DELINEATION OF ANTIGENIC PATHWAYS OF ETHIONINE-INDUCED LIVER-CANCER IN THE RAT

Although oval cell proliferation is observed prior to the appearance o f hepatic nodules and hepatocellular carcinomas during ethionine-induc ed liver carcinogenesis in the rat, the role of these presumptive hepa tic stem cells during the neoplastic process remains controversial. In order to investigate this question, we have used a panel of monoclona l antibodies against antigens associated with normal hepatocytes, oval cells and transplantable hepatocellular carcinomas (THC) to trace ant igenic pathways leading to liver cancer. Male ACI rats were fed a chol ine-deficient diet containing 0.1% DL-ethionine for 4, 16 or 30 weeks. Immunocytochemical analysis of frozen liver sections revealed a subpo pulation of hepatic nodules (7/52), carcinomas (8/15) and lung metasta ses (3/5) containing populations of cells expressing both oval cell, h epatocyte and neoplastic markers. Carcinomas expressing oval cell mark ers often appeared as a mosaic of well-defined patches composed of phe notypically distinct cells. Many of the phenotypes expressed closely m imicked patterns of expression observed in fetal and neonatal liver. T HC derived from primary tumors positive for oval cell antigens (4/5) c ontinued to express these markers. Northern blot analysis and immunocy tochemical analysis revealed that 4/5 primary hepatocellular carcinoma s (PHC) and THC expressed a-fetoprotein (AFP) and albumin transcripts and contained subpopulations expressing AFP together with hepatocyte a nd oval antigens. In contrast, a well-differentiated PHC and its corre sponding THC lacked AFP mRNA and oval cell antigens but showed strong expression of both hepatocyte and neoplastic markers. These results de monstrate that a subpopulation of malignant and metastatic hepatocellu lar carcinomas are comprised of cells expressing multiple oval cell ma rkers in this model system.