletion analysis and one-dimensional V-8 peptide maps. TuAg1 showed variations in molecular weight from 78,000 to 92,000, and a marked heterogeneity in pI, with charge variants ranging between 4.3 and 6.0. The 324.5-epitope was not expressed at detectable levels in any adult normal tissues or during liver regeneration but was transiently expressed during fetal liver development as shown by indirect immunofluorescence analysis of frozen tissue sections. In contrast, the 324.9-epitope was observed on nerve fibers and ganglia and on sperm tails in the adult rat and also appeared independently of the 324.5-epitope during fetal development. Although normal hepatocytes did not express TuAg1, isolated hepatocytes became positive during the first 24 h of primary culture. Attempts to modulate the in vitro expression of TuAg1 were unsuccessful; however, TuAg1 was lost within 7 days following ectopic transplantation of cultured hepatocytes into the pancreas. During the carcinogenic process, TuAg1 was expressed by a rare population of hepatic nodules, by many primary liver tumors, and by all lung metastases and transplantable hepatocellular carcinomas examined to date. Taken together, these observations suggest that the in vivo constitutive expression of this novel oncofetal membrane antigen is closely associated with acquisition of the malignant phenotype during hepatocarcinogenesis.